Announcement

**Lance** · October 20, 2023, 04:56 PM

Weekly U.S. Influenza Surveillance Report

Print
Updated October 20, 2023

Key Updates for Week 41, ending October 14, 2023

Seasonal influenza activity remains low nationally although small increases were reported in some parts of the country. Viruses

Clinical Lab 1.3%

(Trend )

positive for influenza
this week

Public Health Lab
The most frequently reported influenza viruses this week were influenza A(H1N1).

Virus Characterization
Genetic and antigenic characterization are summarized in this report. Illness

Outpatient Respiratory Illness 2.3%

(Trend )

of visits to a health care provider this week were for respiratory illness
(below baseline).

Outpatient Respiratory Illness: Activity Map
This week 0 jurisdictions experienced moderate activity and 2 jurisdictions experienced high activity.

FluSurv-NET 0.3 per 100,000

cumulative hospitalization rate.

NHSN Hospitalizations 1,228

(Trend )

patients admitted to hospitals with influenza
this week.

NCHS Mortality 0.05%

(Trend )

of deaths attributed influenza this week.

Pediatric Deaths 1

death that occurred during the 2022-2023 season was reported this week.

All data are preliminary and may change as more reports are received.

Directional arrows indicate changes between the current week and the previous week. Additional information on the arrows can be found at the bottom of this page.

A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.

Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.

Key Points

Seasonal influenza activity remains low nationally, but there are slight increases in some parts of the country.
Nationally, outpatient respiratory illness is below baseline¹, and all 10 HHS regions are below their respective baselines.
The number of flu hospital admissions remains low.
During week 41, of the 159 viruses reported by public health laboratories, 124 (78.0%) were influenza A, and 35 (22.0%) were influenza B. Of the 99 influenza A viruses subtyped during week 41,93 (93.9%) were influenza A(H1N1), and 6 (6.1%) were A(H3N2).
One influenza-associated pediatric death that occurred during the 2022-2023 season was reported this week.
CDC recommends that everyone ages 6 months and older get an annual flu vaccine, ideally by the end of October.²
There are also prescription flu antiviral drugs that can be used to treat flu illness; those need to be started as early as possible.³
Influenza virus is one of several viruses that contribute to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, influenza, and RSV activity on a weekly basis.

U.S. Virologic Surveillance

Nationally, the percentage of specimens testing positive for influenza in clinical laboratories remained stable (change of <0.5 percentage points) compared to the previous week. In Region 8, this week’s percentage increased compared to the previous week, and in all other regions this week’s percentage remained stable. For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent live attenuated influenza vaccine (LAIV) receipt or found upon further testing to be a vaccine virus are not included, as they are not circulating influenza viruses. Clinical Laboratories

The results of tests performed by clinical laboratories nationwide are summarized below. Data from clinical laboratories (the percentage of specimens tested that are positive for influenza virus) are used to monitor whether influenza activity is increasing or decreasing.

No. of specimens tested	49,312	108,688
No. of positive specimens (%)	661 (1.3%)	1,294 (1.2%)
*Positive specimens by type*
Influenza A	457 (69.1%)	853 (65.9%)
Influenza B	204 (30.9%)	441 (34.1%)

View Chart Data | View Full Screen Public Health Laboratories

The results of tests performed by public health laboratories nationwide are summarized below. Data from public health laboratories are used to monitor the proportion of circulating influenza viruses that belong to each influenza subtype/lineage.

No. of specimens tested	2,374	5,105
No. of positive specimens	159	343
*Positive specimens by type/subtype*
Influenza A	124 (78.0%)	268 (78.1%)
Subtyping Performed	99 (79.8%)	233 (86.9%)
(H1N1)pdm09	93 (93.9%)	214 (91.8%)
H3N2	6 (6.1%)	19 (8.2%)
H3N2v	0 (0%)	0 (0%)
Subtyping not performed	25 (20.2%)	35 (13.1%)
Influenza B	35 (22.0%)	75 (21.9%)
Lineage testing performed	26 (74.3%)	56 (74.7%)
Yamagata lineage	0 (0%)	0 (0%)
Victoria lineage	26 (100%)	56 (100%)
Lineage not performed	9 (25.7%)	19 (25.3%)

View Chart Data | View Full Screen

Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data Influenza Virus Characterization

CDC performs genetic and antigenic characterization of viruses submitted from U.S. state and local public health laboratories according to the Right Size Roadmap submission guidance. These data are used to compare how similar the currently circulating influenza viruses are to the reference viruses representing viruses contained in the current influenza vaccines. The data are also used to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. CDC also tests susceptibility of circulating influenza viruses to antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.

CDC has genetically characterized 477 influenza viruses collected since May 1, 2023.

A/H1	234
		6B.1A.5a	234 (100%)	2a	83 (35.5%)
				2a.1	151 (64.5%)
A/H3	35
		3C.2a1b.2a	35 (100%)	2a.3a	4 (11.4%)
				2a.3a.1	30 (85.7%)
				2b	1 (2.9%)
B/Victoria	208
		V1A	208 (100%)	3a.2	208 (100%)
B/Yamagata	0
		Y3	0	Y3	0 (0%)

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) (H1N1pdm09, H3N2, B/Victoria, and B/Yamagata viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2023-2024 Northern Hemisphere recommended cell or recombinant-based vaccine viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to the reference (vaccine) virus. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than or equal to 8-fold. Viruses selected for antigenic characterization are a subset representing the genetic changes in the surface proteins seen in genetically characterized viruses.

Influenza A Viruses

A (H1N1)pdm09: Sixty-eight A(H1N1)pdm09 viruses were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown A/Wisconsin/67/2022-like reference viruses representing the A(H1N1)pdm09 component for the cell- and recombinant-based influenza vaccines.
A (H3N2): Fourteen A(H3N2) viruses were antigenically characterized by HI or HINT, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus) by ferret antisera to cell-grown A/Darwin/6/2021-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines

Influenza B Viruses

B/Victoria: Forty-five influenza B/Victoria-lineage viruses were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines.
B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.

Antiviral susceptibility data will be reported later this season, when a sufficient number of viruses has been tested. Outpatient Respiratory Illness Surveillance

The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient visits for respiratory illness referred to as influenza-like illness [ILI (fever plus cough or sore throat)], not laboratory-confirmed influenza, and will therefore capture respiratory illness visits due to infection with any pathogen that can present with similar symptoms, including influenza virus, SARS-CoV-2, and RSV. It is important to evaluate syndromic surveillance data, including that from ILINet, in the context of other sources of surveillance data to obtain a complete and accurate picture of influenza, SARS-CoV-2, and other respiratory virus activity. CDC is providing integrated information about COVID-19, influenza and RSV activity on a website that is updated weekly. Information about other respiratory virus activity can be found on CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS) website. Outpatient Respiratory Illness Visits

Nationwide during week 41, 2.3% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This has remained stable (change of ≤ 0.1 percentage points) compared to week 40 and is below the national baseline of 2.9%. All 10 HHS regions are below their respective baselines. Region 8 increased, Region 2 decreased, and all other regions remained stable compared to week 40. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

Outpatient Respiratory Illness Visits by Age Group

About 70% of ILINet participants provide both the number of patient visits for respiratory illness and the total number of patient visits for the week broken out by age group. Data from this subset of providers are used to calculate the percentages of patient visits for respiratory illness by age group.

The percentage of visits for respiratory illness reported in ILINet increased for the 0-4 years and 25-49 years age groups and remained stable (change of ≤ 0.1 percentage point) for all other age groups (4-24 years, 50-64 years, and 65+ years) in week 41 compared to week 40.

View Chart Data | View Full Screen Outpatient Respiratory Illness Activity Map

Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).

Week 41 (Week ending Oct. 14, 2023)	Week 40 (Week ending Oct. 7, 2023)	Week 41 (Week ending Oct. 14, 2023)	Week 40 (Week ending Oct. 7, 2023)
Very High	0	0	0	1
High	2	1	7	3
Moderate	0	2	14	11
Low	4	6	58	57
Minimal	49	46	615	632
Insufficient Data	0	0	235	225

*Data collected in ILINet may disproportionally represent certain populations within a jurisdiction or CBSA, and therefore, may not accurately depict the full picture of influenza activity for the entire jurisdiction or CBSA. Differences in the data presented here by CDC and independently by some health departments likely represent differing levels of data completeness with data presented by the health department likely being the more complete.

Additional information about medically attended visits for ILI for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or ILI Activity Map Hospitalization Surveillance

FluSurv-NET

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in selected counties in 14 states and represents approximately 9% of the U.S. population. FluSurv-NET hospitalization data are preliminary. As data are received each week, prior case counts and rates are updated accordingly.

A total of 103 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2023, and October 14, 2023. The weekly hospitalization rate observed in week 41 was 0.2 per 100,000 population. The overall cumulative hospitalization rate was 0.3 per 100,000 population.

When examining rates by age, the cumulative hospitalization rate per 100,000 population among adults aged 18 years and older was 0.4, while among children aged 0-17 years, the cumulative hospitalization rate per 100,000 population was 0.2.

View Full Screen

In this figure, cumulative rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 season, rates for recent hospitals admissions are subject to reporting delays. As hospitalization data are reviewed each week, prior case counts and rates are updated accordingly.

Additional FluSurv-NET hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods |FluView Interactive: Rates by Age, Sex, and Race/Ethnicity or Data on Patient Characteristics | RESP-NET Interactive National Healthcare Safety Network (NHSN) Hospitalization Surveillance

Hospitals report to NHSN the number of patients admitted with laboratory-confirmed influenza. During week 41, 1,228 patients with laboratory-confirmed influenza were admitted to a hospital. The number of patients admitted to a hospital with laboratory-confirmed influenza slightly increased compared to week 40 (change of >5%). Regions 5, 6, 8, and 9 slightly increased and all other regions remained stable or decreased.

View Chart Data | View Full Screen

Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on October 19, 2023, 0.05% of the deaths that occurred during the week ending October 14, 2023 (week 41), were due to influenza. This percentage remained stable (≤ 0.1 percentage point change) compared to week 40. The data presented are preliminary and may change as more data are received and processed.

The percentages of deaths due to pneumonia and influenza (P&I) and due to pneumonia, influenza, or COVID-19 (PIC) will no longer be displayed in FluView but are available in FluView Interactive.

Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Influenza-Associated Pediatric Mortality

No influenza-associated pediatric deaths occurring during the 2023-2024 season have been reported to CDC.

One influenza-associated pediatric death occurring during the 2022-2023 season was reported to CDC during week 41. The death was associated with an influenza A (H1N1)pdm09 virus and occurred during week 39 (the week ending September 30, 2023). A total of 179 influenza-associated pediatric deaths that occurred during the 2022-2023 season have been reported to CDC.

Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Trend Indicators

Increasing:
Decreasing:
Stable: Indicators Status by System

Clinical Labs: Up or down arrows indicate a change of greater than or equal to 0.5 percentage points in the percent of specimens positive for influenza compared to the previous week.
Outpatient Respiratory Illness (ILINet): Up or down arrows indicate a change of greater than 0.1 percentage points in the percent of visits due to respiratory illness (ILI) compared to the previous week.
NHSN Hospitalizations: Up or down arrows indicate change of greater than or equal to 5% of the number of patients admitted with laboratory-confirmed influenza compared to the previous week.
NCHS Mortality: Up or down arrows indicate change of greater than 0.3 percentage points of the percent of deaths due to PIC compared to the previous week. Reference Footnotes

¹U.S. Influenza Surveillance: Purpose and Methods (2023 Oct). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/weekly/overview.htm#ILINet.

²Grohskopf LA, Blanton LH, Ferdinands JM, Chung JR, Broder KR, Talbot HK. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2023–24 Influenza Season. MMWR Recomm Rep 2023;72(No. RR-2):1–25. DOI: http://dx.doi.org/10.15585/mmwr.rr7202a1

³Influenza Antiviral Medications: Summary for Clinicians (2023 Sept). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/professional...clinicians.htm.

Weekly U.S. Influenza Surveillance Report

https://www.cdc.gov/flu/weekly/index.htm

Learn more about the weekly influenza surveillance report (FluView) prepared by the Influenza Division.

**Lance** · November 3, 2023, 05:08 PM

Weekly U.S. Influenza Surveillance Report

Print
Updated November 3, 2023

Key Updates for Week 43, ending October 28, 2023

Seasonal influenza activity remains low nationally but continues to slightly increase in most parts of the country. Viruses

Clinical Lab 1.9%

(Trend )

positive for influenza
this week

Public Health Lab
The most frequently reported influenza viruses this week were influenza A(H1N1).

Virus Characterization
Genetic and antigenic characterization are summarized in this report. Illness

Outpatient Respiratory Illness 2.7%

(Trend )

of visits to a health care provider this week were for respiratory illness
(below baseline).

Outpatient Respiratory Illness: Activity Map
This week 6 jurisdictions experienced moderate activity and 2 jurisdictions experienced high activity.

FluSurv-NET 0.8 per 100,000

cumulative hospitalization rate.

NHSN Hospitalizations 1,607

(Trend )

patients admitted to hospitals with influenza
this week.

NCHS Mortality 0.06%

(Trend )

of deaths attributed influenza this week.

Pediatric Deaths 3

deaths were reported (2 occurred in 2022-23 season and 1 occurred in 2023-24 season)

All data are preliminary and may change as more reports are received.

Directional arrows indicate changes between the current week and the previous week. Additional information on the arrows can be found at the bottom of this page.

A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.

Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.

Key Points

Seasonal influenza activity remains low nationally but continues to slightly increase in most parts of the country.
Outpatient respiratory illness is below baseline¹nationally and in nine HHS regions. Region 4 (the Southeast of the country) is above its outpatient respiratory illness baseline.
The number of weekly flu hospital admissions remains low but is increasing.
During week 43, of the 189 viruses reported by public health laboratories, 145 (76.7%) were influenza A, and 44 (23.3%) were influenza B. Of the 98 influenza A viruses subtyped during week 43, 84 (85.7%) were influenza A(H1N1), and 14 (14.3%) were A(H3N2).
The first influenza-associated pediatric death occurring during the 2023-24 season was reported this week.
CDC recommends that everyone 6 months and older get an annual flu vaccine.²
There also are prescription flu antiviral drugs that can be used to treat flu illness; those should be started as early as possible.³
Influenza viruses are among several viruses that contribute to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, influenza, and RSV activity on a weekly basis.

U.S. Virologic Surveillance

Nationally, the percentage of respiratory specimens testing positive for influenza in clinical laboratories remained stable (change of <0.5 percentage points) compared to the previous week but is trending upwards. In Regions 1, 3, 6, 8, and 9 the percentage this week increased compared to the previous week, and in all other regions it remained stable. For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent receipt of live attenuated influenza vaccine (LAIV) or found upon further testing to be a vaccine virus are not included, as they are not circulating influenza viruses. Clinical Laboratories

The results of tests performed by clinical laboratories nationwide are summarized below. Data from clinical laboratories (the percentage of specimens tested that are positive for influenza virus) are used to monitor whether influenza activity is increasing or decreasing.

No. of specimens tested	50,459	233,932
No. of positive specimens (%)	964 (1.9%)	3,373 (1.4%)
*Positive specimens by type*
Influenza A	676 (70.1%)	2,275 (67.4%)
Influenza B	288 (29.9%)	1,098 (32.6%)

View Chart Data | View Full Screen Public Health Laboratories

The results of tests performed by public health laboratories nationwide are summarized below. Data from public health laboratories are used to monitor the proportion of circulating influenza viruses that belong to each influenza subtype/lineage.

No. of specimens tested	2,213	10,656
No. of positive specimens	189	901
*Positive specimens by type/subtype*
Influenza A	145 (76.7%)	697 (77.4%)
Subtyping Performed	98 (67.6%)	597 (85.7%)
(H1N1)pdm09	84 (85.7%)	548 (91.8%)
H3N2	14 (14.3%)	49 (8.2%)
H3N2v	0 (0%)	0 (0%)
Subtyping not performed	47 (32.4%)	100 (14.3%)
Influenza B	44 (23.3%)	204 (22.6%)
Lineage testing performed	29 (65.9%)	159 (77.9%)
Yamagata lineage	0 (0%)	0 (0%)
Victoria lineage	29 (100%)	159 (100%)
Lineage not performed	15 (34.1%)	45 (22.1%)

View Chart Data | View Full Screen

Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data Influenza Virus Characterization

CDC performs genetic and antigenic characterization of viruses submitted by U.S. state and local public health laboratories according to the Right Size Roadmap submission guidance. These data are used to compare how similar the currently circulating influenza viruses are to the reference viruses representing viruses contained in current influenza vaccines. The data are also used to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. CDC also tests susceptibility of circulating influenza viruses to influenza antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.

CDC has genetically characterized 559 influenza viruses collected since May 1, 2023.

A/H1	293
		6B.1A.5a	293(100%)	2a	90 (30.7%)
				2a.1	203 (69.3%)
A/H3	41
		3C.2a1b.2a	41 (100%)	2a.3a	4 (9.8%)
				2a.3a.1	36 (87.8%)
				2b	1 (2.4%)
B/Victoria	225
		V1A	225 (100%)	3a.2	225 (100%)
B/Yamagata	0
		Y3	0	Y3	0 (0%)

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) (H1N1pdm09, H3N2, B/Victoria, and B/Yamagata viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2023-2024 Northern Hemisphere recommended cell or recombinant-based vaccine viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to the reference (vaccine) virus. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than or equal to 8-fold. Viruses selected for antigenic characterization are a subset representing the genetic changes in the surface proteins seen in genetically characterized viruses.

Influenza A Viruses

A (H1N1)pdm09: Seventy-six A(H1N1)pdm09 viruses were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown A/Wisconsin/67/2022-like reference viruses representing the A(H1N1)pdm09 component for the cell- and recombinant-based influenza vaccines.
A (H3N2): Twenty-two A(H3N2) viruses were antigenically characterized by HI or HINT, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus) by ferret antisera to cell-grown A/Darwin/6/2021-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.

Influenza B Viruses

B/Victoria: Sixty influenza B/Victoria-lineage viruses were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines.
B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.

Antiviral susceptibility data will be reported later this season, when a sufficient number of viruses has been tested. Outpatient Respiratory Illness Surveillance

The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient visits for respiratory illness referred to as influenza-like illness [ILI (fever plus cough or sore throat)], not laboratory-confirmed influenza, and will therefore capture respiratory illness visits due to infection with pathogens that can present with similar symptoms, including influenza viruses, SARS-CoV-2, and RSV. It is important to evaluate syndromic surveillance data, including that from ILINet, in the context of other sources of surveillance data to obtain a more complete and accurate picture of influenza, SARS-CoV-2, and other respiratory virus activity. CDC is providing integrated information about COVID-19, influenza and RSV activity on a website that is updated weekly. Information about other respiratory virus activity can be found on CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS) website. Outpatient Respiratory Illness Visits

Nationwide during week 43, 2.7% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This has increased (change of > 0.1 percentage points) compared to week 42 but remains below the national baseline of 2.9%. ILI activity is trending upward in 8 of the 10 HHS Regions (Regions 2-9) and is above baseline in Region 4 (the Southeast). Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

Outpatient Respiratory Illness Visits by Age Group

About 70% of ILINet participants provide both the number of patient visits for respiratory illness and the total number of patient visits for the week broken out by age group. Data from this subset of providers are used to calculate the percentages of patient visits for respiratory illness by age group.

The percentage of visits for respiratory illness reported in ILINet is trending upward in all age groups.

View Chart Data | View Full Screen Outpatient Respiratory Illness Activity Map

Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).

Week 43 (Week ending Oct. 28, 2023)	Week 42 (Week ending Oct. 21, 2023)	Week 43 (Week ending Oct. 28, 2023)	Week 42 (Week ending Oct. 21, 2023)
Very High	0	0	1	0
High	2	1	19	9
Moderate	6	3	33	25
Low	8	12	80	78
Minimal	39	39	569	593
Insufficient Data	0	0	227	224

*Data collected in ILINet may disproportionally represent certain populations within a jurisdiction or CBSA, and therefore, may not accurately depict the full picture of influenza activity for the entire jurisdiction or CBSA. Differences in the data presented here by CDC and independently by some health departments likely represent differing levels of data completeness with data presented by the health department likely being the more complete.

Additional information about medically attended visits for ILI for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or ILI Activity Map Hospitalization Surveillance

FluSurv-NET

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in selected counties in 14 states and represents approximately 9% of the U.S. population. FluSurv-NET hospitalization data are preliminary. As data are received each week, prior case counts and rates are updated accordingly.

A total of 238 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2023, and October 28, 2023. The weekly hospitalization rate observed during week 43 was 0.2 per 100,000 population. The overall cumulative hospitalization rate was 0.8 per 100,000 population.

When examining rates by age, the cumulative hospitalization rate per 100,000 population among adults 18 years and older was 0.8, while among children 0-17 years, the cumulative hospitalization rate per 100,000 population was 0.6.

View Full Screen

In this figure, cumulative rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 season, rates for recent hospitals admissions are subject to reporting delays. As hospitalization data are reviewed each week, prior case counts and rates are updated accordingly.

Additional FluSurv-NET hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods |FluView Interactive: Rates by Age, Sex, and Race/Ethnicity or Data on Patient Characteristics | RESP-NET Interactive National Healthcare Safety Network (NHSN) Hospitalization Surveillance

Hospitals report to NHSN the number of patients admitted with laboratory-confirmed influenza. During week 43, 1,607 patients with laboratory-confirmed influenza were admitted to a hospital. The number of patients admitted to a hospital with laboratory-confirmed influenza for the week slightly increased compared to week 42 (change of >5%) nationally and in Regions 1, 2, 9, and 10. All other regions remained stable or decreased.

View Chart Data | View Full Screen

Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on November 2, 2023, 0.06% of the deaths that occurred during the week ending October 28, 2023 (week 43), were due to influenza. This percentage remained stable (≤ 0.1 percentage point change) compared to week 42. The data presented are preliminary and may change as more data are received and processed.

The percentages of deaths due to pneumonia and influenza (P&I) and due to pneumonia, influenza, or COVID-19 (PIC) will no longer be displayed in FluView but are available in FluView Interactive.

Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Influenza-Associated Pediatric Mortality

Three influenza-associated pediatric deaths were reported to CDC during week 43.

One death occurred during week 40 (the week ending October 7, 2023) that was associated with an influenza A virus for which no subtyping was performed. This is the first influenza-associated pediatric death occurring during the 2023-2024 season that has been reported to CDC.

Two deaths occurring during the 2022-2023 season were also reported, which brings the total number of pediatric deaths for last season to 181. One death was associated with an influenza A virus for which no subtyping was performed and occurred during week 7 (the week ending February 18, 2023). The other death was associated with an influenza B virus with no lineage determined and occurred during week 38 (the week ending September 23, 2023).

Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Trend Indicators

Increasing:
Decreasing:
Stable: Indicators Status by System

Clinical Labs: Up or down arrows indicate a change of greater than or equal to 0.5 percentage points in the percent of specimens positive for influenza compared to the previous week.
Outpatient Respiratory Illness (ILINet): Up or down arrows indicate a change of greater than 0.1 percentage points in the percent of visits due to respiratory illness (ILI) compared to the previous week.
NHSN Hospitalizations: Up or down arrows indicate change of greater than or equal to 5% of the number of patients admitted with laboratory-confirmed influenza compared to the previous week.
NCHS Mortality: Up or down arrows indicate change of greater than 0.1 percentage points of the percent of deaths due to influenza compared to the previous week. Reference Footnotes

¹U.S. Influenza Surveillance: Purpose and Methods (2023 Oct). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/weekly/overview.htm#ILINet.

²Grohskopf LA, Blanton LH, Ferdinands JM, Chung JR, Broder KR, Talbot HK. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2023–24 Influenza Season. MMWR Recomm Rep 2023;72(No. RR-2):1–25. DOI: http://dx.doi.org/10.15585/mmwr.rr7202a1

³Influenza Antiviral Medications: Summary for Clinicians (2023 Sept). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/professional...clinicians.htm.

Weekly U.S. Influenza Surveillance Report

https://www.cdc.gov/flu/weekly/index.htm

Learn more about the weekly influenza surveillance report (FluView) prepared by the Influenza Division.

**Lance** · November 10, 2023, 03:52 PM

Weekly U.S. Influenza Surveillance Report

Print
Updated November 3, 2023

Key Updates for Week 43, ending October 28, 2023

Seasonal influenza activity remains low nationally but continues to slightly increase in most parts of the country. Viruses

Clinical Lab 1.9%

(Trend )

positive for influenza
this week

Public Health Lab
The most frequently reported influenza viruses this week were influenza A(H1N1).

Virus Characterization
Genetic and antigenic characterization are summarized in this report. Illness

Outpatient Respiratory Illness 2.7%

(Trend )

of visits to a health care provider this week were for respiratory illness
(below baseline).

Outpatient Respiratory Illness: Activity Map
This week 6 jurisdictions experienced moderate activity and 2 jurisdictions experienced high activity.

FluSurv-NET 0.8 per 100,000

cumulative hospitalization rate.

NHSN Hospitalizations 1,607

(Trend )

patients admitted to hospitals with influenza
this week.

NCHS Mortality 0.06%

(Trend )

of deaths attributed influenza this week.

Pediatric Deaths 3

deaths were reported (2 occurred in 2022-23 season and 1 occurred in 2023-24 season)

All data are preliminary and may change as more reports are received.

Directional arrows indicate changes between the current week and the previous week. Additional information on the arrows can be found at the bottom of this page.

A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.

Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.

Key Points

Seasonal influenza activity remains low nationally but continues to slightly increase in most parts of the country.
Outpatient respiratory illness is below baseline¹nationally and in nine HHS regions. Region 4 (the Southeast of the country) is above its outpatient respiratory illness baseline.
The number of weekly flu hospital admissions remains low but is increasing.
During week 43, of the 189 viruses reported by public health laboratories, 145 (76.7%) were influenza A, and 44 (23.3%) were influenza B. Of the 98 influenza A viruses subtyped during week 43, 84 (85.7%) were influenza A(H1N1), and 14 (14.3%) were A(H3N2).
The first influenza-associated pediatric death occurring during the 2023-24 season was reported this week.
CDC recommends that everyone 6 months and older get an annual flu vaccine.²
There also are prescription flu antiviral drugs that can be used to treat flu illness; those should be started as early as possible.³
Influenza viruses are among several viruses that contribute to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, influenza, and RSV activity on a weekly basis.

U.S. Virologic Surveillance

Nationally, the percentage of respiratory specimens testing positive for influenza in clinical laboratories remained stable (change of <0.5 percentage points) compared to the previous week but is trending upwards. In Regions 1, 3, 6, 8, and 9 the percentage this week increased compared to the previous week, and in all other regions it remained stable. For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent receipt of live attenuated influenza vaccine (LAIV) or found upon further testing to be a vaccine virus are not included, as they are not circulating influenza viruses. Clinical Laboratories

The results of tests performed by clinical laboratories nationwide are summarized below. Data from clinical laboratories (the percentage of specimens tested that are positive for influenza virus) are used to monitor whether influenza activity is increasing or decreasing.

No. of specimens tested	50,459	233,932
No. of positive specimens (%)	964 (1.9%)	3,373 (1.4%)
*Positive specimens by type*
Influenza A	676 (70.1%)	2,275 (67.4%)
Influenza B	288 (29.9%)	1,098 (32.6%)

View Chart Data | View Full Screen Public Health Laboratories

The results of tests performed by public health laboratories nationwide are summarized below. Data from public health laboratories are used to monitor the proportion of circulating influenza viruses that belong to each influenza subtype/lineage.

No. of specimens tested	2,213	10,656
No. of positive specimens	189	901
*Positive specimens by type/subtype*
Influenza A	145 (76.7%)	697 (77.4%)
Subtyping Performed	98 (67.6%)	597 (85.7%)
(H1N1)pdm09	84 (85.7%)	548 (91.8%)
H3N2	14 (14.3%)	49 (8.2%)
H3N2v	0 (0%)	0 (0%)
Subtyping not performed	47 (32.4%)	100 (14.3%)
Influenza B	44 (23.3%)	204 (22.6%)
Lineage testing performed	29 (65.9%)	159 (77.9%)
Yamagata lineage	0 (0%)	0 (0%)
Victoria lineage	29 (100%)	159 (100%)
Lineage not performed	15 (34.1%)	45 (22.1%)

View Chart Data | View Full Screen

Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data Influenza Virus Characterization

CDC performs genetic and antigenic characterization of viruses submitted by U.S. state and local public health laboratories according to the Right Size Roadmap submission guidance. These data are used to compare how similar the currently circulating influenza viruses are to the reference viruses representing viruses contained in current influenza vaccines. The data are also used to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. CDC also tests susceptibility of circulating influenza viruses to influenza antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.

CDC has genetically characterized 559 influenza viruses collected since May 1, 2023.

A/H1	293
		6B.1A.5a	293(100%)	2a	90 (30.7%)
				2a.1	203 (69.3%)
A/H3	41
		3C.2a1b.2a	41 (100%)	2a.3a	4 (9.8%)
				2a.3a.1	36 (87.8%)
				2b	1 (2.4%)
B/Victoria	225
		V1A	225 (100%)	3a.2	225 (100%)
B/Yamagata	0
		Y3	0	Y3	0 (0%)

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) (H1N1pdm09, H3N2, B/Victoria, and B/Yamagata viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2023-2024 Northern Hemisphere recommended cell or recombinant-based vaccine viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to the reference (vaccine) virus. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than or equal to 8-fold. Viruses selected for antigenic characterization are a subset representing the genetic changes in the surface proteins seen in genetically characterized viruses.

Influenza A Viruses

A (H1N1)pdm09: Seventy-six A(H1N1)pdm09 viruses were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown A/Wisconsin/67/2022-like reference viruses representing the A(H1N1)pdm09 component for the cell- and recombinant-based influenza vaccines.
A (H3N2): Twenty-two A(H3N2) viruses were antigenically characterized by HI or HINT, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus) by ferret antisera to cell-grown A/Darwin/6/2021-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.

Influenza B Viruses

B/Victoria: Sixty influenza B/Victoria-lineage viruses were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines.
B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.

Antiviral susceptibility data will be reported later this season, when a sufficient number of viruses has been tested. Outpatient Respiratory Illness Surveillance

The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient visits for respiratory illness referred to as influenza-like illness [ILI (fever plus cough or sore throat)], not laboratory-confirmed influenza, and will therefore capture respiratory illness visits due to infection with pathogens that can present with similar symptoms, including influenza viruses, SARS-CoV-2, and RSV. It is important to evaluate syndromic surveillance data, including that from ILINet, in the context of other sources of surveillance data to obtain a more complete and accurate picture of influenza, SARS-CoV-2, and other respiratory virus activity. CDC is providing integrated information about COVID-19, influenza and RSV activity on a website that is updated weekly. Information about other respiratory virus activity can be found on CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS) website. Outpatient Respiratory Illness Visits

Nationwide during week 43, 2.7% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This has increased (change of > 0.1 percentage points) compared to week 42 but remains below the national baseline of 2.9%. ILI activity is trending upward in 8 of the 10 HHS Regions (Regions 2-9) and is above baseline in Region 4 (the Southeast). Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

Outpatient Respiratory Illness Visits by Age Group

About 70% of ILINet participants provide both the number of patient visits for respiratory illness and the total number of patient visits for the week broken out by age group. Data from this subset of providers are used to calculate the percentages of patient visits for respiratory illness by age group.

The percentage of visits for respiratory illness reported in ILINet is trending upward in all age groups.

View Chart Data | View Full Screen Outpatient Respiratory Illness Activity Map

Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).

Week 43 (Week ending Oct. 28, 2023)	Week 42 (Week ending Oct. 21, 2023)	Week 43 (Week ending Oct. 28, 2023)	Week 42 (Week ending Oct. 21, 2023)
Very High	0	0	1	0
High	2	1	19	9
Moderate	6	3	33	25
Low	8	12	80	78
Minimal	39	39	569	593
Insufficient Data	0	0	227	224

*Data collected in ILINet may disproportionally represent certain populations within a jurisdiction or CBSA, and therefore, may not accurately depict the full picture of influenza activity for the entire jurisdiction or CBSA. Differences in the data presented here by CDC and independently by some health departments likely represent differing levels of data completeness with data presented by the health department likely being the more complete.

Additional information about medically attended visits for ILI for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or ILI Activity Map Hospitalization Surveillance

FluSurv-NET

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in selected counties in 14 states and represents approximately 9% of the U.S. population. FluSurv-NET hospitalization data are preliminary. As data are received each week, prior case counts and rates are updated accordingly.

A total of 238 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2023, and October 28, 2023. The weekly hospitalization rate observed during week 43 was 0.2 per 100,000 population. The overall cumulative hospitalization rate was 0.8 per 100,000 population.

When examining rates by age, the cumulative hospitalization rate per 100,000 population among adults 18 years and older was 0.8, while among children 0-17 years, the cumulative hospitalization rate per 100,000 population was 0.6.

View Full Screen

In this figure, cumulative rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 season, rates for recent hospitals admissions are subject to reporting delays. As hospitalization data are reviewed each week, prior case counts and rates are updated accordingly.

Additional FluSurv-NET hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods |FluView Interactive: Rates by Age, Sex, and Race/Ethnicity or Data on Patient Characteristics | RESP-NET Interactive National Healthcare Safety Network (NHSN) Hospitalization Surveillance

Hospitals report to NHSN the number of patients admitted with laboratory-confirmed influenza. During week 43, 1,607 patients with laboratory-confirmed influenza were admitted to a hospital. The number of patients admitted to a hospital with laboratory-confirmed influenza for the week slightly increased compared to week 42 (change of >5%) nationally and in Regions 1, 2, 9, and 10. All other regions remained stable or decreased.

View Chart Data | View Full Screen

Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on November 2, 2023, 0.06% of the deaths that occurred during the week ending October 28, 2023 (week 43), were due to influenza. This percentage remained stable (≤ 0.1 percentage point change) compared to week 42. The data presented are preliminary and may change as more data are received and processed.

The percentages of deaths due to pneumonia and influenza (P&I) and due to pneumonia, influenza, or COVID-19 (PIC) will no longer be displayed in FluView but are available in FluView Interactive.

Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Influenza-Associated Pediatric Mortality

Three influenza-associated pediatric deaths were reported to CDC during week 43.

One death occurred during week 40 (the week ending October 7, 2023) that was associated with an influenza A virus for which no subtyping was performed. This is the first influenza-associated pediatric death occurring during the 2023-2024 season that has been reported to CDC.

Two deaths occurring during the 2022-2023 season were also reported, which brings the total number of pediatric deaths for last season to 181. One death was associated with an influenza A virus for which no subtyping was performed and occurred during week 7 (the week ending February 18, 2023). The other death was associated with an influenza B virus with no lineage determined and occurred during week 38 (the week ending September 23, 2023).

Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Trend Indicators

Increasing:
Decreasing:
Stable: Indicators Status by System

Clinical Labs: Up or down arrows indicate a change of greater than or equal to 0.5 percentage points in the percent of specimens positive for influenza compared to the previous week.
Outpatient Respiratory Illness (ILINet): Up or down arrows indicate a change of greater than 0.1 percentage points in the percent of visits due to respiratory illness (ILI) compared to the previous week.
NHSN Hospitalizations: Up or down arrows indicate change of greater than or equal to 5% of the number of patients admitted with laboratory-confirmed influenza compared to the previous week.
NCHS Mortality: Up or down arrows indicate change of greater than 0.1 percentage points of the percent of deaths due to influenza compared to the previous week. Reference Footnotes

¹U.S. Influenza Surveillance: Purpose and Methods (2023 Oct). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/weekly/overview.htm#ILINet.

²Grohskopf LA, Blanton LH, Ferdinands JM, Chung JR, Broder KR, Talbot HK. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2023–24 Influenza Season. MMWR Recomm Rep 2023;72(No. RR-2):1–25. DOI: http://dx.doi.org/10.15585/mmwr.rr7202a1

³Influenza Antiviral Medications: Summary for Clinicians (2023 Sept). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/professional...clinicians.htm.
https://www.cdc.gov/flu/weekly/index.htm

**Lance** · November 17, 2023, 04:30 PM

Weekly U.S. Influenza Surveillance Report

Print
Updated November 17, 2023

Key Updates for Week 45, ending November 11, 2023

Seasonal influenza activity continues to increase in most parts of the country, most notably in the South Central, Southeast, and West Coast regions. Viruses

Clinical Lab 4.0%

(Trend )

positive for influenza
this week

Public Health Lab
The most frequently reported influenza viruses this week were influenza A(H1N1).

Virus Characterization
Genetic and antigenic characterization are summarized in this report. Illness

Outpatient Respiratory Illness 3.5%

(Trend )

of visits to a health care provider this week were for respiratory illness
(above baseline).

Outpatient Respiratory Illness: Activity Map
This week 8 jurisdictions experienced moderate activity and 9 jurisdictions experienced high or very high activity.

FluSurv-NET 1.7 per 100,000

cumulative hospitalization rate.

NHSN Hospitalizations 2,721

(Trend )

patients admitted to hospitals with influenza this week.

NCHS Mortality 0.10%

(Trend )

of deaths attributed to influenza this week.

Pediatric Deaths 0

influenza-associated deaths reported this week.

All data are preliminary and may change as more reports are received.

Directional arrows indicate changes between the current week and the previous week. Additional information on the arrows can be found at the bottom of this page.

A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.

Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.

Key Points

Seasonal influenza activity continues to increase in most parts of the country, most notably in the South Central, Southeast, and West Coast regions.
Outpatient respiratory illness is above baseline¹nationally for the second week and is at or above baseline in five of 10 HHS Regions. Region 3 (Mid-Atlantic) is at its region-specific outpatient respiratory illness baseline and Regions 2, 4, 6, and 9 (New York/New Jersey/Puerto Rico/Virgin Islands, Southeast, South Central, and West Coast) are above their region-specific baselines.
The number of weekly flu hospital admissions continues to increase.
During week 45, of the 310 viruses reported by public health laboratories, 235 (75.8%) were influenza A and 75 (24.2%) were influenza B. Of the 133 influenza A viruses subtyped during week 45, 116 (87.2%) were influenza A(H1N1) and 17 (12.8%) were A(H3N2).
CDC estimates that there have been at least 780,000 illnesses, 8,000 hospitalizations, and 490 deaths from flu so far this season.
CDC recommends that everyone 6 months and older get an annual flu vaccine.²
There also are prescription flu antiviral drugs that can be used to treat flu illness; those should be started as early as possible and are especially important for higher risk patients.³
Influenza viruses are among several viruses that contribute to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, influenza, and RSV activity on a weekly basis.

U.S. Virologic Surveillance

Nationally and in HHS Regions 1, 2, 4, 5, 6, 7, 8, 9 and 10, the percentage of respiratory specimens testing positive for influenza in clinical laboratories increased (change of ≥0.5 percentage points) compared to the previous week. In Region 3 the percentage remained stable compared to the previous week but is trending upwards. The regions with the highest percent positivity were Regions 8 (7.4%), 4 (7.0%), 6 (6.5%) and 9 (5.3%). For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent receipt of live attenuated influenza vaccine (LAIV) or found upon further testing to be a vaccine virus are not included, as they are not circulating influenza viruses. Clinical Laboratories

The results of tests performed by clinical laboratories nationwide are summarized below. Data from clinical laboratories (the percentage of specimens tested that are positive for influenza virus) are used to monitor whether influenza activity is increasing or decreasing.

No. of specimens tested	74,217	396,399
No. of positive specimens (%)	3,002 (4.0%)	8,508 (2.2%)
*Positive specimens by type*
Influenza A	2,144 (71.4%)	5,912 (69.5%)
Influenza B	858 (28.6%)	2,596 (30.5%)

View Chart Data | View Full Screen Public Health Laboratories

The results of tests performed by public health laboratories nationwide are summarized below. Data from public health laboratories are used to monitor the proportion of circulating influenza viruses that belong to each influenza subtype/lineage.

No. of specimens tested	2,136	16,551
No. of positive specimens	310	1,909
*Positive specimens by type/subtype*
Influenza A	235 (75.8%)	1,501 (78.6%)
Subtyping Performed	133 (56.6%)	1,166 (77.7%)
(H1N1)pdm09	116 (87.2%)	1,053 (90.3%)
H3N2	17 (12.8%)	113 (9.7%)
H3N2v	0 (0%)	0 (0%)
Subtyping not performed	102 (43.4%)	335 (22.3%)
Influenza B	75 (24.2%)	408 (21.4%)
Lineage testing performed	52 (69.3%)	340 (83.3%)
Yamagata lineage	0 (0%)	0 (0%)
Victoria lineage	52 (100%)	340 (100%)
Lineage not performed	23 (30.7%)	68 (16.7%)

View Chart Data | View Full Screen

Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data Influenza Virus Characterization

CDC performs genetic and antigenic characterization of viruses submitted by U.S. state and local public health laboratories according to the Right Size Roadmap submission guidance. These data are used to compare how similar the currently circulating influenza viruses are to the reference viruses representing viruses contained in current influenza vaccines. The data are also used to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. CDC also tests susceptibility of circulating influenza viruses to influenza antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.

CDC has genetically characterized 567 influenza viruses collected since May 1, 2023.

A/H1	297
		6B.1A.5a	297 (100%)	2a	90 (30.3%)
				2a.1	207 (69.7%)
A/H3	41
		3C.2a1b.2a	41 (100%)	2a.3a	4 (9.8%)
				2a.3a.1	36 (87.8%)
				2b	1 (2.4%)
B/Victoria	229
		V1A	229 (100%)	3a.2	229 (100%)
B/Yamagata	0
		Y3	0	Y3	0 (0%)

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) (H1N1pdm09, H3N2, B/Victoria, and B/Yamagata viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2023-2024 Northern Hemisphere recommended cell or recombinant-based vaccine viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to the reference (vaccine) virus. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than or equal to 8-fold. Viruses selected for antigenic characterization are a subset representing the genetic changes in the surface proteins seen in genetically characterized viruses.

Influenza A Viruses

A (H1N1)pdm09: One hundred and nine A(H1N1)pdm09 viruses were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown A/Wisconsin/67/2022-like reference viruses representing the A(H1N1)pdm09 component for the cell- and recombinant-based influenza vaccines.
A (H3N2): Twenty-three A(H3N2) viruses were antigenically characterized by HI or HINT, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer in HI or reacting at titers that were less than or equal to 8-fold of the homologous virus in HINT) by ferret antisera to cell-grown A/Darwin/6/2021-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.

Influenza B Viruses

B/Victoria: Seventy-three influenza B/Victoria-lineage virus were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines.
B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.

Antiviral susceptibility data will be reported later this season, when a sufficient number of viruses has been tested. Outpatient Respiratory Illness Surveillance

The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient visits for respiratory illness referred to as influenza-like illness [ILI (fever plus cough or sore throat)], not laboratory-confirmed influenza, and will therefore capture respiratory illness visits due to infection with pathogens that can present with similar symptoms, including influenza viruses, SARS-CoV-2, and RSV. It is important to evaluate syndromic surveillance data, including that from ILINet, in the context of other sources of surveillance data to obtain a more complete and accurate picture of influenza, SARS-CoV-2, and other respiratory virus activity. CDC is providing integrated information about COVID-19, influenza and RSV activity on a website that is updated weekly. Information about other respiratory virus activity can be found on CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS) website. Outpatient Respiratory Illness Visits

Nationwide during week 45, 3.5% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This has increased (change of > 0.1 percentage points) compared to week 44 and is above the national baseline of 2.9%. ILI activity is trending upward in 9 of the 10 HHS Regions (Regions 1, 2, 3, 4, 5, 6, 7, 8, and 9). Five regions are at or above their region-specific baselines (Regions 2, 3, 4, 6, and 9) this week. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

Outpatient Respiratory Illness Visits by Age Group

About 70% of ILINet participants provide both the number of patient visits for respiratory illness and the total number of patient visits for the week broken out by age group. Data from this subset of providers are used to calculate the percentages of patient visits for respiratory illness by age group.

The percentage of visits for respiratory illness reported in ILINet has increased (change of > 0.1 percentage points) in all age groups in week 45 compared to week 44.

View Chart Data | View Full Screen Outpatient Respiratory Illness Activity Map

Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).

Week 45 (Week ending Nov. 11, 2023)	Week 44 (Week ending Nov. 4, 2023)	Week 45 (Week ending Nov. 11, 2023)	Week 44 (Week ending Nov. 4, 2023)
Very High	1	0	9	1
High	8	4	41	31
Moderate	8	6	62	37
Low	10	10	125	104
Minimal	27	35	453	532
Insufficient Data	1	0	239	224

*Data collected in ILINet may disproportionally represent certain populations within a jurisdiction or CBSA, and therefore, may not accurately depict the full picture of influenza activity for the entire jurisdiction or CBSA. Differences in the data presented here by CDC and independently by some health departments likely represent differing levels of data completeness with data presented by the health department likely being the more complete.

Additional information about medically attended visits for ILI for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or ILI Activity Map Hospitalization Surveillance

FluSurv-NET

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in selected counties in 14 states and represents approximately 9% of the U.S. population. FluSurv-NET hospitalization data are preliminary. As data are received each week, prior case counts and rates are updated accordingly.

A total of 527 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2023, and November 11, 2023; 425 (80.6%) were associated with influenza A virus, 93 (17.6%) with influenza B virus, 4 (0.8%) with influenza A virus and influenza B virus co-infection, and 5 (0.9%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 76 (83.5%) were A(H1N1)pdm09 virus and 14 (15.4%) were A(H3N2).

The weekly hospitalization rate observed during week 45 was 0.5 per 100,000 population. The overall cumulative hospitalization rate was 1.7 per 100,000 population. This cumulative hospitalization rate is the second highest cumulative in-season hospitalization rate observed during week 45, after the 2022-2023 season (8.1). Cumulative in-season hospitalization rates observed during week 45 from 2010-2011 through 2021-2022 ranged from 0.1 to 0.9.

When examining rates by age, the highest cumulative hospitalization rate per 100,000 population was among adults 65 years and older (4.3), followed by children 0-4 years (2.4) and adults 50-64 years (2.1). When examining unadjusted rates by race and ethnicity, the highest hospitalization rate per 100,000 population was among non-Hispanic Black persons (2.7), followed by Hispanic persons (1.9), non-Hispanic American Indian or Alaska Native persons (1.6), non-Hispanic Asian persons (1.4), and non-Hispanic White persons (1.3).

View Full Screen

In this figure, cumulative rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 season, rates for recent hospitals admissions are subject to reporting delays. As hospitalization data are reviewed each week, prior case counts and rates are updated accordingly.

View Full Screen

In this figure, weekly rates for all seasons prior to the 2023-24 season reflect end-of-season rates. For the 2023-24 season, rates for recent hospital admissions are subject to reporting delays and are shown as a dashed line for the current season. As hospitalization data are received each week, prior case counts and rates are updated accordingly.

Additional FluSurv-NET hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods |FluView Interactive: Rates by Age, Sex, and Race/Ethnicity or Data on Patient Characteristics | RESP-NET Interactive National Healthcare Safety Network (NHSN) Hospitalization Surveillance

Hospitals report to NHSN the weekly number of patients admitted with laboratory-confirmed influenza. During week 45, 2,721 patients with laboratory-confirmed influenza were admitted to a hospital. The number of patients admitted to a hospital with laboratory-confirmed influenza for week 45 increased compared to week 44 (change of >5%) nationally and in 9 of the 10 HHS Regions (2, 3, 4, 5, 6, 7, 8, 9, and 10). In Region 1, the number of hospitalizations remained stable in week 45 compared to week 44.

View Chart Data | View Full Screen

Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on November 16, 2023, 0.10% of the deaths that occurred during the week ending November 11, 2023 (week 45), were due to influenza. This percentage remained stable (≤ 0.1 percentage point change) compared to week 44. The data presented are preliminary and may change as more data are received and processed.

The percentages of deaths due to pneumonia and influenza (P&I) and due to pneumonia, influenza, or COVID-19 (PIC) will no longer be displayed in FluView but are available in FluView Interactive.

Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Influenza-Associated Pediatric Mortality

No influenza-associated pediatric deaths were reported to CDC during week 45.

One influenza-associated pediatric death occurring during the 2023-2024 season has been reported to CDC.

Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Trend Indicators

Increasing:
Decreasing:
Stable: Indicators Status by System

Clinical Labs: Up or down arrows indicate a change of greater than or equal to 0.5 percentage points in the percent of specimens positive for influenza compared to the previous week.
Outpatient Respiratory Illness (ILINet): Up or down arrows indicate a change of greater than 0.1 percentage points in the percent of visits due to respiratory illness (ILI) compared to the previous week.
NHSN Hospitalizations: Up or down arrows indicate change of greater than or equal to 5% of the number of patients admitted with laboratory-confirmed influenza compared to the previous week.
NCHS Mortality: Up or down arrows indicate change of greater than 0.1 percentage points of the percent of deaths due to influenza compared to the previous week. Reference Footnotes

¹U.S. Influenza Surveillance: Purpose and Methods (2023 Oct). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/weekly/overview.htm#ILINet.

²Grohskopf LA, Blanton LH, Ferdinands JM, Chung JR, Broder KR, Talbot HK. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2023–24 Influenza Season. MMWR Recomm Rep 2023;72(No. RR-2):1–25. DOI: http://dx.doi.org/10.15585/mmwr.rr7202a1

³Influenza Antiviral Medications: Summary for Clinicians (2023 Sept). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/professional...clinicians.htm.
https://www.cdc.gov/flu/weekly/index.htm

**Lance** · December 1, 2023, 05:26 PM

Weekly U.S. Influenza Surveillance Report

Print
Updated December 1, 2023

Key Updates for Week 47, ending November 25, 2023

Seasonal influenza activity continues to increase in most parts of the country, most notably in the South Central, Southeast, Mountain, and West Coast regions. Viruses

Clinical Lab 6.2%

(Trend )

positive for influenza
this week

Public Health Lab
The most frequently reported influenza viruses this week were influenza A(H1N1).

Virus Characterization
Genetic and antigenic characterization and antiviral susceptibility are summarized in this report. Illness

Outpatient Respiratory Illness 3.9%

(Trend )

of visits to a health care provider this week were for respiratory illness
(above baseline).

Outpatient Respiratory Illness: Activity Map
This week 10 jurisdictions experienced moderate activity and 13 jurisdictions experienced high or very high activity.

FluSurv-NET 3.7 per 100,000

cumulative hospitalization rate.

NHSN Hospitalizations 4,268

(Trend )

patients admitted to hospitals with influenza this week.

NCHS Mortality 0.1%

(Trend )

of deaths attributed to influenza this week.

Pediatric Deaths 5

influenza-associated deaths reported this week for a
total of 8 so far this season

All data are preliminary and may change as more reports are received.

Directional arrows indicate changes between the current week and the previous week. Additional information on the arrows can be found at the bottom of this page.

A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.

Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.

Key Points

Seasonal influenza activity continues to increase in most parts of the country, most notably in the South Central, Southeast, Mountain, and West Coast regions.
Outpatient respiratory illness is above baseline¹nationally for the fourth week and is at or above baseline in eight of 10 HHS Regions. Region 7 (Central) is at its region-specific outpatient respiratory illness baseline and regions 1, 2, 4, 5, 6, 8, and 9 (New England, New York/New Jersey/Puerto Rico/Virgin Islands, Midwest, Southeast, South Central, Mountain, and West Coast) are above their region-specific baselines.
The number of weekly flu hospital admissions continues to increase.
In week 47, 5 pediatric deaths associated with influenza were reported. This brings the 2023-2024 season total to 8 pediatric deaths.
During week 47, of the 379 viruses reported by public health laboratories, 311 (82.1%) were influenza A and 68 (17.9%) were influenza B. Of the 202 influenza A viruses subtyped during week 47, 166 (82.2%) were influenza A(H1N1) and 32 (17.8%) were A(H3N2).
CDC estimates that there have been at least 1.8 million illnesses, 17,000 hospitalizations, and 1,100 deaths from flu so far this season.
CDC recommends that everyone 6 months and older get an annual flu vaccine, as there are still vaccines available.²Now is still a good time to get a vaccine, if you haven’t already.
There also are prescription flu antiviral drugs that can be used to treat flu illness; those should be started as early as possible and are especially important for higher risk patients.³
Influenza viruses are among several viruses that contribute to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, influenza, and RSV activity on a weekly basis.

U.S. Virologic Surveillance

Nationally and in HHS Regions 1, 2, 3, 4, 8, 9, and 10, the percentage of respiratory specimens testing positive for influenza in clinical laboratories increased (change of ≥0.5 percentage points) compared to the previous week. In all other regions the percentage remained stable compared to the previous week but is trending upwards. The regions with the highest percent positivity were regions 8 (13.2%), 4 (10.3%), 6 (7.7%), and 9 (7.2%). Influenza A(H1N1)pdm09 is the predominant virus circulating in all regions, however a larger proportion (38%) of the viruses detected by public health labs have been influenza B in regions 4 and 6 compared to other regions (9-19%). For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent receipt of live attenuated influenza vaccine (LAIV) or found upon further testing to be a vaccine virus are not included, as they are not circulating influenza viruses. Clinical Laboratories

The results of tests performed by clinical laboratories nationwide are summarized below. Data from clinical laboratories (the percentage of specimens tested that are positive for influenza virus) are used to monitor whether influenza activity is increasing or decreasing.

No. of specimens tested	69,884	581,944
No. of positive specimens (%)	4,336 (6.2%)	17,814 (3.1%)
*Positive specimens by type*
Influenza A	3,257 (75.1%)	12,873 (72.3%)
Influenza B	1,079 (24.9%)	4,941 (27.7%)

View Chart Data | View Full Screen Public Health Laboratories

The results of tests performed by public health laboratories nationwide are summarized below. Data from public health laboratories are used to monitor the proportion of circulating influenza viruses that belong to each influenza subtype/lineage.

No. of specimens tested	1,798	22,823
No. of positive specimens	379	3,561
*Positive specimens by type/subtype*
Influenza A	311 (82.1%)	2,845 (79.9%)
Subtyping Performed	202 (65.0%)	2,250 (79.1%)
(H1N1)pdm09	166 (82.2%)	1,969 (87.5%)
H3N2	36 (17.8%)	281 (12.5%)
H3N2v	0 (0.0%)	0 (0.0%)
Subtyping not performed	109 (35.0%)	595 (20.9%)
Influenza B	68 (17.9%)	716 (20.1%)
Lineage testing performed	47 (69.1%)	561 (78.4%)
Yamagata lineage	0 (0.0%)	0 (0.0%)
Victoria lineage	47 (100%)	561 (100%)
Lineage not performed	21 (30.9%)	155 (21.6%)

View Chart Data | View Full Screen

Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data Influenza Virus Characterization

CDC performs genetic and antigenic characterization of viruses submitted by U.S. state and local public health laboratories according to the Right Size Roadmap submission guidance. These data are used to compare how similar the currently circulating influenza viruses are to the reference viruses representing viruses contained in current influenza vaccines. The data are also used to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. CDC also tests susceptibility of circulating influenza viruses to influenza antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.

CDC has genetically characterized 818 influenza viruses collected since May 1, 2023.

A/H1	441
		6B.1A.5a	441 (100%)	2a	141 (32.0%)
				2a.1	300 (68.0%)
A/H3	81
		3C.2a1b.2a	81 (100%)	2a.1b	1 (1.2%)
				2a.3a	4 (4.9%)
				2a.3a.1	75 (92.6%)
				2b	1 (1.2%)
B/Victoria	296
		V1A	296 (100%)	3a.2	296 (100%)
B/Yamagata	0
		Y3	0	Y3	0 (0%)

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) (H1N1pdm09, H3N2, B/Victoria, and B/Yamagata viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2023-2024 Northern Hemisphere recommended cell or recombinant-based vaccine viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to the reference (vaccine) virus. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than or equal to 8-fold. Viruses selected for antigenic characterization are a subset representing the genetic changes in the surface proteins seen in genetically characterized viruses.

Influenza A Viruses

A(H1N1)pdm09: One hundred and thirty-four A(H1N1)pdm09 viruses were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown A/Wisconsin/67/2022-like reference viruses representing the A(H1N1)pdm09 component for the cell- and recombinant-based influenza vaccines.
A(H3N2): Twenty-three A(H3N2) viruses were antigenically characterized by HI or HINT, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer in HI or reacting at titers that were less than or equal to 8-fold of the homologous virus in HINT) by ferret antisera to cell-grown A/Darwin/6/2021-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.

Influenza B Viruses

B/Victoria: Eighty influenza B/Victoria-lineage virus were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines.
B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.

Assessment of Virus Susceptibility to Antiviral Medications

CDC assesses susceptibility of influenza viruses to the antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir using next generation sequence analysis supplemented by laboratory assays. Information about antiviral susceptibility test methods can be found at U.S. Influenza Surveillance: Purpose and Methods | CDC.

Viruses collected in the U.S. since October 01, 2023 were tested for antiviral susceptibility as follows:

Neuraminidase Inhibitors	Oseltamivir	Viruses Tested	265	156	41	68
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Peramivir	Viruses Tested	265	156	41	68
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Zanamivir	Viruses Tested	265	156	41	68
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
PA Cap-Dependent Endonuclease Inhibitor	Baloxavir	Viruses Tested	262	153	41	68
Decreased Susceptibility	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)

High levels of resistance to the adamantanes (amantadine and rimantadine) persist among influenza A(H1N1)pdm09 and influenza A(H3N2) viruses (the adamantanes are not effective against influenza B viruses). Therefore, use of these antivirals for treatment and prevention of influenza A virus infection is not recommended and data from adamantane resistance testing are not presented. Outpatient Respiratory Illness Surveillance

The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient visits for respiratory illness referred to as influenza-like illness [ILI (fever plus cough or sore throat)], not laboratory-confirmed influenza, and will therefore capture respiratory illness visits due to infection with pathogens that can present with similar symptoms, including influenza viruses, SARS-CoV-2, and RSV. It is important to evaluate syndromic surveillance data, including that from ILINet, in the context of other sources of surveillance data to obtain a more complete and accurate picture of influenza, SARS-CoV-2, and other respiratory virus activity. CDC is providing integrated information about COVID-19, influenza and RSV activity on a website that is updated weekly. Information about other respiratory virus activity can be found on CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS) website. Outpatient Respiratory Illness Visits

Nationwide during Week 47, 3.9% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This has increased compared to week 47 and is above the national baseline of 2.9% for the fourth consecutive week. ILI activity increased in 7 of the 10 HHS Regions (regions 1, 5, 6, 7, 8, 9, and 10) during week 47 compared to week 48 and, while stable compared to last week, is trending upward in Region 4. Percent of visits for ILI decreased in Region 3, but that may be attributed to low reporting during week 47. Eight regions are at or above their region-specific baselines (regions 1, 2, 4, 5, 6, 7, 8, and 9) this week. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

Outpatient Respiratory Illness Visits by Age Group

About 70% of ILINet participants provide both the number of patient visits for respiratory illness and the total number of patient visits for the week broken out by age group. Data from this subset of providers are used to calculate the percentages of patient visits for respiratory illness by age group.

The percentage of visits for respiratory illness reported in ILINet has increased (change of > 0.1 percentage points) in two age groups (5-24 years and 50-64 years) in Week 47 compared to week 46 and is trending upward in all other age groups (0-4 years, 25-49 years, 65+ years).

View Chart Data | View Full Screen Outpatient Respiratory Illness Activity Map

Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).

Week 47 (Week ending Nov. 25, 2023)	Week 46 (Week ending Nov. 18, 2023)	Week 47 (Week ending Nov. 25, 2023)	Week 46 (Week ending Nov. 18, 2023)
Very High	2	2	10	12
High	11	10	56	59
Moderate	10	9	82	71
Low	11	9	156	128
Minimal	19	25	385	433
Insufficient Data	2	0	240	226

*Data collected in ILINet may disproportionally represent certain populations within a jurisdiction or CBSA, and therefore, may not accurately depict the full picture of influenza activity for the entire jurisdiction or CBSA. Differences in the data presented here by CDC and independently by some health departments likely represent differing levels of data completeness with data presented by the health department likely being the more complete.

Additional information about medically attended visits for ILI for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or ILI Activity Map Hospitalization Surveillance

FluSurv-NET

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in select counties in 14 states and represents approximately 9% of the U.S. population. FluSurv-NET hospitalization data are preliminary. As data are received each week, prior case counts and rates are updated accordingly.

A total of 1,131 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2023, and November 25, 2023; 923 (81.6%) were associated with influenza A virus, 162 (14.3%) with influenza B virus, 4 (0.4%) with influenza A virus and influenza B virus co-infection, and 42 (3.7%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 185 (83.3%) were A(H1N1)pdm09 and 36 (16.2%) were A(H3N2).

The weekly hospitalization rate observed in week 47 was 0.9 per 100,000 population. The overall cumulative hospitalization rate was 3.7 per 100,000 population. This cumulative hospitalization rate is the second highest cumulative in-season hospitalization rate observed in week 47, following the 2022-2023 season (16.6). Cumulative in-season hospitalization rates observed in week 46 from 2010-2011 through 2021-2022 ranged from 0.1 to 2.0.

When examining rates by age, the highest cumulative hospitalization rate per 100,000 population was among adults aged 65 years and older (8.9), followed by adults aged 50-64 years (4.9), and children aged 0-4 years (4.0). When examining unadjusted rates by race and ethnicity, the highest rate of hospitalization per 100,000 population was among non-Hispanic Black persons (6.4), followed by non-Hispanic American Indian or Alaska Native persons (4.4), Hispanic persons (3.7), non-Hispanic Asian persons (2.7), and non-Hispanic White persons (2.6).

View Full Screen

In this figure, cumulative rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 season, rates for recent hospitals admissions are subject to reporting delays. As hospitalization data are reviewed each week, prior case counts and rates are updated accordingly.

View Full Screen

In this figure, weekly rates for all seasons prior to the 2023-24 season reflect end-of-season rates. For the 2023-24 season, rates for recent hospital admissions are subject to reporting delays and are shown as a dashed line for the current season. As hospitalization data are received each week, prior case counts and rates are updated accordingly.

Additional FluSurv-NET hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods |FluView Interactive: Rates by Age, Sex, and Race/Ethnicity or Data on Patient Characteristics | RESP-NET Interactive National Healthcare Safety Network (NHSN) Hospitalization Surveillance

Hospitals report to NHSN the weekly number of patients admitted with laboratory-confirmed influenza. During Week 47, 4,268 patients with laboratory-confirmed influenza were admitted to a hospital. The number of patients admitted to a hospital with laboratory-confirmed influenza for Week 47 increased compared to Week 46 (change of >5%) nationally and in 9 of the 10 HHS Regions (1- 9). In Region 10, the number of hospitalizations slightly decreased compared to week 46.

View Chart Data | View Full Screen

Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on November 30, 2023, 0.1% of the deaths that occurred during the week ending November 25, 2023 (week 47), were due to influenza. This percentage remained stable (≤ 0.1 percentage point change) compared to Week 46, but is trending upwards slightly since early October. The data presented are preliminary and may change as more data are received and processed.

The percentages of deaths due to pneumonia and influenza (P&I) and due to pneumonia, influenza, or COVID-19 (PIC) will no longer be displayed in FluView but are available in FluView Interactive.

Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Influenza-Associated Pediatric Mortality

Five influenza-associated pediatric deaths occurring during the 2023-2024 season were reported to CDC during week 47. The deaths occurred during weeks 43, 45, and 46 of 2023 (weeks ending October 28, November 11, and November 25, respectively). Two deaths were associated with influenza A(H1N1) viruses and three deaths were associated with influenza B viruses with no lineage determined.

A total of 8 influenza-associated pediatric deaths occurring during the 2023-2024 season have been reported to CDC.

Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Trend Indicators

Increasing:
Decreasing:
Stable: Indicators Status by System

Clinical Labs: Up or down arrows indicate a change of greater than or equal to 0.5 percentage points in the percent of specimens positive for influenza compared to the previous week.
Outpatient Respiratory Illness (ILINet): Up or down arrows indicate a change of greater than 0.1 percentage points in the percent of visits due to respiratory illness (ILI) compared to the previous week.
NHSN Hospitalizations: Up or down arrows indicate change of greater than or equal to 5% of the number of patients admitted with laboratory-confirmed influenza compared to the previous week.
NCHS Mortality: Up or down arrows indicate change of greater than 0.1 percentage points of the percent of deaths due to influenza compared to the previous week. Reference Footnotes

¹U.S. Influenza Surveillance: Purpose and Methods (2023 Oct). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/weekly/overview.htm#ILINet.

²Grohskopf LA, Blanton LH, Ferdinands JM, Chung JR, Broder KR, Talbot HK. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2023–24 Influenza Season. MMWR Recomm Rep 2023;72(No. RR-2):1–25. DOI: http://dx.doi.org/10.15585/mmwr.rr7202a1

³Influenza Antiviral Medications: Summary for Clinicians (2023 Sept). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/professional...clinicians.htm.
https://www.cdc.gov/flu/weekly/index.htm

**Lance** · December 8, 2023, 05:31 PM

Weekly U.S. Influenza Surveillance Report

Print
Updated December 8, 2023

Key Updates for Week 48, ending December 2, 2023

Seasonal influenza activity continues to increase in most parts of the country, with the southeast and south-central areas of the country reporting the highest levels of activity. Viruses

Clinical Lab 6.8%

(Trend )

positive for influenza
this week

Public Health Lab
The most frequently reported influenza viruses this week were influenza A(H1N1).

Virus Characterization
Genetic and antigenic characterization and antiviral susceptibility are summarized in this report. Illness

Outpatient Respiratory Illness 4.0%

(Trend )

of visits to a health care provider this week were for respiratory illness
(above baseline).

Outpatient Respiratory Illness: Activity Map
This week 9 jurisdictions experienced moderate activity and 16 jurisdictions experienced high or very high activity.

FluSurv-NET 5.5 per 100,000

cumulative hospitalization rate.

NHSN Hospitalizations 5,753

(Trend )

patients admitted to hospitals with influenza this week.

NCHS Mortality 0.2%

(Trend )

of deaths attributed to influenza this week.

Pediatric Deaths 4

influenza-associated deaths were reported
this week for a season total of 12.

All data are preliminary and may change as more reports are received.

Directional arrows indicate changes between the current week and the previous week. Additional information on the arrows can be found at the bottom of this page.

A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.

Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.

Key Points

Seasonal flu activity continues to increase in most parts of the country, with the southeast and south-central areas of the country reporting the highest levels of activity.
Outpatient respiratory illness is above baseline¹nationally for the fifth week and is at or above baseline in all 10 HHS Regions. Region 10 (Pacific Northwest) is at its region-specific outpatient respiratory illness baseline, and regions 1-9 are above their region-specific baselines.
The number of weekly flu hospital admissions continues to increase.
Four influenza-associated pediatric deaths were reported during Week 48, bringing the 2023-2024 season total to 12 pediatric deaths.
During Week 48, of the 547 viruses reported by public health laboratories, 437 (79.9%) were influenza A and 110 (20.1%) were influenza B. Of the 294 influenza A viruses subtyped during week 48, 218 (74.1%) were influenza A(H1N1) and 76 (25.9%) were A(H3N2).
CDC estimates that there have been at least 2.6 million illnesses, 26,000 hospitalizations, and 1,600 deaths from flu so far this season.
CDC recommends that everyone 6 months and older get an annual flu vaccine, as there are still vaccines available.²Now is still a good time to get vaccinated if you haven’t already.
There also are prescription flu antiviral drugs that can be used to treat flu illness; those should be started as early as possible and are especially important for higher risk patients.³
Flu viruses are among several viruses that contribute to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, flu, and RSV activity on a weekly basis.

U.S. Virologic Surveillance

Nationally and in HHS Regions 2, 3, 4, 5, 7, and 9, the percentage of respiratory specimens testing positive for influenza in clinical laboratories increased (change of ≥0.5 percentage points) compared to the previous week. Regions 1 and 10 continue trending upward while region 6 and 8 remained stable compared to the previous week. The regions with the highest percent positivity were regions 8 (13.1%), 4 (10.9%), 6 (8.9%), and 9 (8.6%). Since Week 40, influenza A(H1N1)pdm09 has been the predominant virus circulating in all regions. A larger combined proportion (37%) of the viruses detected by public health labs have been influenza B in regions 4 and 6 compared to other regions (7-20%). For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent receipt of live attenuated influenza vaccine (LAIV) or found upon further testing to be a vaccine virus are not included, as they are not circulating influenza viruses. Clinical Laboratories

The results of tests performed by clinical laboratories nationwide are summarized below. Data from clinical laboratories (the percentage of specimens tested that are positive for influenza virus) are used to monitor whether influenza activity is increasing or decreasing.

No. of specimens tested	93,944	690,802
No. of positive specimens (%)	6,415 (6.8%)	25,243 (3.7%)
*Positive specimens by type*
Influenza A	5,152 (80.3%)	18,772 (74.4%)
Influenza B	1,263 (19.7%)	6,471 (25.6%)

View Chart Data | View Full Screen Public Health Laboratories

The results of tests performed by public health laboratories nationwide are summarized below. Data from public health laboratories are used to monitor the proportion of circulating influenza viruses that belong to each influenza subtype/lineage.

No. of specimens tested	2,828	27,138
No. of positive specimens	547	4,778
*Positive specimens by type/subtype*
Influenza A	437 (79.9%)	3,843 (80.4%)
Subtyping Performed	294 (67.3%)	3,107 (80.8%)
(H1N1)pdm09	218 (74.1%)	2,656 (85.5%)
H3N2	76 (25.9%)	451 (14.5%)
H3N2v	0 (0.0%)	0 (0.0%)
Subtyping not performed	143 (32.7%)	736 (19.2%)
Influenza B	110 (20.1%)	935 (19.6%)
Lineage testing performed	84 (76.4%)	764 (81.7%)
Yamagata lineage	0 (0.0%)	0 (0.0%)
Victoria lineage	84 (100%)	764 (100%)
Lineage not performed	26 (23.6%)	171 (18.3%)

View Chart Data | View Full Screen

Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data Influenza Virus Characterization

CDC performs genetic and antigenic characterization of viruses submitted by U.S. state and local public health laboratories according to the Right Size Roadmap submission guidance. These data are used to compare how similar the currently circulating influenza viruses are to the reference viruses representing viruses contained in current influenza vaccines. The data are also used to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. CDC also tests susceptibility of circulating influenza viruses to influenza antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.

CDC has genetically characterized 1,005 influenza viruses collected since May 1, 2023.

A/H1	541
		6B.1A.5a	541 (100%)	2a	170 (31.4%)
				2a.1	371 (68.6%)
A/H3	113
		3C.2a1b.2a	113 (100%)	2a.1b	1 (0.9%)
				2a.3a	5 (4.4%)
				2a.3a.1	106 (93.8%)
				2b	1 (0.9%)
B/Victoria	351
		V1A	351 (100%)	3a.2	351 (100%)
B/Yamagata	0
		Y3	0	Y3	0 (0%)

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) (H1N1pdm09, H3N2, B/Victoria, and B/Yamagata viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2023-2024 Northern Hemisphere recommended cell or recombinant-based vaccine viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to the reference (vaccine) virus. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than or equal to 8-fold. Viruses selected for antigenic characterization are a subset representing the genetic changes in the surface proteins seen in genetically characterized viruses.

Influenza A Viruses

A (H1N1)pdm09: One hundred and thirty-four A(H1N1)pdm09 viruses were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown A/Wisconsin/67/2022-like reference viruses representing the A(H1N1)pdm09 component for the cell- and recombinant-based influenza vaccines.
A (H3N2): Fifty-one A(H3N2) viruses were antigenically characterized by HI or HINT, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer in HI or reacting at titers that were less than or equal to 8-fold of the homologous virus in HINT) by ferret antisera to cell-grown A/Darwin/6/2021-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.

Influenza B Viruses

B/Victoria: Eighty influenza B/Victoria-lineage virus were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines.
B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.

Assessment of Virus Susceptibility to Antiviral Medications

CDC assesses susceptibility of influenza viruses to the antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir using next generation sequence analysis supplemented by laboratory assays. Information about antiviral susceptibility test methods can be found at U.S. Influenza Surveillance: Purpose and Methods | CDC.

Viruses collected in the U.S. since October 01, 2023 were tested for antiviral susceptibility as follows:

Neuraminidase Inhibitors	Oseltamivir	Viruses Tested	421	233	67	121
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Peramivir	Viruses Tested	421	233	67	121
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Zanamivir	Viruses Tested	421	233	67	121
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
PA Cap-Dependent Endonuclease Inhibitor	Baloxavir	Viruses Tested	416	230	67	119
Decreased Susceptibility	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)

High levels of resistance to the adamantanes (amantadine and rimantadine) persist among influenza A(H1N1)pdm09 and influenza A(H3N2) viruses (the adamantanes are not effective against influenza B viruses). Therefore, use of these antivirals for treatment and prevention of influenza A virus infection is not recommended and data from adamantane resistance testing are not presented. Outpatient Respiratory Illness Surveillance

The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient visits for respiratory illness referred to as influenza-like illness [ILI (fever plus cough or sore throat)], not laboratory-confirmed influenza, and will therefore capture respiratory illness visits due to infection with pathogens that can present with similar symptoms, including influenza viruses, SARS-CoV-2, and RSV. It is important to evaluate syndromic surveillance data, including that from ILINet, in the context of other sources of surveillance data to obtain a more complete and accurate picture of influenza, SARS-CoV-2, and other respiratory virus activity. CDC is providing integrated information about COVID-19, influenza and RSV activity on a website that is updated weekly. Information about other respiratory virus activity can be found on CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS) website. Outpatient Respiratory Illness Visits

Nationwide during Week 48, 4.0% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This has remained stable compared to Week 47 and is above the national baseline of 2.9% for the fifth consecutive week. ILI activity increased in regions 1, 3, 5, and 9, decreased slightly in Region 6, and remained stable but trending upward in all other regions (regions 2, 4, 7, 8, and 10) during Week 48 compared to Week 47. All regions are at or above their region-specific baselines this week. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

Outpatient Respiratory Illness Visits by Age Group

About 70% of ILINet participants provide both the number of patient visits for respiratory illness and the total number of patient visits for the week broken out by age group. Data from this subset of providers are used to calculate the percentages of patient visits for respiratory illness by age group.

The percentage of visits for respiratory illness reported in ILINet has increased (change of > 0.1 percentage points) in the 25-49 years, 50-64 years, and 65+ years age groups, decreased in the 5-24 years age group, and remained stable in the 0-4 years age group in Week 48 compared to Week 47.

View Chart Data | View Full Screen Outpatient Respiratory Illness Activity Map

Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).

Week 48 (Week ending Dec. 2, 2023)	Week 47 (Week ending Nov. 25, 2023)	Week 48 (Week ending Dec. 2, 2023)	Week 47 (Week ending Nov. 25, 2023)
Very High	2	2	7	10
High	14	13	86	58
Moderate	9	9	82	89
Low	12	12	149	156
Minimal	18	19	346	392
Insufficient Data	0	0	259	224

*Data collected in ILINet may disproportionally represent certain populations within a jurisdiction or CBSA, and therefore, may not accurately depict the full picture of influenza activity for the entire jurisdiction or CBSA. Differences in the data presented here by CDC and independently by some health departments likely represent differing levels of data completeness with data presented by the health department likely being the more complete.

Additional information about medically attended visits for ILI for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or ILI Activity Map Hospitalization Surveillance

FluSurv-NET

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in select counties in 14 states and represents approximately 9% of the U.S. population. FluSurv-NET hospitalization data are preliminary. As data are received each week, prior case counts and rates are updated accordingly.

A total of 1,696 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2023, and December 2, 2023; 1,452 (85.6%) were associated with influenza A virus, 229 (13.5%) with influenza B virus, 4 (0.2%) with influenza A virus and influenza B virus co-infection, and 11 (0.6%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 273 (83.7%) were A(H1N1)pdm09 and 52 (16.0%) were A(H3N2).

The weekly hospitalization rate observed in Week 48 was 1.4 per 100,000 population. The overall cumulative hospitalization rate for the season was 5.5 per 100,000 population. This cumulative hospitalization rate is the second highest cumulative in-season hospitalization rate observed in Week 48, following the 2022-2023 season (26.0). Cumulative in-season hospitalization rates observed in Week 48 from 2010-2011 through 2021-2022 ranged from 0.2 to 2.7.

When examining rates by age, the highest cumulative hospitalization rate per 100,000 population was among adults aged 65 years and older (13.6), followed by adults aged 50-64 years (7.1) and children aged 0-4 years (5.9). When examining unadjusted rates by race and ethnicity, the highest rate of hospitalization per 100,000 population was among non-Hispanic Black persons (9.9), followed by non-Hispanic American Indian or Alaska Native persons (5.5), Hispanic persons (5.2), non-Hispanic Asian/Pacific Islander persons (4.2), and non-Hispanic White persons (3.9).

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In this figure, cumulative rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 season, rates for recent hospitals admissions are subject to reporting delays. As hospitalization data are reviewed each week, prior case counts and rates are updated accordingly.

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In this figure, weekly rates for all seasons prior to the 2023-24 season reflect end-of-season rates. For the 2023-24 season, rates for recent hospital admissions are subject to reporting delays and are shown as a dashed line for the current season. As hospitalization data are received each week, prior case counts and rates are updated accordingly.

Additional FluSurv-NET hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods |FluView Interactive: Rates by Age, Sex, and Race/Ethnicity or Data on Patient Characteristics | RESP-NET Interactive National Healthcare Safety Network (NHSN) Hospitalization Surveillance

Hospitals report to NHSN the weekly number of patients admitted with laboratory-confirmed influenza. During Week 48, 5,753 patients with laboratory-confirmed influenza were admitted to a hospital. The number of patients admitted to a hospital with laboratory-confirmed influenza for Week 48 increased compared to Week 47 (change of >5%) nationally and in all 10 HHS Regions (1- 10).

View Chart Data | View Full Screen

Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on December 7, 2023, 0.2% of the deaths that occurred during the week ending December 2, 2023 (Week 48), were due to influenza. This percentage remained stable (< 0.1 percentage point change) compared to Week 47 but is trending upward. The data presented are preliminary and may change as more data are received and processed.

The percentages of deaths due to pneumonia and influenza (P&I) and due to pneumonia, influenza, or COVID-19 (PIC) will no longer be displayed in FluView but are available in FluView Interactive.

Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Influenza-Associated Pediatric Mortality

Four influenza-associated pediatric deaths occurring during the 2023-2024 season were reported to CDC during Week 48. The deaths occurred during weeks 45, 46 and 47 of 2023 (the weeks ending November 11, November 18, and November 25, respectively). Three deaths were associated with influenza A(H1N1) viruses and one death was associated with an influenza B virus with no lineage determined.

A total of 12 influenza-associated pediatric deaths occurring during the 2023-2024 season have been reported to CDC.

Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Trend Indicators

Increasing:
Decreasing:
Stable: Indicators Status by System

Clinical Labs: Up or down arrows indicate a change of greater than or equal to 0.5 percentage points in the percent of specimens positive for influenza compared to the previous week.
Outpatient Respiratory Illness (ILINet): Up or down arrows indicate a change of greater than 0.1 percentage points in the percent of visits due to respiratory illness (ILI) compared to the previous week.
NHSN Hospitalizations: Up or down arrows indicate change of greater than or equal to 5% of the number of patients admitted with laboratory-confirmed influenza compared to the previous week.
NCHS Mortality: Up or down arrows indicate change of greater than 0.1 percentage points of the percent of deaths due to influenza compared to the previous week. Reference Footnotes

¹U.S. Influenza Surveillance: Purpose and Methods (2023 Oct). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/weekly/overview.htm#ILINet.

²Grohskopf LA, Blanton LH, Ferdinands JM, Chung JR, Broder KR, Talbot HK. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2023–24 Influenza Season. MMWR Recomm Rep 2023;72(No. RR-2):1–25. DOI: http://dx.doi.org/10.15585/mmwr.rr7202a1

³Influenza Antiviral Medications: Summary for Clinicians (2023 Sept). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/professional...clinicians.htm.

Weekly U.S. Influenza Surveillance Report

https://www.cdc.gov/flu/weekly/index.htm

Learn more about the weekly influenza surveillance report (FluView) prepared by the Influenza Division.

**Lance** · December 15, 2023, 03:58 PM

Weekly U.S. Influenza Surveillance Report

Print
Updated December 15, 2023

Key Updates for Week 49, ending December 9, 2023

Seasonal influenza activity is elevated in most parts of the country, with the southeast, south-central, and west coast areas of the country reporting the highest levels of activity. Viruses

Clinical Lab 10.2%

(Trend )

positive for influenza
this week

Public Health Lab
The most frequently reported influenza viruses this week were influenza A(H1N1).

Virus Characterization
Genetic and antigenic characterization and antiviral susceptibility are summarized in this report. Illness

Outpatient Respiratory Illness 4.4%

(Trend )

of visits to a health care provider this week were for respiratory illness
(above baseline).

Outpatient Respiratory Illness: Activity Map
This week 9 jurisdictions experienced moderate activity and 18 jurisdictions experienced high or very high activity.

FluSurv-NET 8.0 per 100,000

cumulative hospitalization rate.

NHSN Hospitalizations 7,090

(Trend )

patients admitted to hospitals with influenza this week.

NCHS Mortality 0.2%

(Trend )

of deaths attributed to influenza this week.

Pediatric Deaths 2

influenza-associated deaths were reported
this week for a season total of 14.

All data are preliminary and may change as more reports are received.

Directional arrows indicate changes between the current week and the previous week. Additional information on the arrows can be found at the bottom of this page.

A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.

Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.

Key Points

Seasonal influenza activity is elevated in most parts of the country, with the southeast, south-central, and west coast areas of the country reporting the highest levels of activity.
Outpatient respiratory illness is above baseline¹nationally for the sixth consecutive week and is above baseline in all 10 HHS Regions.
The number of weekly flu hospital admissions continues to increase.
Two influenza-associated pediatric deaths were reported during Week 49, bringing the 2023-2024 season total to 14 pediatric deaths.
During Week 49, of the 971 viruses reported by public health laboratories, 806 (83%) were influenza A and 165 (17%) were influenza B. Of the 500 influenza A viruses subtyped during week 49, 374 (74.8%) were influenza A(H1N1) and 126 (25.2%) were A(H3N2).
CDC estimates that there have been at least 3.7 million illnesses, 38,000 hospitalizations, and 2,300 deaths from flu so far this season.
CDC recommends that everyone 6 months and older get an annual flu vaccine, as there are still vaccines available.²Now is still a good time to get vaccinated if you haven’t already.
There also are prescription flu antiviral drugs that can be used to treat flu illness; those should be started as early as possible and are especially important for higher risk patients.³
Flu viruses are among several viruses that contribute to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, flu, and RSV activity on a weekly basis.

U.S. Virologic Surveillance

Nationally and in all HHS Regions, the percentage of respiratory specimens testing positive for influenza in clinical laboratories increased (change of ≥0.5 percentage points) compared to the previous week. The regions with the highest percent positivity were regions 8 (15.3%), 6 (13.0%), 4 (11.7%), and 9 (11.6%). Since Week 40, influenza A(H1N1)pdm09 has been the predominant virus circulating in all regions. However, a larger combined proportion (34%) of the viruses detected by public health labs have been influenza B in regions 4 and 6 compared to other regions (5-20%). For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent receipt of live attenuated influenza vaccine (LAIV) or found upon further testing to be a vaccine virus are not included, as they are not circulating influenza viruses. Clinical Laboratories

The results of tests performed by clinical laboratories nationwide are summarized below. Data from clinical laboratories (the percentage of specimens tested that are positive for influenza virus) are used to monitor whether influenza activity is increasing or decreasing.

No. of specimens tested	90,439	810,104
No. of positive specimens (%)	9,212 (10.2%)	36,366 (4.5%)
*Positive specimens by type*
Influenza A	7,570 (82.2%)	27,751 (76.3%)
Influenza B	1,642 (17.8%)	8,615 (23.7%)

View Chart Data | View Full Screen Public Health Laboratories

The results of tests performed by public health laboratories nationwide are summarized below. Data from public health laboratories are used to monitor the proportion of circulating influenza viruses that belong to each influenza subtype/lineage.

No. of specimens tested	3,060	32,227
No. of positive specimens	971	6,596
*Positive specimens by type/subtype*
Influenza A	806 (83.0%)	5,345 (81.0%)
Subtyping Performed	500 (62.0%)	4,209 (78.7%)
(H1N1)pdm09	374 (74.8%)	3,519 (83.6%)
H3N2	126 (25.2%)	690 (16.4%)
H3N2v	0 (0.0%)	0 (0.0%)
Subtyping not performed	306 (38.0%)	1,136 (21.3%)
Influenza B	165 (17.0%)	1,251 (19.0%)
Lineage testing performed	95 (57.6%)	1,000 (79.9%)
Yamagata lineage	0 (0.0%)	0 (0.0%)
Victoria lineage	95 (100%)	1,000 (100%)
Lineage not performed	70 (42.4%)	251 (20.1%)

View Chart Data | View Full Screen

Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data Influenza Virus Characterization

CDC performs genetic and antigenic characterization of U.S. viruses submitted from state and local public health laboratories according to the Right Size Roadmap submission guidance. These data are used to compare how similar the currently circulating influenza viruses are to the reference viruses representing viruses contained in the current influenza vaccines. The data are also used to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. CDC also tests susceptibility of circulating influenza viruses to antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.

CDC has genetically characterized 484 influenza viruses collected since October 1, 2023.

A/H1	274
		6B.1A.5a	274 (100%)	2a	95 (34.7%)
				2a.1	179 (65.3%)
A/H3	77
		3C.2a1b.2a	77 (100%)	2a.1b	1 (1.3%)
				2a.3a	1 (1.3%)
				2a.3a.1	74 (96.1%)
				2b	1 (1.3%)
B/Victoria	133
		V1A	133 (100%)	3a.2	133 (100%)
B/Yamagata	0
		Y3	0	Y3	0 (0%)

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) (H1N1pdm09, H3N2, B/Victoria, and B/Yamagata viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2023-2024 Northern Hemisphere recommended cell or recombinant-based vaccine viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to the reference (vaccine) virus. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than or equal to 8-fold. Viruses selected for antigenic characterization are a subset representing the genetic changes in the surface proteins seen in genetically characterized viruses.

Influenza A Viruses

A (H1N1)pdm09: Nineteen A(H1N1)pdm09 viruses were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown A/Wisconsin/67/2022-like reference viruses representing the A(H1N1)pdm09 component for the cell- and recombinant-based influenza vaccines.
A (H3N2): Twenty-four A(H3N2) viruses were antigenically characterized by HI or HINT, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer in HI or reacting at titers that were less than or equal to 8-fold of the homologous virus in HINT) by ferret antisera to cell-grown A/Darwin/6/2021-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.

Influenza B Viruses

B/Victoria: Three influenza B/Victoria-lineage virus were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines.
B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.

Assessment of Virus Susceptibility to Antiviral Medications

CDC assesses susceptibility of influenza viruses to the antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir using next generation sequence analysis supplemented by laboratory assays. Information about antiviral susceptibility test methods can be found at U.S. Influenza Surveillance: Purpose and Methods | CDC.

Viruses collected in the U.S. since October 01, 2023, were tested for antiviral susceptibility as follows:

Neuraminidase Inhibitors	Oseltamivir	Viruses Tested	494	278	78	138
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Peramivir	Viruses Tested	494	278	78	138
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Zanamivir	Viruses Tested	494	278	78	138
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
PA Cap-Dependent Endonuclease Inhibitor	Baloxavir	Viruses Tested	489	276	78	135
Decreased Susceptibility	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)

High levels of resistance to the adamantanes (amantadine and rimantadine) persist among influenza A(H1N1)pdm09 and influenza A(H3N2) viruses (the adamantanes are not effective against influenza B viruses). Therefore, use of these antivirals for treatment and prevention of influenza A virus infection is not recommended and data from adamantane resistance testing are not presented. Outpatient Respiratory Illness Surveillance

The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient visits for respiratory illness referred to as influenza-like illness [ILI (fever plus cough or sore throat)], not laboratory-confirmed influenza, and will therefore capture respiratory illness visits due to infection with pathogens that can present with similar symptoms, including influenza viruses, SARS-CoV-2, and RSV. It is important to evaluate syndromic surveillance data, including that from ILINet, in the context of other sources of surveillance data to obtain a more complete and accurate picture of influenza, SARS-CoV-2, and other respiratory virus activity. CDC is providing integrated information about COVID-19, influenza, and RSV activity on a website that is updated weekly. Information about other respiratory virus activity can be found on CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS) website. Outpatient Respiratory Illness Visits

Nationwide during Week 49, 4.4% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This has increased compared to Week 48 and is above the national baseline of 2.9% for the sixth consecutive week. During week 49 compared to week 48, ILI activity increased in regions 1-4 and 6-10 and remained stable but trending upward in region 5. All regions are above their region-specific baselines this week. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

Outpatient Respiratory Illness Visits by Age Group

About 70% of ILINet participants provide both the number of patient visits for respiratory illness and the total number of patient visits for the week broken out by age group. Data from this subset of providers are used to calculate the percentages of patient visits for respiratory illness by age group.

The percentage of visits for respiratory illness reported in ILINet has increased (change of > 0.1 percentage points) in the 0-4 years, 5-24 years, and 50-64 years age groups, and remained stable, but trending upward in the 25-49 years and 65+ years age groups in Week 49 compared to Week 48.

Outpatient Respiratory Illness Activity Map

Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).

Week 49 (Week ending Dec. 9, 2023)	Week 48 (Week ending Dec. 2, 2023)	Week 49 (Week ending Dec. 9, 2023)	Week 48 (Week ending Dec. 2, 2023)
Very High	5	2	15	7
High	13	13	107	91
Moderate	9	11	110	87
Low	15	11	160	158
Minimal	12	18	310	362
Insufficient Data	1	0	227	224

*Data collected in ILINet may disproportionally represent certain populations within a jurisdiction or CBSA, and therefore, may not accurately depict the full picture of influenza activity for the entire jurisdiction or CBSA. Differences in the data presented here by CDC and independently by some health departments likely represent differing levels of data completeness with data presented by the health department likely being the more complete.

Additional information about medically attended visits for ILI for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or ILI Activity Map Hospitalization Surveillance

FluSurv-NET

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in select counties in 14 states and represents approximately 9% of the U.S. population. FluSurv-NET hospitalization data are preliminary. As data are received each week, prior case counts and rates are updated accordingly.

A total of 2,449 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2023, and December 9, 2023; 2,115 (86.4%) were associated with influenza A virus, 313 (12.8%) with influenza B virus, 5 (0.2%) with influenza A virus and influenza B virus co-infection, and 16 (0.7%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 369 (81.5%) were A(H1N1)pdm09 and 84 (18.5%) were A(H3N2).

The weekly hospitalization rate observed in Week 49 was 1.8 per 100,000 population. The overall cumulative hospitalization rate was 8.0 per 100,000 population. This cumulative hospitalization rate is the second highest cumulative in-season hospitalization rate observed in Week 49, following the 2022-2023 season (32.7). Cumulative in-season hospitalization rates observed in Week 49 from 2010-2011 through 2021-2022 ranged from 0.2 to 3.9.

When examining rates by age, the highest cumulative hospitalization rate per 100,000 population was among adults aged 65 years and older (20.3), followed by adults aged 50-64 years (10.0) and children aged 0-4 years (8.2). When examining unadjusted rates by race and ethnicity, the highest rate of hospitalization per 100,000 population was among non-Hispanic Black persons (14.6), followed by non-Hispanic American Indian or Alaska Native persons (8.8), Hispanic persons (7.1), non-Hispanic Asian/Pacific Islander persons (5.9), and non-Hispanic White persons (5.7).

View Full Screen

In this figure, cumulative rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 season, rates for recent hospitals admissions are subject to reporting delays. As hospitalization data are reviewed each week, prior case counts and rates are updated accordingly.

View Full Screen

In this figure, weekly rates for all seasons prior to the 2023-24 season reflect end-of-season rates. For the 2023-24 season, rates for recent hospital admissions are subject to reporting delays and are shown as a dashed line for the current season. As hospitalization data are received each week, prior case counts and rates are updated accordingly.

Additional FluSurv-NET hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods |FluView Interactive: Rates by Age, Sex, and Race/Ethnicity or Data on Patient Characteristics | RESP-NET Interactive National Healthcare Safety Network (NHSN) Hospitalization Surveillance

Hospitals report to NHSN the weekly number of patients admitted with laboratory-confirmed influenza. During Week 49, 7,090 patients with laboratory-confirmed influenza were admitted to a hospital. The number of patients admitted to a hospital with laboratory-confirmed influenza for Week 49 increased compared to Week 48 (change of >5%) nationally and in all 10 HHS Regions (1- 10).

View Chart Data | View Full Screen

Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on December 14, 2023, 0.2% of the deaths that occurred during the week ending December 9, 2023 (Week 49), were due to influenza. This percentage remained stable (< 0.1 percentage point change) compared to Week 48 but has been trending upward for several weeks. The data presented are preliminary and may change as more data are received and processed.

The percentages of deaths due to pneumonia and influenza (P&I) and due to pneumonia, influenza, or COVID-19 (PIC) will no longer be displayed in FluView but are available in FluView Interactive.

Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Influenza-Associated Pediatric Mortality

Two influenza-associated pediatric deaths occurring during the 2023-2024 season were reported to CDC during Week 49. One death was associated with an influenza B virus with no lineage determined and occurred during Week 46 (the week ending November 18, 2023). The other death was associated with an influenza A(H3) virus and occurred during Week 49 (the week ending December 9, 2023).

A total of 14 influenza-associated pediatric deaths occurring during the 2023-2024 season have been reported to CDC.

Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Trend Indicators

Increasing:
Decreasing:
Stable: Indicators Status by System

Clinical Labs: Up or down arrows indicate a change of greater than or equal to 0.5 percentage points in the percent of specimens positive for influenza compared to the previous week.
Outpatient Respiratory Illness (ILINet): Up or down arrows indicate a change of greater than 0.1 percentage points in the percent of visits due to respiratory illness (ILI) compared to the previous week.
NHSN Hospitalizations: Up or down arrows indicate change of greater than or equal to 5% of the number of patients admitted with laboratory-confirmed influenza compared to the previous week.
NCHS Mortality: Up or down arrows indicate change of greater than 0.1 percentage points of the percent of deaths due to influenza compared to the previous week. Reference Footnotes

¹U.S. Influenza Surveillance: Purpose and Methods (2023 Oct). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/weekly/overview.htm#ILINet.

²Grohskopf LA, Blanton LH, Ferdinands JM, Chung JR, Broder KR, Talbot HK. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2023–24 Influenza Season. MMWR Recomm Rep 2023;72(No. RR-2):1–25. DOI: http://dx.doi.org/10.15585/mmwr.rr7202a1

³Influenza Antiviral Medications: Summary for Clinicians (2023 Sept). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/professional...clinicians.htm.

Additional National and International Influenza Surveillance Information

FluView Interactive: FluView includes enhanced web-based interactive applications that can provide dynamic visuals of the influenza data collected and analyzed by CDC. These FluView Interactive applications allow people to create customized, visual interpretations of influenza data, as well as make comparisons across flu seasons, regions, age groups and a variety of other demographics.

National Institute for Occupational Safety and Health: Monthly surveillance data on the prevalence of health-related workplace absenteeism among full-time workers in the United States are available from NIOSH.

U.S. State and local influenza surveillance: Select a jurisdiction below to access the latest local influenza information.
https://www.cdc.gov/flu/weekly/index.htm

**Lance** · December 22, 2023, 06:57 PM

Weekly U.S. Influenza Surveillance Report

Print
Updated December 22, 2023

Key Updates for Week 50, ending December 16, 2023

Seasonal influenza activity is elevated and continues to increase in most parts of the country. Viruses

Clinical Lab 12.8%

(Trend )

positive for influenza
this week

Public Health Lab
The most frequently reported influenza viruses this week were influenza A(H1N1).

Virus Characterization
Genetic and antigenic characterization and antiviral susceptibility are summarized in this report. Illness

Outpatient Respiratory Illness 5.1%

(Trend )

of visits to a health care provider this week were for respiratory illness
(above baseline).

Outpatient Respiratory Illness: Activity Map
This week 11 jurisdictions experienced moderate activity and 25 jurisdictions experienced high or very high activity.

FluSurv-NET 11.3 per 100,000

cumulative hospitalization rate.

NHSN Hospitalizations 9,825

(Trend )

patients admitted to hospitals with influenza this week.

NCHS Mortality 0.3%

(Trend )

of deaths attributed to influenza this week.

Pediatric Deaths 0

influenza-associated deaths were reported
this week. A total of 14 influenza-associated deaths have been reported.

All data are preliminary and may change as more reports are received.

Directional arrows indicate changes between the current week and the previous week. Additional information on the arrows can be found at the bottom of this page.

A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.

Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.

Key Points

Seasonal influenza activity is elevated and continues to increase in most parts of the country.
Outpatient respiratory illness is above baseline¹nationally for the seventh consecutive week and is above baseline in all 10 HHS Regions.
The number of weekly flu hospital admissions continues to increase.
During Week 50, of the 1,050 viruses reported by public health laboratories, 860 (81.9%) were influenza A and 190 (18.1%) were influenza B. Of the 481 influenza A viruses subtyped during week 50, 376 (78.2%) were influenza A(H1N1) and 105 (21.8%) were A(H3N2).
CDC estimates that there have been at least 5.3 million illnesses, 54,000 hospitalizations, and 3,200 deaths from flu so far this season.
CDC recommends that everyone 6 months and older get an annual flu vaccine, as there are still vaccines available.²Now is still a good time to get vaccinated if you haven’t already.
There also are prescription flu antiviral drugs that can be used to treat flu illness; those should be started as early as possible and are especially important for higher risk patients.³
Flu viruses are among several viruses that contribute to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, flu, and RSV activity on a weekly basis.

U.S. Virologic Surveillance

Nationally and in all HHS Regions, the percentage of respiratory specimens testing positive for influenza in clinical laboratories increased (change of ≥0.5 percentage points) compared to the previous week. The regions with the highest percent positivity were regions 8 (23.6%), 6 (17.5%), 4 (15.5%), and 9 (14.3%). Since Week 40, influenza A(H1N1)pdm09 has been the predominant virus circulating in all regions. However, the distribution of circulating viruses varies by region. For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent receipt of live attenuated influenza vaccine (LAIV) or found upon further testing to be a vaccine virus are not included, as they are not circulating influenza viruses. Clinical Laboratories

The results of tests performed by clinical laboratories nationwide are summarized below. Data from clinical laboratories (the percentage of specimens tested that are positive for influenza virus) are used to monitor whether influenza activity is increasing or decreasing.

No. of specimens tested	105,774	949,819
No. of positive specimens (%)	13,576 (12.8%)	52,825 (5.6%)
*Positive specimens by type*
Influenza A	11,243 (82.8%)	41,302 (78.2%)
Influenza B	2,333 (17.2%)	11,523 (21.8%)

View Chart Data | View Full Screen Public Health Laboratories

The results of tests performed by public health laboratories nationwide are summarized below. Data from public health laboratories are used to monitor the proportion of circulating influenza viruses that belong to each influenza subtype/lineage.

No. of specimens tested	2,974	36,500
No. of positive specimens	1,050	8,385
*Positive specimens by type/subtype*
Influenza A	860 (81.9%)	6,809 (81.2%)
Subtyping Performed	481 (55.9%)	5,405 (79.4%)
(H1N1)pdm09	376 (78.2%)	4,481 (82.9%)
H3N2	105 (21.8%)	924 (17.1%)
H3N2v	0 (0.0%)	0 (0.0%)
Subtyping not performed	379 (44.1%)	1,404 (20.6%)
Influenza B	190 (18.1%)	1,576 (18.8%)
Lineage testing performed	97 (51.1%)	1,240 (78.7%)
Yamagata lineage	0 (0.0%)	0 (0.0%)
Victoria lineage	97 (100%)	1,240 (100%)
Lineage not performed	93 (48.9%)	336 (21.3%)

View Chart Data | View Full Screen

Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data Influenza Virus Characterization

CDC performs genetic and antigenic characterization of U.S. viruses submitted from state and local public health laboratories according to the Right Size Roadmap submission guidance. These data are used to compare how similar the currently circulating influenza viruses are to the reference viruses representing viruses contained in the current influenza vaccines. The data are also used to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. CDC also tests susceptibility of circulating influenza viruses to antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.

CDC has genetically characterized 580 influenza viruses collected since October 1, 2023.

A/H1	321
		6B.1A.5a	321 (100%)	2a	103 (32.1%)
				2a.1	218 (67.9%)
A/H3	105
		3C.2a1b.2a	105 (100%)	2a.1b	1 (1.0%)
				2a.3a	1 (1.0%)
				2a.3a.1	102 (97.1%)
				2b	1 (1.0%)
B/Victoria	154
		V1A	154 (100%)	3a.2	154 (100%)
B/Yamagata	0
		Y3	0	Y3	0 (0%)

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) (H1N1pdm09, H3N2, B/Victoria, and B/Yamagata viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2023-2024 Northern Hemisphere recommended cell or recombinant-based vaccine viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to the reference (vaccine) virus. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than or equal to 8-fold. Viruses selected for antigenic characterization are a subset representing the genetic changes in the surface proteins seen in genetically characterized viruses.

Influenza A Viruses

A (H1N1)pdm09: Nineteen A(H1N1)pdm09 viruses were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown A/Wisconsin/67/2022-like reference viruses representing the A(H1N1)pdm09 component for the cell- and recombinant-based influenza vaccines.
A (H3N2): Forty-nine A(H3N2) viruses were antigenically characterized by HI or HINT, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer in HI or reacting at titers that were less than or equal to 8-fold of the homologous virus in HINT) by ferret antisera to cell-grown A/Darwin/6/2021-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.

Influenza B Viruses

B/Victoria: Nineteen influenza B/Victoria-lineage virus were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines.
B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.

Assessment of Virus Susceptibility to Antiviral Medications

CDC assesses susceptibility of influenza viruses to the antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir using next generation sequence analysis supplemented by laboratory assays. Information about antiviral susceptibility test methods can be found at U.S. Influenza Surveillance: Purpose and Methods | CDC.

Viruses collected in the U.S. since October 01, 2023, were tested for antiviral susceptibility as follows:

Neuraminidase Inhibitors	Oseltamivir	Viruses Tested	583	321	105	157
Reduced Inhibition	1 (0.2%)	1 (0.3%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Peramivir	Viruses Tested	583	321	105	157
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Zanamivir	Viruses Tested	583	321	105	157
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
PA Cap-Dependent Endonuclease Inhibitor	Baloxavir	Viruses Tested	528	304	87	137
Decreased Susceptibility	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)

One A(H1N1)pdm09 virus had NA-S247N and NA-I223V amino acid substitutions and showed reduced inhibition by oseltamivir.

High levels of resistance to the adamantanes (amantadine and rimantadine) persist among influenza A(H1N1)pdm09 and influenza A(H3N2) viruses (the adamantanes are not effective against influenza B viruses). Therefore, use of these antivirals for treatment and prevention of influenza A virus infection is not recommended and data from adamantane resistance testing are not presented. Outpatient Respiratory Illness Surveillance

The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient visits for respiratory illness referred to as influenza-like illness [ILI (fever plus cough or sore throat)], not laboratory-confirmed influenza, and will therefore capture respiratory illness visits due to infection with pathogens that can present with similar symptoms, including influenza viruses, SARS-CoV-2, and RSV. It is important to evaluate syndromic surveillance data, including that from ILINet, in the context of other sources of surveillance data to obtain a more complete and accurate picture of influenza, SARS-CoV-2, and other respiratory virus activity. CDC is providing integrated information about COVID-19, influenza, and RSV activity on a website that is updated weekly. Information about other respiratory virus activity can be found on CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS) website. Outpatient Respiratory Illness Visits

Nationwide during Week 50, 5.1% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This has increased compared to Week 49 and is above the national baseline of 2.9% for the seventh consecutive week. During Week 50 compared to week 49, ILI activity increased in all regions. All regions are above their region-specific baselines this week. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

Outpatient Respiratory Illness Visits by Age Group

About 70% of ILINet participants provide both the number of patient visits for respiratory illness and the total number of patient visits for the week broken out by age group. Data from this subset of providers are used to calculate the percentages of patient visits for respiratory illness by age group.

The percentage of visits for respiratory illness reported in ILINet has increased (change of > 0.1 percentage points) in all age groups in Week 50 compared to Week 49.

Outpatient Respiratory Illness Activity Map

Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).

Week 50 (Week ending Dec. 16, 2023)	Week 49 (Week ending Dec. 9, 2023)	Week 50 (Week ending Dec. 16, 2023)	Week 49 (Week ending Dec. 9, 2023)
Very High	10	5	40	15
High	15	14	135	106
Moderate	11	9	107	112
Low	9	15	162	160
Minimal	10	12	253	310
Insufficient Data	0	0	232	226

*Data collected in ILINet may disproportionally represent certain populations within a jurisdiction or CBSA, and therefore, may not accurately depict the full picture of influenza activity for the entire jurisdiction or CBSA. Differences in the data presented here by CDC and independently by some health departments likely represent differing levels of data completeness with data presented by the health department likely being the more complete.

Additional information about medically attended visits for ILI for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or ILI Activity Map Hospitalization Surveillance

FluSurv-NET

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in select counties in 14 states and represents approximately 9% of the U.S. population. FluSurv-NET hospitalization data are preliminary. As data are received each week, prior case counts and rates are updated accordingly.

A total of 3,455 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2023 and December 16, 2023; 3,000 (86.8%) were associated with influenza A virus, 415 (12.0%) with influenza B virus, 9 (0.3%) with influenza A virus and influenza B virus co-infection, and 31 (0.9%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 513 (79.4%) were A(H1N1)pdm09 and 133 (20.6%) were A(H3N2).

The weekly hospitalization rate observed in week 50 was 2.5 per 100,000 population. The overall cumulative hospitalization rate was 11.3 per 100,000 population. This cumulative hospitalization rate is the second highest cumulative in-season hospitalization rate observed in week 50, following the 2022-2023 season (39.9). Cumulative in-season hospitalization rates observed in week 50 from 2010-2011 through 2021-2022 ranged from 0.3 to 6.2.

When examining rates by age, the highest cumulative hospitalization rate per 100,000 population was among adults aged 65 years and older (28.1), followed by adults aged 50-64 years (13.9) and children aged 0-4 years (12.4). When examining unadjusted rates by race and ethnicity, the highest rate of hospitalization per 100,000 population was among non-Hispanic Black persons (20.2), followed by non-Hispanic American Indian or Alaska Native persons (13.2), Hispanic persons (9.8), non-Hispanic White persons (7.7), and non-Hispanic Asian/Pacific Islander persons (7.2).

View Full Screen

In this figure, cumulative rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 season, rates for recent hospitals admissions are subject to reporting delays. As hospitalization data are reviewed each week, prior case counts and rates are updated accordingly.

View Full Screen

In this figure, weekly rates for all seasons prior to the 2023-24 season reflect end-of-season rates. For the 2023-24 season, rates for recent hospital admissions are subject to reporting delays and are shown as a dashed line for the current season. As hospitalization data are received each week, prior case counts and rates are updated accordingly.

Additional FluSurv-NET hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods |FluView Interactive: Rates by Age, Sex, and Race/Ethnicity or Data on Patient Characteristics | RESP-NET Interactive National Healthcare Safety Network (NHSN) Hospitalization Surveillance

Hospitals report to NHSN the weekly number of patients admitted with laboratory-confirmed influenza. During Week 50, 9,825 patients with laboratory-confirmed influenza were admitted to a hospital. The number of patients admitted to a hospital with laboratory-confirmed influenza for Week 50 increased compared to Week 49 (change of >5%) nationally and in 9 of the 10 HHS Regions (1-6 and 8-10) and in Region 7, the number of hospitalizations remained stable.

View Chart Data | View Full Screen

Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on December 21, 2023, 0.3% of the deaths that occurred during the week ending December 16, 2023 (Week 50), were due to influenza. This percentage remained stable (< 0.1 percentage point change) compared to Week 49 but has been trending upward for several weeks. The data presented are preliminary and may change as more data are received and processed.

The percentages of deaths due to pneumonia and influenza (P&I) and due to pneumonia, influenza, or COVID-19 (PIC) will no longer be displayed in FluView but are available in FluView Interactive.

Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Influenza-Associated Pediatric Mortality

No influenza-associated pediatric deaths were reported to CDC during week 50.

A total of 14 influenza-associated pediatric deaths occurring during the 2023-2024 season have been reported to CDC.

Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Trend Indicators

Increasing:
Decreasing:
Stable: Indicators Status by System

Clinical Labs: Up or down arrows indicate a change of greater than or equal to 0.5 percentage points in the percent of specimens positive for influenza compared to the previous week.
Outpatient Respiratory Illness (ILINet): Up or down arrows indicate a change of greater than 0.1 percentage points in the percent of visits due to respiratory illness (ILI) compared to the previous week.
NHSN Hospitalizations: Up or down arrows indicate change of greater than or equal to 5% of the number of patients admitted with laboratory-confirmed influenza compared to the previous week.
NCHS Mortality: Up or down arrows indicate change of greater than 0.1 percentage points of the percent of deaths due to influenza compared to the previous week. Reference Footnotes

¹U.S. Influenza Surveillance: Purpose and Methods (2023 Oct). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/weekly/overview.htm#ILINet.

²Grohskopf LA, Blanton LH, Ferdinands JM, Chung JR, Broder KR, Talbot HK. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2023–24 Influenza Season. MMWR Recomm Rep 2023;72(No. RR-2):1–25. DOI: http://dx.doi.org/10.15585/mmwr.rr7202a1

³Influenza Antiviral Medications: Summary for Clinicians (2023 Sept). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/professional...clinicians.htm.

Additional National and International Influenza Surveillance Information

FluView Interactive: FluView includes enhanced web-based interactive applications that can provide dynamic visuals of the influenza data collected and analyzed by CDC. These FluView Interactive applications allow people to create customized, visual interpretations of influenza data, as well as make comparisons across flu seasons, regions, age groups and a variety of other demographics.

National Institute for Occupational Safety and Health: Monthly surveillance data on the prevalence of health-related workplace absenteeism among full-time workers in the United States are available from NIOSH.

U.S. State and local influenza surveillance: Select a jurisdiction below to access the latest local influenza information.
https://www.cdc.gov/flu/weekly/index.htm

**Lance** · December 29, 2023, 07:21 PM

Weekly U.S. Influenza Surveillance Report

Print
Updated December 29, 2023

Key Updates for Week 51, ending December 23, 2023

Seasonal influenza activity is elevated and continues to increase in most parts of the country. Viruses

Clinical Lab 16.1%

(Trend )

positive for influenza
this week

Public Health Lab
The most frequently reported influenza viruses this week were influenza A(H1N1)pdm09.

Virus Characterization
Genetic and antigenic characterization and antiviral susceptibility are summarized in this report. Illness

Outpatient Respiratory Illness 6.1%

(Trend )

of visits to a health care provider this week were for respiratory illness
(above baseline).

Outpatient Respiratory Illness: Activity Map
This week 6 jurisdictions experienced moderate activity and 33 jurisdictions experienced high or very high activity.

FluSurv-NET 15.4 per 100,000

cumulative hospitalization rate.

NHSN Hospitalizations 14,732

(Trend )

patients admitted to hospitals with influenza this week.

NCHS Mortality 0.5%

(Trend )

of deaths attributed to influenza this week.

Pediatric Deaths 7

influenza-associated deaths were reported
(1 occurred during 2022-2023 season and 6 occurred during 2023-2024 season)

All data are preliminary and may change as more reports are received.

Directional arrows indicate changes between the current week and the previous week. Additional information on the arrows can be found at the bottom of this page.

A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.

Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.

Key Points

Seasonal influenza activity is elevated and continues to increase in most parts of the country.
Outpatient respiratory illness is above baseline¹nationally for the eighth consecutive week and is above baseline in all 10 HHS Regions.
The number of weekly flu hospital admissions continues to increase.
During Week 51, of the 875 viruses reported by public health laboratories, 748 (85.5%) were influenza A and 127 (14.5%) were influenza B. Of the 391 influenza A viruses subtyped during Week 51, 309 (79.0%) were influenza A(H1N1) and 82 (21.0%) were A(H3N2).
Six influenza-associated pediatric deaths occurring during the 2023-2024 influenza season were reported during Week 51, bringing the 2023-2024 season total to 20 pediatric deaths.
CDC estimates that there have been at least 7.1 million illnesses, 73,000 hospitalizations, and 4,500 deaths from flu so far this season.
CDC recommends that everyone 6 months and older get an annual flu vaccine as long as influenza viruses are spreading.²Vaccination now can still provide benefit this season.
There also are prescription flu antiviral drugs that can treat flu illness; those should be started as early as possible and are especially important for higher risk patients.³
Flu viruses are among several viruses contributing to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, flu, and RSV activity on a weekly basis.

U.S. Virologic Surveillance

Nationally the percentage of respiratory specimens testing positive for influenza in clinical laboratories increased (change of ≥0.5 percentage points) compared to the previous week. The regions with the highest percent positivity were regions 8 (25.4%), 6 (25.0%), 4 (20.7%), and 3 (20.2%). Since Week 40, influenza A(H1N1)pdm09 has been the predominant virus circulating in all regions. However, the distribution of circulating viruses varies by region. For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent receipt of live attenuated influenza vaccine (LAIV) or found upon further testing to be a vaccine virus are not included, as they are not circulating influenza viruses. Clinical Laboratories

The results of tests performed by clinical laboratories nationwide are summarized below. Data from clinical laboratories (the percentage of specimens tested that are positive for influenza virus) are used to monitor whether influenza activity is increasing or decreasing.

No. of specimens tested	79,353	1,060,095
No. of positive specimens (%)	12,775 (16.1%)	67,851 (6.4%)
*Positive specimens by type*
Influenza A	10,401 (81.4%)	53,548 (78.9%)
Influenza B	2,374 (18.6%)	14,303 (21.1%)

View Chart Data | View Full Screen Public Health Laboratories

The results of tests performed by public health laboratories nationwide are summarized below. Data from public health laboratories are used to monitor the proportion of circulating influenza viruses that belong to each influenza subtype/lineage.

No. of specimens tested	2,795	40,596
No. of positive specimens	875	10,143
*Positive specimens by type/subtype*
Influenza A	748 (85.5%)	8,270 (81.5%)
Subtyping Performed	391 (52.3%)	6,444 (77.9%)
(H1N1)pdm09	309 (79.0%)	5,309 (82.4%)
H3N2	82 (21.0%)	1,135 (17.6%)
H3N2v	0 (0.0%)	0 (0.0%)
Subtyping not performed	357 (47.7%)	1,826 (22.1%)
Influenza B	127 (14.5%)	1,873 (18.5%)
Lineage testing performed	60 (47.2%)	1,470 (78.5%)
Yamagata lineage	0 (0.0%)	0 (0.0%)
Victoria lineage	60 (100%)	1,470 (100%)
Lineage not performed	67 (52.8%)	403 (21.5%)

View Chart Data | View Full Screen

Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data Influenza Virus Characterization

CDC performs genetic and antigenic characterization of U.S. viruses submitted from state and local public health laboratories according to the Right Size Roadmap submission guidance. These data are used to compare how similar the currently circulating influenza viruses are to the reference viruses representing viruses contained in the current influenza vaccines. The data are also used to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. CDC also tests susceptibility of circulating influenza viruses to antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.

CDC has genetically characterized 687 influenza viruses collected since October 1, 2023.

A/H1	361
		6B.1A.5a	361 (100%)	2a	110 (30.5%)
				2a.1	251 (69.5%)
A/H3	121
		3C.2a1b.2a	121 (100%)	2a.1b	1 (0.8%)
				2a.3a	1 (0.8%)
				2a.3a.1	118 (97.5%)
				2b	1 (0.8%)
B/Victoria	205
		V1A	205 (100%)	3a.2	205 (100%)
B/Yamagata	0
		Y3	0	Y3	0 (0%)

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) (H1N1pdm09, H3N2, B/Victoria, and B/Yamagata viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2023-2024 Northern Hemisphere recommended cell or recombinant-based vaccine viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to the reference (vaccine) virus. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than or equal to 8-fold. Viruses selected for antigenic characterization are a subset representing the genetic changes in the surface proteins seen in genetically characterized viruses.

Influenza A Viruses

A (H1N1)pdm09: Fifty A(H1N1)pdm09 viruses were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown A/Wisconsin/67/2022-like reference viruses representing the A(H1N1)pdm09 component for the cell- and recombinant-based influenza vaccines.
A (H3N2): Forty-nine A(H3N2) viruses were antigenically characterized by HI or HINT, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer in HI or reacting at titers that were less than or equal to 8-fold of the homologous virus in HINT) by ferret antisera to cell-grown A/Darwin/6/2021-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.

Influenza B Viruses

B/Victoria: Nineteen influenza B/Victoria-lineage virus were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines.
B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.

Assessment of Virus Susceptibility to Antiviral Medications

CDC assesses susceptibility of influenza viruses to the antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir using next generation sequence analysis supplemented by laboratory assays. Information about antiviral susceptibility test methods can be found at U.S. Influenza Surveillance: Purpose and Methods | CDC.

Viruses collected in the U.S. since October 01, 2023, were tested for antiviral susceptibility as follows:

Neuraminidase Inhibitors	Oseltamivir	Viruses Tested	694	362	121	211
Reduced Inhibition	1 (0.1%)	1 (0.3%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Peramivir	Viruses Tested	694	362	121	211
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Zanamivir	Viruses Tested	694	362	121	211
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
PA Cap-Dependent Endonuclease Inhibitor	Baloxavir	Viruses Tested	687	360	120	207
Decreased Susceptibility	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)

One A(H1N1)pdm09 virus had NA-S247N and NA-I223V amino acid substitutions and showed reduced inhibition by oseltamivir.

High levels of resistance to the adamantanes (amantadine and rimantadine) persist among influenza A(H1N1)pdm09 and influenza A(H3N2) viruses (the adamantanes are not effective against influenza B viruses). Therefore, use of these antivirals for treatment and prevention of influenza A virus infection is not recommended and data from adamantane resistance testing are not presented. Outpatient Respiratory Illness Surveillance

The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient visits for respiratory illness referred to as influenza-like illness [ILI (fever plus cough or sore throat)], not laboratory-confirmed influenza, and will therefore capture respiratory illness visits due to infection with pathogens that can present with similar symptoms, including influenza viruses, SARS-CoV-2, and RSV. It is important to evaluate syndromic surveillance data, including that from ILINet, in the context of other sources of surveillance data to obtain a more complete and accurate picture of influenza, SARS-CoV-2, and other respiratory virus activity. CDC is providing integrated information about COVID-19, influenza, and RSV activity on a website that is updated weekly. Information about other respiratory virus activity can be found on CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS) website. Outpatient Respiratory Illness Visits

Nationwide during Week 51, 6.1% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This has increased compared to Week 50 and has remained above the national baseline of 2.9% since Week 44. All regions are above their region-specific baselines this week. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

Outpatient Respiratory Illness Visits by Age Group

About 70% of ILINet participants provide both the number of patient visits for respiratory illness and the total number of patient visits for the week broken out by age group. Data from this subset of providers are used to calculate the percentages of patient visits for respiratory illness by age group.

The percentage of visits for respiratory illness reported in ILINet has increased (change of > 0.1 percentage points) in all age groups in Week 51 compared to Week 50.

Outpatient Respiratory Illness Activity Map

Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).

Week 51 (Week ending Dec. 23, 2023)	Week 50 (Week ending Dec. 16, 2023)	Week 51 (Week ending Dec. 23, 2023)	Week 50 (Week ending Dec. 16, 2023)
Very High	14	10	75	41
High	19	14	163	135
Moderate	6	12	89	107
Low	5	8	145	159
Minimal	9	11	205	258
Insufficient Data	2	0	252	229

*Data collected in ILINet may disproportionally represent certain populations within a jurisdiction or CBSA, and therefore, may not accurately depict the full picture of influenza activity for the entire jurisdiction or CBSA. Differences in the data presented here by CDC and independently by some health departments likely represent differing levels of data completeness with data presented by the health department likely being the more complete.

Additional information about medically attended visits for ILI for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or ILI Activity Map Hospitalization Surveillance

FluSurv-NET

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in select counties in 14 states and represents approximately 9% of the U.S. population. FluSurv-NET hospitalization data are preliminary. As data are received each week, prior case counts and rates are updated accordingly.

A total of 4,697 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2023, and December 23, 2023; 4,105 (87.4%) were associated with influenza A virus, 529 (11.3%) with influenza B virus, 12 (0.3%) with influenza A virus and influenza B virus co-infection, and 51 (1.1%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 706 (79.8%) were A(H1N1)pdm09 and 179 (20.2%) were A(H3N2).

The weekly hospitalization rate observed during Week 51 was 2.8 per 100,000 population. The overall cumulative hospitalization rate was 15.4 per 100,000 population. This cumulative hospitalization rate is the second highest cumulative in-season hospitalization rate observed during Week 51, following the 2022-2023 season (44.3). Cumulative in-season hospitalization rates observed during Week 51 from 2010-2011 through 2021-2022 ranged from 0.3 to 9.7.

When examining rates by age, the highest cumulative hospitalization rate per 100,000 population was among adults aged 65 years and older (38.8), followed by adults aged 50-64 years (18.9) and children aged 0-4 years (16.6). When examining unadjusted rates by race and ethnicity, the highest rate of hospitalization per 100,000 population was among non-Hispanic Black persons (29.0), followed by non-Hispanic American Indian or Alaska Native persons (17.6), Hispanic persons (12.8), non-Hispanic White persons (11.2), and non-Hispanic Asian/Pacific Islander persons (9.9).

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In this figure, cumulative rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 season, rates for recent hospitals admissions are subject to reporting delays. As hospitalization data are reviewed each week, prior case counts and rates are updated accordingly.

View Full Screen

In this figure, weekly rates for all seasons prior to the 2023-24 season reflect end-of-season rates. For the 2023-24 season, rates for recent hospital admissions are subject to reporting delays and are shown as a dashed line for the current season. As hospitalization data are received each week, prior case counts and rates are updated accordingly.

Additional FluSurv-NET hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods |FluView Interactive: Rates by Age, Sex, and Race/Ethnicity or Data on Patient Characteristics | RESP-NET Interactive National Healthcare Safety Network (NHSN) Hospitalization Surveillance

Hospitals report to NHSN the weekly number of patients admitted with laboratory-confirmed influenza. During Week 51, 14,732 patients with laboratory-confirmed influenza were admitted to a hospital. The number of patients admitted to a hospital with laboratory-confirmed influenza for Week 51 increased compared to Week 50 (change of >5%) nationally and in all the 10 HHS Regions.

View Chart Data | View Full Screen

Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on December 28, 2023, 0.5% of the deaths that occurred during the week ending December 23, 2023 (Week 51), were due to influenza. This percentage increased (≥ 0.1 percentage point change) compared to Week 50. The data presented are preliminary and may change as more data are received and processed.

The percentages of deaths due to pneumonia and influenza (P&I) and due to pneumonia, influenza, or COVID-19 (PIC) will no longer be displayed in FluView but are available in FluView Interactive.

Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Influenza-Associated Pediatric Mortality

Seven influenza-associated pediatric deaths were reported to CDC during week 51.

Six deaths occurred during Weeks 49, 50, and 51 (the weeks ending December 9, December 16, and December 23 of 2023, respectively). Four of the deaths were associated with influenza A viruses. Two of the influenza A viruses had subtyping performed; both were A(H1N1) viruses. Two deaths were associated with influenza B viruses with no lineage determined.

One death occurring during the 2022-2023 season was also reported, which brings the total number of pediatric deaths for last season to 183. The death was associated with an influenza B virus with no lineage determined and occurred during Week 33 (the week ending August 19, 2023).

Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Trend Indicators

Increasing:
Decreasing:
Stable: Indicators Status by System

Clinical Labs: Up or down arrows indicate a change of greater than or equal to 0.5 percentage points in the percent of specimens positive for influenza compared to the previous week.
Outpatient Respiratory Illness (ILINet): Up or down arrows indicate a change of greater than 0.1 percentage points in the percent of visits due to respiratory illness (ILI) compared to the previous week.
NHSN Hospitalizations: Up or down arrows indicate change of greater than or equal to 5% of the number of patients admitted with laboratory-confirmed influenza compared to the previous week.
NCHS Mortality: Up or down arrows indicate change of greater than 0.1 percentage points of the percent of deaths due to influenza compared to the previous week. Reference Footnotes

¹U.S. Influenza Surveillance: Purpose and Methods (2023 Oct). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/weekly/overview.htm#ILINet.

²Grohskopf LA, Blanton LH, Ferdinands JM, Chung JR, Broder KR, Talbot HK. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2023–24 Influenza Season. MMWR Recomm Rep 2023;72(No. RR-2):1–25. DOI: http://dx.doi.org/10.15585/mmwr.rr7202a1

³Influenza Antiviral Medications: Summary for Clinicians (2023 Sept). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/professional...clinicians.htm.

Additional National and International Influenza Surveillance Information

FluView Interactive: FluView includes enhanced web-based interactive applications that can provide dynamic visuals of the influenza data collected and analyzed by CDC. These FluView Interactive applications allow people to create customized, visual interpretations of influenza data, as well as make comparisons across flu seasons, regions, age groups and a variety of other demographics.

National Institute for Occupational Safety and Health: Monthly surveillance data on the prevalence of health-related workplace absenteeism among full-time workers in the United States are available from NIOSH.

U.S. State and local influenza surveillance: Select a jurisdiction below to access the latest local influenza information.
https://www.cdc.gov/flu/weekly/index.htm

**Lance** · January 5, 2024, 06:43 PM

Weekly U.S. Influenza Surveillance Report

Print
Updated January 5, 2024

Key Updates for Week 52, ending December 30, 2023

Seasonal influenza activity is elevated and continues to increase in most parts of the country. Viruses

Clinical Lab 17.5%

(Trend )

positive for influenza
this week

Public Health Lab
The most frequently reported influenza viruses this week were influenza A(H1N1)pdm09.

Virus Characterization
Genetic and antigenic characterization and antiviral susceptibility are summarized in this report. Illness

Outpatient Respiratory Illness 6.9%

(Trend )

of visits to a health care provider this week were for respiratory illness
(above baseline).

Outpatient Respiratory Illness: Activity Map
This week 6 jurisdictions experienced moderate activity and 39 jurisdictions experienced high or very high activity.

FluSurv-NET 22.3 per 100,000

cumulative hospitalization rate.

NHSN Hospitalizations 20,066

(Trend )

patients admitted to hospitals with influenza this week.

NCHS Mortality 0.9%

(Trend )

of deaths attributed to influenza this week.

Pediatric Deaths 7

influenza-associated deaths were reported
this week for a total of 27 deaths this season.

All data are preliminary and may change as more reports are received.

Directional arrows indicate changes between the current week and the previous week. Additional information on the arrows can be found at the bottom of this page.

A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.

Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.

Key Points

Seasonal influenza activity is elevated and continues to increase in most parts of the country.
Outpatient respiratory illness has been above baseline¹nationally since November and is above baseline in all 10 HHS Regions.
The number of weekly flu hospital admissions continues to increase.
During Week 52, of the 651 viruses reported by public health laboratories, 581 (89.2%) were influenza A and 70 (10.8%) were influenza B. Of the 342 influenza A viruses subtyped during Week 52, 300 (87.7%) were influenza A(H1N1) and 42 (12.3%) were A(H3N2).
Seven influenza-associated pediatric deaths were reported during Week 52, bringing the 2023-2024 season total to 27 pediatric deaths.
CDC estimates that there have been at least 10 million illnesses, 110,000 hospitalizations, and 6,500 deaths from flu so far this season.
CDC recommends that everyone 6 months and older get an annual flu vaccine as long as influenza viruses are spreading.²Vaccination now can still provide benefit this season.
There also are prescription flu antiviral drugs that can treat flu illness; those should be started as early as possible and are especially important for higher risk patients.³
Flu viruses are among several viruses contributing to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, flu, and RSV activity on a weekly basis.

U.S. Virologic Surveillance

Nationally the percentage of respiratory specimens testing positive for influenza in clinical laboratories increased (change of ≥0.5 percentage points) compared to the previous week. The regions with the highest percent positivity were regions 8 (29.2%), 4 (21.8%), 6 (21.4%), and 3 (21.2%). Since Week 40, influenza A(H1N1)pdm09 has been the predominant virus circulating in all regions. However, the distribution of circulating viruses varies by region. For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent receipt of live attenuated influenza vaccine (LAIV) or found upon further testing to be a vaccine virus are not included, as they are not circulating influenza viruses. Clinical Laboratories

The results of tests performed by clinical laboratories nationwide are summarized below. Data from clinical laboratories (the percentage of specimens tested that are positive for influenza virus) are used to monitor whether influenza activity is increasing or decreasing.

No. of specimens tested	102,294	1,186,063
No. of positive specimens (%)	17,925 (17.5%)	88,881 (7.5%)
*Positive specimens by type*
Influenza A	14,732 (82.2%)	70,485 (79.3%)
Influenza B	3,193 (17.8%)	18,396 (20.7%)

View Chart Data | View Full Screen Public Health Laboratories

The results of tests performed by public health laboratories nationwide are summarized below. Data from public health laboratories are used to monitor the proportion of circulating influenza viruses that belong to each influenza subtype/lineage.

No. of specimens tested	2,300	44,620
No. of positive specimens	651	11,639
*Positive specimens by type/subtype*
Influenza A	581 (89.2%)	9,552 (82.1%)
Subtyping Performed	342 (58.9%)	7,622 (79.8%)
(H1N1)pdm09	300 (87.7%)	6,254 (82.1%)
H3N2	42 (12.3%)	1,368 (17.9%)
H3N2v	0 (0.0%)	0 (0.0%)
Subtyping not performed	239 (41.1%)	1,930 (20.2%)
Influenza B	70 (10.8%)	2,087 (17.9%)
Lineage testing performed	43 (61.4%)	1,692 (81.1%)
Yamagata lineage	0 (0.0%)	0 (0.0%)
Victoria lineage	43 (100%)	1,692 (100%)
Lineage not performed	27 (38.6%)	395 (18.9%)

View Chart Data | View Full Screen

Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data Influenza Virus Characterization

CDC performs genetic and antigenic characterization of U.S. viruses submitted from state and local public health laboratories according to the Right Size Roadmap submission guidance. These data are used to compare how similar the currently circulating influenza viruses are to the reference viruses representing viruses contained in the current influenza vaccines. The data are also used to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. CDC also tests susceptibility of circulating influenza viruses to antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.

CDC has genetically characterized 776 influenza viruses collected since October 1, 2023.

A/H1	394
		6B.1A.5a	394 (100%)	2a	118 (29.9%)
				2a.1	276 (70.1%)
A/H3	149
		3C.2a1b.2a	149 (100%)	2a.1b	1 (0.7%)
				2a.3a	1 (0.7%)
				2a.3a.1	146 (98.0%)
				2b	1 (0.7%)
B/Victoria	233
		V1A	233 (100%)	3a.2	233 (100%)
B/Yamagata	0
		Y3	0	Y3	0 (0%)

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) (H1N1pdm09, H3N2, B/Victoria, and B/Yamagata viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2023-2024 Northern Hemisphere recommended cell or recombinant-based vaccine viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to the reference (vaccine) virus. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than or equal to 8-fold. Viruses selected for antigenic characterization are a subset representing the genetic changes in the surface proteins seen in genetically characterized viruses.

Influenza A Viruses

A (H1N1)pdm09: Fifty A(H1N1)pdm09 viruses were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown A/Wisconsin/67/2022-like reference viruses representing the A(H1N1)pdm09 component for the cell- and recombinant-based influenza vaccines.
A (H3N2): Sixty-eight A(H3N2) viruses were antigenically characterized by HI or HINT, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer in HI or reacting at titers that were less than or equal to 8-fold of the homologous virus in HINT) by ferret antisera to cell-grown A/Darwin/6/2021-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.

Influenza B Viruses

B/Victoria: Nineteen influenza B/Victoria-lineage virus were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines.
B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.

Assessment of Virus Susceptibility to Antiviral Medications

CDC assesses susceptibility of influenza viruses to the antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir using next generation sequence analysis supplemented by laboratory assays. Information about antiviral susceptibility test methods can be found at U.S. Influenza Surveillance: Purpose and Methods | CDC.

Viruses collected in the U.S. since October 01, 2023, were tested for antiviral susceptibility as follows:

Neuraminidase Inhibitors	Oseltamivir	Viruses Tested	780	394	147	239
Reduced Inhibition	1 (0.1%)	1 (0.3%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Peramivir	Viruses Tested	780	394	147	239
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Zanamivir	Viruses Tested	780	394	147	239
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
PA Cap-Dependent Endonuclease Inhibitor	Baloxavir	Viruses Tested	762	389	145	228
Decreased Susceptibility	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)

One A(H1N1)pdm09 virus had NA-S247N and NA-I223V amino acid substitutions and showed reduced inhibition by oseltamivir.

High levels of resistance to the adamantanes (amantadine and rimantadine) persist among influenza A(H1N1)pdm09 and influenza A(H3N2) viruses (the adamantanes are not effective against influenza B viruses). Therefore, use of these antivirals for treatment and prevention of influenza A virus infection is not recommended and data from adamantane resistance testing are not presented. Outpatient Respiratory Illness Surveillance

The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient visits for respiratory illness referred to as influenza-like illness [ILI (fever plus cough or sore throat)], not laboratory-confirmed influenza, and will therefore capture respiratory illness visits due to infection with pathogens that can present with similar symptoms, including influenza viruses, SARS-CoV-2, and RSV. It is important to evaluate syndromic surveillance data, including that from ILINet, in the context of other sources of surveillance data to obtain a more complete and accurate picture of influenza, SARS-CoV-2, and other respiratory virus activity. CDC is providing integrated information about COVID-19, influenza, and RSV activity on a website that is updated weekly. Information about other respiratory virus activity can be found on CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS) website. Outpatient Respiratory Illness Visits

Nationwide during Week 52, 6.9% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This has increased compared to Week 51 and has remained above the national baseline of 2.9% since Week 44. All regions are above their region-specific baselines this week. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

Outpatient Respiratory Illness Visits by Age Group

About 70% of ILINet participants provide both the number of patient visits for respiratory illness and the total number of patient visits for the week broken out by age group. Data from this subset of providers are used to calculate the percentages of patient visits for respiratory illness by age group.

The percentage of visits for respiratory illness reported in ILINet has increased (change of > 0.1 percentage points) in the 0-4 years, 25-49 years, 50-64 years, and 65+ years age groups, and decreased slightly in the 5-24 years age group in Week 52 compared to Week 51.

Outpatient Respiratory Illness Activity Map

Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).

Week 52 (Week ending Dec. 30, 2023)	Week 51 (Week ending Dec. 23, 2023)	Week 52 (Week ending Dec. 30, 2023)	Week 51 (Week ending Dec. 23, 2023)
Very High	22	16	94	77
High	17	18	171	168
Moderate	6	7	143	98
Low	6	6	126	148
Minimal	2	8	149	208
Insufficient Data	2	0	246	230

*Data collected in ILINet may disproportionally represent certain populations within a jurisdiction or CBSA, and therefore, may not accurately depict the full picture of influenza activity for the entire jurisdiction or CBSA. Differences in the data presented here by CDC and independently by some health departments likely represent differing levels of data completeness with data presented by the health department likely being the more complete.

Additional information about medically attended visits for ILI for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or ILI Activity Map Hospitalization Surveillance

FluSurv-NET

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in select counties in 14 states and represents approximately 9% of the U.S. population. FluSurv-NET hospitalization data are preliminary. As data are received each week, prior case counts and rates are updated accordingly.

A total of 6,829 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2023, and December 30, 2023; 6,034 (88.4%) were associated with influenza A virus, 728 (10.7%) with influenza B virus, 15 (0.2%) with influenza A virus and influenza B virus co-infection, and 52 (0.8%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 951 (78.7%) were A(H1N1)pdm09 and 258 (21.3%) were A(H3N2).

The weekly hospitalization rate observed in Week 52 was 3.9 per 100,000 population. The overall cumulative hospitalization rate was 22.3 per 100,000 population. This cumulative hospitalization rate is the second highest cumulative in-season hospitalization rate observed in Week 52, following the 2022-2023 season (48.6). Cumulative in-season hospitalization rates observed in Week 52 from 2010-2011 through 2021-2022 ranged from 0.3 to 13.8.

When examining rates by age, the highest cumulative hospitalization rate per 100,000 population was among adults aged 65 years and older (58.2), followed by adults aged 50-64 years (27.6) and children aged 0-4 years (23.7). When examining unadjusted rates by race and ethnicity, the highest rate of hospitalization per 100,000 population was among non-Hispanic Black persons (40.0), followed by non-Hispanic American Indian or Alaska Native persons (22.5), Hispanic persons (17.0), non-Hispanic White persons (16.2), and non-Hispanic Asian/Pacific Islander persons (13.0).

View Full Screen

In this figure, cumulative rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 season, rates for recent hospitals admissions are subject to reporting delays. As hospitalization data are reviewed each week, prior case counts and rates are updated accordingly.

View Full Screen

In this figure, weekly rates for all seasons prior to the 2023-24 season reflect end-of-season rates. For the 2023-24 season, rates for recent hospital admissions are subject to reporting delays and are shown as a dashed line for the current season. As hospitalization data are received each week, prior case counts and rates are updated accordingly.

Additional FluSurv-NET hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods |FluView Interactive: Rates by Age, Sex, and Race/Ethnicity or Data on Patient Characteristics | RESP-NET Interactive National Healthcare Safety Network (NHSN) Hospitalization Surveillance

Hospitals report to NHSN the weekly number of patients admitted with laboratory-confirmed influenza. During Week 52, 20,066 patients with laboratory-confirmed influenza were admitted to a hospital. The number of patients admitted to a hospital with laboratory-confirmed influenza for Week 52 increased compared to Week 51 (change of >5%) nationally and in 9 of the 10 HHS regions (1-8 and 10). The number of hospital admissions remained stable this week compared to last in Region 9.

View Chart Data | View Full Screen

Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on January 5, 2023, 0.9% of the deaths that occurred during the week ending December 30, 2023 (Week 52), were due to influenza. This percentage increased (≥ 0.1 percentage point change) compared to Week 51. The data presented are preliminary and may change as more data are received and processed.

The percentages of deaths due to pneumonia and influenza (P&I) and due to pneumonia, influenza, or COVID-19 (PIC) will no longer be displayed in FluView but are available in FluView Interactive.

Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Influenza-Associated Pediatric Mortality

Seven influenza-associated pediatric deaths occurring during the 2023-2024 season were reported to CDC during Week 52. The deaths occurred during Weeks 50, 51, and 52 of 2023 (the weeks ending December 16, December 23, and December 30, respectively). Five deaths were associated with influenza A viruses. Three of the influenza A viruses had subtyping performed; all three were A(H1N1) viruses. Two deaths were associated with influenza B viruses with no lineage determined.

A total of 27 influenza-associated pediatric deaths occurring during the 2023-2024 season have been reported to CDC.

Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Trend Indicators

Increasing:
Decreasing:
Stable: Indicators Status by System

Clinical Labs: Up or down arrows indicate a change of greater than or equal to 0.5 percentage points in the percent of specimens positive for influenza compared to the previous week.
Outpatient Respiratory Illness (ILINet): Up or down arrows indicate a change of greater than 0.1 percentage points in the percent of visits due to respiratory illness (ILI) compared to the previous week.
NHSN Hospitalizations: Up or down arrows indicate change of greater than or equal to 5% of the number of patients admitted with laboratory-confirmed influenza compared to the previous week.
NCHS Mortality: Up or down arrows indicate change of greater than 0.1 percentage points of the percent of deaths due to influenza compared to the previous week. Reference Footnotes

¹U.S. Influenza Surveillance: Purpose and Methods (2023 Oct). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/weekly/overview.htm#ILINet.

²Grohskopf LA, Blanton LH, Ferdinands JM, Chung JR, Broder KR, Talbot HK. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2023–24 Influenza Season. MMWR Recomm Rep 2023;72(No. RR-2):1–25. DOI: http://dx.doi.org/10.15585/mmwr.rr7202a1

³Influenza Antiviral Medications: Summary for Clinicians (2023 Sept). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/professional...clinicians.htm.

Additional National and International Influenza Surveillance Information

FluView Interactive: FluView includes enhanced web-based interactive applications that can provide dynamic visuals of the influenza data collected and analyzed by CDC. These FluView Interactive applications allow people to create customized, visual interpretations of influenza data, as well as make comparisons across flu seasons, regions, age groups and a variety of other demographics.

National Institute for Occupational Safety and Health: Monthly surveillance data on the prevalence of health-related workplace absenteeism among full-time workers in the United States are available from NIOSH.

U.S. State and local influenza surveillance: Select a jurisdiction below to access the latest local influenza information.
https://www.cdc.gov/flu/weekly/index.htm

**Lance** · January 12, 2024, 04:09 PM

Weekly U.S. Influenza Surveillance Report

Print
Updated January 12, 2024

Key Updates for Week 1, ending January 6, 2024

Seasonal influenza activity remains elevated in most parts of the country. Viruses

Clinical Lab 14.0%

(Trend )

positive for influenza
this week

Public Health Lab
The most frequently reported influenza viruses this week were influenza A(H1N1)pdm09.

Virus Characterization
Genetic and antigenic characterization and antiviral susceptibility are summarized in this report. Illness

Outpatient Respiratory Illness 5.7%

(Trend )

of visits to a health care provider this week were for respiratory illness
(above baseline).

Outpatient Respiratory Illness: Activity Map
This week 7 jurisdictions experienced moderate activity and 37 jurisdictions experienced high or very high activity.

FluSurv-NET 31.7 per 100,000

cumulative hospitalization rate.

NHSN Hospitalizations 18,526

(Trend )

patients admitted to hospitals with influenza this week.

NCHS Mortality 1.3%

(Trend )

of deaths attributed to influenza this week.

Pediatric Deaths 13

influenza-associated deaths were reported
this week for a total of 40 deaths this season.

All data are preliminary and may change as more reports are received.

Directional arrows indicate changes between the current week and the previous week. Additional information on the arrows can be found at the bottom of this page.

A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.

Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.

Key Points

Seasonal influenza activity remains elevated in most parts of the country.
After several weeks of increases in key flu indicators a single week of decrease has been noted. CDC will continue to monitor for a second period of increased influenza activity that often occurs after the winter holidays.
Outpatient respiratory illness has been above baseline¹nationally since November and is above baseline in all 10 HHS Regions.
The number of weekly flu hospital admissions decreased slightly.
During Week 1, of the 1,036 viruses reported by public health laboratories, 830 (80.1%) were influenza A and 206 (19.9%) were influenza B. Of the 511 influenza A viruses subtyped during Week 1, 407 (79.6%) were influenza A(H1N1) and 104 (20.4%) were A(H3N2).
Thirteen influenza-associated pediatric deaths were reported during Week 1, bringing the 2023-2024 season total to 40 pediatric deaths.
CDC estimates that there have been at least 14 million illnesses, 150,000 hospitalizations, and 9,400 deaths from flu so far this season.
CDC recommends that everyone 6 months and older get an annual flu vaccine as long as influenza viruses are spreading.²Vaccination now can still provide benefit this season.
There also are prescription flu antiviral drugs that can treat flu illness; those should be started as early as possible and are especially important for higher risk patients.³
Flu viruses are among several viruses contributing to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, flu, and RSV activity on a weekly basis.

U.S. Virologic Surveillance

Nationally, the percentage of respiratory specimens testing positive for influenza in clinical laboratories decreased (change of ≥0.5 percentage points) compared to the previous week. The regions with the highest percent positivity were regions 6 (22.7%), 8 (22.4%), 3 (18.8%), and 4 (15.0%). Since Week 40, influenza A(H1N1)pdm09 has been the predominant virus circulating in all regions. However, the distribution of circulating viruses varies by region. For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent receipt of live attenuated influenza vaccine (LAIV) or found upon further testing to be a vaccine virus are not included, as they are not circulating influenza viruses. Clinical Laboratories

The results of tests performed by clinical laboratories nationwide are summarized below. Data from clinical laboratories (the percentage of specimens tested that are positive for influenza virus) are used to monitor whether influenza activity is increasing or decreasing.

No. of specimens tested	107,334	1,412,848
No. of positive specimens (%)	15,018 (14.0%)	123,398 (8.7%)
*Positive specimens by type*
Influenza A	11,942 (79.5%)	98,983 (80.2%)
Influenza B	3,076 (20.5%)	24,415 (19.8%)

View Chart Data | View Full Screen Public Health Laboratories

The results of tests performed by public health laboratories nationwide are summarized below. Data from public health laboratories are used to monitor the proportion of circulating influenza viruses that belong to each influenza subtype/lineage.

No. of specimens tested	3,028	50,191
No. of positive specimens	1,036	14,255
*Positive specimens by type/subtype*
Influenza A	830 (80.1%)	11,743 (82.4%)
Subtyping Performed	511 (61.6%)	9,392 (80.0%)
(H1N1)pdm09	407 (79.6%)	7,638 (81.3%)
H3N2	104 (20.4%)	1,754 (18.7%)
H3N2v	0 (0.0%)	0 (0.0%)
Subtyping not performed	319 (38.4%)	2,351 (20.0%)
Influenza B	206 (19.9%)	2,512 (17.6%)
Lineage testing performed	138 (67.0%)	2,028 (80.7%)
Yamagata lineage	0 (0.0%)	0 (0.0%)
Victoria lineage	138 (100%)	2,028 (100%)
Lineage not performed	68 (33.0%)	484 (19.3%)

View Chart Data | View Full Screen

Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data Influenza Virus Characterization

CDC performs genetic and antigenic characterization of U.S. viruses submitted from state and local public health laboratories according to the Right Size Roadmap submission guidance. These data are used to compare how similar the currently circulating influenza viruses are to the reference viruses representing viruses contained in the current influenza vaccines. The data are also used to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. CDC also tests susceptibility of circulating influenza viruses to antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.

CDC has genetically characterized 904 influenza viruses collected since October 1, 2023.

A/H1	455
		6B.1A.5a	455 (100%)	2a	133 (29.2%)
				2a.1	322 (70.8%)
A/H3	202
		3C.2a1b.2a	202 (100%)	2a.1b	1 (0.5%)
				2a.3a	1 (0.5%)
				2a.3a.1	199 (98.5%)
				2b	1 (0.5%)
B/Victoria	247
		V1A	247 (100%)	3a.2	247 (100%)
B/Yamagata	0
		Y3	0	Y3	0 (0%)

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) (H1N1pdm09, H3N2, B/Victoria, and B/Yamagata viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2023-2024 Northern Hemisphere recommended cell- or recombinant-based vaccine viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to the reference (vaccine) virus. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than or equal to 8-fold. Viruses selected for antigenic characterization are a subset representing the genetic changes in the surface proteins seen in genetically characterized viruses.

Influenza A Viruses

A (H1N1)pdm09: Eighty-three A(H1N1)pdm09 viruses were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown A/Wisconsin/67/2022-like reference viruses representing the A(H1N1)pdm09 component for the cell- and recombinant-based influenza vaccines.
A (H3N2): Sixty-eight A(H3N2) viruses were antigenically characterized by HI or HINT, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer in HI or reacting at titers that were less than or equal to 8-fold of the homologous virus in HINT) by ferret antisera to cell-grown A/Darwin/6/2021-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.

Influenza B Viruses

B/Victoria: Forty-three influenza B/Victoria-lineage virus were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines.
B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.

Assessment of Virus Susceptibility to Antiviral Medications

CDC assesses susceptibility of influenza viruses to the antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir using next generation sequence analysis supplemented by laboratory assays. Information about antiviral susceptibility test methods can be found at U.S. Influenza Surveillance: Purpose and Methods | CDC.

Viruses collected in the U.S. since October 01, 2023, were tested for antiviral susceptibility as follows:

Neuraminidase Inhibitors	Oseltamivir	Viruses Tested	893	422	209	262
Reduced Inhibition	1 (0.1%)	1 (0.2%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	1 (0.1%)	1 (0.2%)	0 (0.0%)	0 (0.0%)
Peramivir	Viruses Tested	893	422	209	262
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	1 (0.1%)	1 (0.2%)	0 (0.0%)	0 (0.0%)
Zanamivir	Viruses Tested	893	422	209	262
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
PA Cap-Dependent Endonuclease Inhibitor	Baloxavir	Viruses Tested	869	417	200	252
Decreased Susceptibility	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)

One A(H1N1)pdm09 virus had NA-S247N and NA-I223V amino acid substitutions and showed reduced inhibition by oseltamivir. Another A(H1N1)pdm09 virus had NA-H275Y amino acid substitution and showed highly reduced inhibition by oseltamivir and peramivir.

High levels of resistance to the adamantanes (amantadine and rimantadine) persist among influenza A(H1N1)pdm09 and influenza A(H3N2) viruses (the adamantanes are not effective against influenza B viruses). Therefore, use of these antivirals for treatment and prevention of influenza A virus infection is not recommended, and data from adamantane resistance testing are not presented. Outpatient Respiratory Illness Surveillance

The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient visits for respiratory illness referred to as influenza-like illness [ILI (fever plus cough or sore throat)], not laboratory-confirmed influenza, and will therefore capture respiratory illness visits due to infection with pathogens that can present with similar symptoms, including influenza viruses, SARS-CoV-2, and RSV. It is important to evaluate syndromic surveillance data, including that from ILINet, in the context of other sources of surveillance data to obtain a more complete and accurate picture of influenza, SARS-CoV-2, and other respiratory virus activity. CDC is providing integrated information about COVID-19, influenza, and RSV activity on a website that is updated weekly. Information about other respiratory virus activity can be found on CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS) website. Outpatient Respiratory Illness Visits

Nationwide during Week 1, 5.7% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This has decreased compared to Week 52 and is above the national baseline of 2.9%. All regions have decreased compared to Week 52 and remain above their region-specific baselines this week. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

Outpatient Respiratory Illness Visits by Age Group

About 70% of ILINet participants provide both the number of patient visits for respiratory illness and the total number of patient visits for the week broken out by age group. Data from this subset of providers are used to calculate the percentages of patient visits for respiratory illness by age group.

The percentage of visits for respiratory illness reported in ILINet decreased (change of > 0.1 percentage points) in all age groups during Week 1 compared to Week 52.

Outpatient Respiratory Illness Activity Map

Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).

Week 1 (Week ending Jan. 6, 2023)	Week 52 (Week ending Dec. 30, 2023)	Week 1 (Week ending Jan. 6, 2023)	Week 52 (Week ending Dec. 30, 2023)
Very High	14	22	51	93
High	23	19	167	177
Moderate	7	8	121	140
Low	8	4	158	132
Minimal	3	2	198	151
Insufficient Data	0	0	234	236

*Data collected in ILINet may disproportionally represent certain populations within a jurisdiction or CBSA, and therefore, may not accurately depict the full picture of influenza activity for the entire jurisdiction or CBSA. Differences in the data presented here by CDC and independently by some health departments likely represent differing levels of data completeness with data presented by the health department likely being the more complete.

Additional information about medically attended visits for ILI for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or ILI Activity Map Hospitalization Surveillance

FluSurv-NET

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in select counties in 14 states and represents approximately 9% of the U.S. population. FluSurv-NET hospitalization data are preliminary. As data are received each week, prior case counts and rates are updated accordingly.

A total of 9,676 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2023, and January 6, 2024; 8,555 (88.4%) were associated with influenza A virus, 1,019 (10.5%) with influenza B virus, 19 (0.2%) with influenza A virus and influenza B virus co-infection, and 83 (0.9%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 1,296 (78.4%) were A(H1N1)pdm09 and 358 (21.6%) were A(H3N2).

The weekly hospitalization rate observed in Week 1 was 4.5 per 100,000 population. The weekly hospitalization rate observed during Week 52 is the fourth highest peak weekly rate observed during all seasons going back to 2010-2011, following the 2014-2015, 2017-2018, and 2022-2023 seasons. The overall cumulative hospitalization rate was 31.7 per 100,000 population. This cumulative hospitalization rate is the third highest cumulative hospitalization rate when compared against previous end-of-season rates for Week 1, while it is the second highest cumulative in-season hospitalization rate observed in Week 1, following the 2022-2023 season (54.4). Cumulative in-season hospitalization rates observed in Week 1 from 2010-2011 through 2021-2022 ranged from 0.4 to 30.0.

When examining rates by age, the highest cumulative hospitalization rate per 100,000 population was among adults aged 65 years and older (85.3), followed by adults aged 50-64 years (39.2) and children aged 0-4 years (32.4). When examining age-adjusted rates by race and ethnicity, the highest rate of hospitalization per 100,000 population was among non-Hispanic Black persons (60.2), followed by non-Hispanic American Indian or Alaska Native persons (32.4), Hispanic persons (30.5), non-Hispanic White persons (21.9), and non-Hispanic Asian/Pacific Islander persons (18.8).

View Full Screen

In this figure, cumulative rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 season, rates for recent hospitals admissions are subject to reporting delays. As hospitalization data are reviewed each week, prior case counts and rates are updated accordingly.

View Full Screen

In this figure, weekly rates for all seasons prior to the 2023-24 season reflect end-of-season rates. For the 2023-24 season, rates for recent hospital admissions are subject to reporting delays and are shown as a dashed line for the current season. As hospitalization data are received each week, prior case counts and rates are updated accordingly.

Additional FluSurv-NET hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods |FluView Interactive: Rates by Age, Sex, and Race/Ethnicity or Data on Patient Characteristics | RESP-NET Interactive National Healthcare Safety Network (NHSN) Hospitalization Surveillance

Hospitals report to NHSN the weekly number of patients admitted with laboratory-confirmed influenza. During Week 1, 18,526 patients with laboratory-confirmed influenza were admitted to a hospital. The number of patients admitted to a hospital with laboratory-confirmed influenza for Week 1 decreased compared to Week 52 (change of >5%) nationally and in 7 of the 10 HHS regions (2-5 and 7-9). The number of hospital admissions increased in Region 1 and remained stable this week compared to last in regions 6 and 10.

View Chart Data | View Full Screen

Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on January 11, 2024, 1.3% of the deaths that occurred during the week ending January 6, 2024 (Week 1), were due to influenza. This percentage increased (≥ 0.1 percentage point change) compared to Week 52. The data presented are preliminary and may change as more data are received and processed.

The percentages of deaths due to pneumonia and influenza (P&I) and due to pneumonia, influenza, or COVID-19 (PIC) will no longer be displayed in FluView but are available in FluView Interactive.

Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Influenza-Associated Pediatric Mortality

Thirteen influenza-associated pediatric deaths occurring during the 2023-2024 season were reported to CDC during Week 1. The deaths occurred between Week 49 of 2023 (the week ending December 9, 2023) and Week 1 of 2024 (the week ending January 6, 2024). Six deaths were associated with influenza A viruses. Two of the influenza A viruses had subtyping performed; one was an A(H1N1) virus and one was an A(H3N2) virus. Seven deaths were associated with influenza B viruses. Three of the influenza B viruses had lineage determined and all were B/Victoria viruses.

A total of 40 influenza-associated pediatric deaths occurring during the 2023-2024 season have been reported to CDC.

Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Trend Indicators

Increasing:
Decreasing:
Stable: Indicators Status by System

Clinical Labs: Up or down arrows indicate a change of greater than or equal to 0.5 percentage points in the percent of specimens positive for influenza compared to the previous week.
Outpatient Respiratory Illness (ILINet): Up or down arrows indicate a change of greater than 0.1 percentage points in the percent of visits due to respiratory illness (ILI) compared to the previous week.
NHSN Hospitalizations: Up or down arrows indicate change of greater than or equal to 5% of the number of patients admitted with laboratory-confirmed influenza compared to the previous week.
NCHS Mortality: Up or down arrows indicate change of greater than 0.1 percentage points of the percent of deaths due to influenza compared to the previous week. Reference Footnotes

¹U.S. Influenza Surveillance: Purpose and Methods (2023 Oct). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/weekly/overview.htm#ILINet.

²Grohskopf LA, Blanton LH, Ferdinands JM, Chung JR, Broder KR, Talbot HK. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2023–24 Influenza Season. MMWR Recomm Rep 2023;72(No. RR-2):1–25. DOI: http://dx.doi.org/10.15585/mmwr.rr7202a1

³Influenza Antiviral Medications: Summary for Clinicians (2023 Sept). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/professional...clinicians.htm.

Additional National and International Influenza Surveillance Information

FluView Interactive: FluView includes enhanced web-based interactive applications that can provide dynamic visuals of the influenza data collected and analyzed by CDC. These FluView Interactive applications allow people to create customized, visual interpretations of influenza data, as well as make comparisons across flu seasons, regions, age groups and a variety of other demographics.

National Institute for Occupational Safety and Health: Monthly surveillance data on the prevalence of health-related workplace absenteeism among full-time workers in the United States are available from NIOSH.

U.S. State and local influenza surveillance: Select a jurisdiction below to access the latest local influenza information.
https://www.cdc.gov/flu/weekly/index.htm

**Lance** · January 19, 2024, 06:37 PM

Weekly U.S. Influenza Surveillance Report

Print
Updated January 19, 2024

Key Updates for Week 2, ending January 13, 2024

Seasonal influenza activity remains elevated in most parts of the country. Viruses

Clinical Lab 13.7%

(Trend )

positive for influenza
this week

Public Health Lab
The most frequently reported influenza viruses this week were influenza A(H1N1)pdm09.

Virus Characterization
Genetic and antigenic characterization and antiviral susceptibility are summarized in this report. Illness

Outpatient Respiratory Illness 4.7%

(Trend )

of visits to a health care provider this week were for respiratory illness
(above baseline).

Outpatient Respiratory Illness: Activity Map
This week 14 jurisdictions experienced moderate activity and 25 jurisdictions experienced high or very high activity.

FluSurv-NET 37.3 per 100,000

cumulative hospitalization rate.

NHSN Hospitalizations 14,874 (Trend )

patients admitted to hospitals with influenza this week.

NCHS Mortality 1.2%

(Trend )

of deaths attributed to influenza this week.

Pediatric Deaths 7

influenza-associated deaths were reported
this week for a total of 47 deaths this season.

All data are preliminary and may change as more reports are received.

Directional arrows indicate changes between the current week and the previous week. Additional information on the arrows can be found at the bottom of this page.

A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.

Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.

Key Points

Seasonal influenza activity remains elevated in most parts of the country.
After several weeks of increases in key flu indicators through the end of 2023, two weeks of decreasing or stable trends nationally have been noted. CDC will continue to monitor for a second period of increased influenza activity that often occurs after the winter holidays.
Outpatient respiratory illness has been above baseline¹nationally since November and is above baseline in all 10 HHS regions.
The number of weekly flu hospital admissions has decreased slightly for two consecutive weeks.
During Week 2, of the 727 viruses reported by public health laboratories, 591 (81.3%) were influenza A and 136 (18.7%) were influenza B. Of the 264 influenza A viruses subtyped during Week 2, 193 (73.1%) were influenza A(H1N1) and 71 (26.9%) were A(H3N2).
Seven influenza-associated pediatric deaths were reported during Week 2, bringing the 2023-2024 season total to 47 pediatric deaths.
CDC estimates that there have been at least 16 million illnesses, 180,000 hospitalizations, and 11,000 deaths from flu so far this season.
CDC recommends that everyone 6 months and older get an annual flu vaccine as long as influenza viruses are spreading.²Vaccination now can still provide benefit this season.
There also are prescription flu antiviral drugs that can treat flu illness; those should be started as early as possible and are especially important for higher risk patients.³
Flu viruses are among several viruses contributing to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, flu, and RSV activity on a weekly basis.

U.S. Virologic Surveillance

Nationally, the percentage of respiratory specimens testing positive for influenza in clinical laboratories remained stable (change of ≥0.5 percentage points) compared to the previous week. The regions with the highest percent positivity were regions 6 (23.4%), 8 (22.6%), 4 (14.8%), and 7 (14.8%). Since Week 40, influenza A(H1N1)pdm09 has been the predominant virus circulating in all regions. However, the distribution of circulating viruses varies by region. For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent receipt of live attenuated influenza vaccine (LAIV) or found upon further testing to be a vaccine virus are not included, as they are not circulating influenza viruses. Clinical Laboratories

The results of tests performed by clinical laboratories nationwide are summarized below. Data from clinical laboratories (the percentage of specimens tested that are positive for influenza virus) are used to monitor whether influenza activity is increasing or decreasing.

No. of specimens tested	94,815	1,511,701
No. of positive specimens (%)	13,006 (13.7%)	139,669 (9.2%)
*Positive specimens by type*
Influenza A	10,330 (79.4%)	112,121 (80.3%)
Influenza B	2,676 (20.6%)	27,548 (19.7%)

View Chart Data | View Full Screen Public Health Laboratories

The results of tests performed by public health laboratories nationwide are summarized below. Data from public health laboratories are used to monitor the proportion of circulating influenza viruses that belong to each influenza subtype/lineage.

No. of specimens tested	2,779	54,318
No. of positive specimens	727	15,826
*Positive specimens by type/subtype*
Influenza A	591 (81.3%)	13,054 (82.5%)
Subtyping Performed	264 (44.7%)	10,362 (79.4%)
(H1N1)pdm09	193 (73.1%)	8,373 (80.8%)
H3N2	71 (26.9%)	1,989 (19.2%)
H3N2v	0 (0.0%)	0 (0.0%)
Subtyping not performed	327 (55.3%)	2,692 (20.6%)
Influenza B	136 (18.7%)	2,772 (17.5%)
Lineage testing performed	60 (44.1%)	2,222 (80.2%)
Yamagata lineage	0 (0.0%)	0 (0.0%)
Victoria lineage	60 (100%)	2,222 (100%)
Lineage not performed	76 (55.9%)	550 (19.8%)

View Chart Data | View Full Screen

Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data Influenza Virus Characterization

CDC performs genetic and antigenic characterization of U.S. viruses submitted from state and local public health laboratories according to the Right Size Roadmap submission guidance. These data are used to compare how similar the currently circulating influenza viruses are to the reference viruses representing viruses contained in the current influenza vaccines. The data are also used to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. CDC also tests susceptibility of circulating influenza viruses to antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.

CDC has genetically characterized 1,069 influenza viruses collected since October 1, 2023.

A/H1	522
		6B.1A.5a	522 (100%)	2a	153 (29.3%)
				2a.1	369 (70.7%)
A/H3	256
		3C.2a1b.2a	256 (100%)	2a.1b	1 (0.4%)
				2a.3a	1 (0.4%)
				2a.3a.1	253 (98.8%)
				2b	1 (0.5%)
B/Victoria	291
		V1A	291 (100%)	3a.2	291 (100%)
B/Yamagata	0
		Y3	0	Y3	0 (0%)

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) (H1N1pdm09, H3N2, B/Victoria, and B/Yamagata viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2023-2024 Northern Hemisphere recommended cell or recombinant-based vaccine viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to the reference (vaccine) virus. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than or equal to 8-fold. Viruses selected for antigenic characterization are a subset representing the genetic changes in the surface proteins seen in genetically characterized viruses.

Influenza A Viruses

A (H1N1)pdm09: Eighty-three A(H1N1)pdm09 viruses were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown A/Wisconsin/67/2022-like reference viruses representing the A(H1N1)pdm09 component for the cell- and recombinant-based influenza vaccines.
A (H3N2): Ninety-one A(H3N2) viruses were antigenically characterized by HI or HINT, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer in HI or reacting at titers that were less than or equal to 8-fold of the homologous virus in HINT) by ferret antisera to cell-grown A/Darwin/6/2021-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.

Influenza B Viruses

B/Victoria: Sixty-one influenza B/Victoria-lineage virus were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines.
B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.

Assessment of Virus Susceptibility to Antiviral Medications

CDC assesses susceptibility of influenza viruses to the antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir using next generation sequence analysis supplemented by laboratory assays. Information about antiviral susceptibility test methods can be found at U.S. Influenza Surveillance: Purpose and Methods | CDC.

Viruses collected in the U.S. since October 01, 2023, were tested for antiviral susceptibility as follows:

Neuraminidase Inhibitors	Oseltamivir	Viruses Tested	1075	522	257	296
Reduced Inhibition	1 (0.1%)	1 (0.2%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	1 (0.1%)	1 (0.2%)	0 (0.0%)	0 (0.0%)
Peramivir	Viruses Tested	1075	522	257	296
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	1 (0.1%)	1 (0.2%)	0 (0.0%)	0 (0.0%)
Zanamivir	Viruses Tested	1075	522	257	296
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
PA Cap-Dependent Endonuclease Inhibitor	Baloxavir	Viruses Tested	1063	513	254	296
Decreased Susceptibility	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)

One A(H1N1)pdm09 virus had NA-S247N and NA-I223V amino acid substitutions and showed reduced inhibition by oseltamivir. Another A(H1N1)pdm09 virus had NA-H275Y amino acid substitution and showed highly reduced inhibition by oseltamivir and peramivir.

High levels of resistance to the adamantanes (amantadine and rimantadine) persist among influenza A(H1N1)pdm09 and influenza A(H3N2) viruses (the adamantanes are not effective against influenza B viruses). Therefore, use of these antivirals for treatment and prevention of influenza A virus infection is not recommended and data from adamantane resistance testing are not presented. Outpatient Respiratory Illness Surveillance

The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient visits for respiratory illness referred to as influenza-like illness [ILI (fever plus cough or sore throat)], not laboratory-confirmed influenza and will therefore capture respiratory illness visits due to infection with pathogens that can present with similar symptoms, including influenza viruses, SARS-CoV-2, and RSV. It is important to evaluate syndromic surveillance data, including that from ILINet, in the context of other sources of surveillance data to obtain a more complete and accurate picture of influenza, SARS-CoV-2, and other respiratory virus activity. CDC is providing integrated information about COVID-19, influenza, and RSV activity on a website that is updated weekly. Information about other respiratory virus activity can be found on CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS) website. Outpatient Respiratory Illness Visits

Nationwide during Week 2, 4.7% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This has decreased compared to Week 1 and is above the national baseline of 2.9%. All regions have decreased compared to Week 1 and remain above their region-specific baselines this week. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

Outpatient Respiratory Illness Visits by Age Group

About 70% of ILINet participants provide both the number of patient visits for respiratory illness and the total number of patient visits for the week broken out by age group. Data from this subset of providers are used to calculate the percentages of patient visits for respiratory illness by age group.

The percentage of visits for respiratory illness reported in ILINet decreased (change of > 0.1 percentage points) in all age groups during Week 2 compared to Week 1.

Outpatient Respiratory Illness Activity Map

Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).

Week 2 (Week ending Jan. 13, 2024)	Week 1 (Week ending Jan. 6, 2024)	Week 2 (Week ending Jan. 13, 2024)	Week 1 (Week ending Jan. 13, 2024)
Very High	8	14	21	52
High	17	23	126	167
Moderate	14	6	101	121
Low	10	9	190	160
Minimal	6	3	252	202
Insufficient Data	0	0	239	227

*Data collected in ILINet may disproportionally represent certain populations within a jurisdiction or CBSA, and therefore, may not accurately depict the full picture of influenza activity for the entire jurisdiction or CBSA. Differences in the data presented here by CDC and independently by some health departments likely represent differing levels of data completeness with data presented by the health department likely being the more complete.

Additional information about medically attended visits for ILI for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or ILI Activity Map Hospitalization Surveillance

FluSurv-NET

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in select counties in 14 states and represents approximately 9% of the U.S. population. FluSurv-NET hospitalization data are preliminary. As data are received each week, prior case counts and rates are updated accordingly.

A total of 11,414 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2023, and January 13, 2024; 10,101 (88.5%) were associated with influenza A virus, 1,196 (10.5%) with influenza B virus, 25 (0.2%) with influenza A virus and influenza B virus co-infection, and 92 (0.8%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 1,591 (77.6%) were A(H1N1)pdm09 and 458 (22.4%) were A(H3N2).

The weekly hospitalization rate observed in Week 2 was 3.2 per 100,000 population. The weekly hospitalization rate observed during Week 2 is the fourth highest peak weekly rate observed during all seasons going back to 2010-2011 following the 2014-2015, 2017-2018, and 2022-2023 seasons. The overall cumulative hospitalization rate was 37.3 per 100,000 population. This cumulative hospitalization rate is the fourth highest cumulative hospitalization rate when compared against previous end-of-season rates for Week 2, while it is the second highest cumulative in-season hospitalization rate observed in Week 2, following the 2022-2023 season (56.7). Cumulative in-season hospitalization rates observed in Week 2 from 2010-2011 through 2021-2022 ranged from 0.5 to 36.3.

When examining rates by age, the highest cumulative hospitalization rate per 100,000 population was among adults aged 65 years and older (99.8), followed by adults aged 50-64 years (46.8) and children aged 0-4 years (39.2). When examining age-adjusted rates by race and ethnicity, the highest rate of hospitalization per 100,000 population was among non-Hispanic Black persons (73.0), followed by non-Hispanic American Indian or Alaska Native persons (36.5), Hispanic persons (34.7), non-Hispanic White persons (26.2), and non-Hispanic Asian/Pacific Islander persons (22.1).

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In this figure, cumulative rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 season, rates for recent hospitals admissions are subject to reporting delays. As hospitalization data are reviewed each week, prior case counts and rates are updated accordingly.

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In this figure, weekly rates for all seasons prior to the 2023-24 season reflect end-of-season rates. For the 2023-24 season, rates for recent hospital admissions are subject to reporting delays and are shown as a dashed line for the current season. As hospitalization data are received each week, prior case counts and rates are updated accordingly.

Additional FluSurv-NET hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods |FluView Interactive: Rates by Age, Sex, and Race/Ethnicity or Data on Patient Characteristics | RESP-NET Interactive National Healthcare Safety Network (NHSN) Hospitalization Surveillance

Hospitals report to NHSN the weekly number of patients admitted with laboratory-confirmed influenza. During Week 2, 14,874 patients with laboratory-confirmed influenza were admitted to a hospital. The number of patients admitted to a hospital with laboratory-confirmed influenza for Week 2 decreased compared to Week 1 (change of >5%) nationally and in all 10 HHS regions.

View Chart Data | View Full Screen

Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on January 18, 2024, 1.2% of the deaths that occurred during the week ending January 13, 2024 (Week 2), were due to influenza. This percentage decreased (≥ 0.1 percentage point change) compared to Week 1. The data presented are preliminary and may change as more data are received and processed.

The percentages of deaths due to pneumonia and influenza (P&I) and due to pneumonia, influenza, or COVID-19 (PIC) will no longer be displayed in FluView but are available in FluView Interactive.

Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Influenza-Associated Pediatric Mortality

Seven influenza-associated pediatric deaths occurring during the 2023-2024 season were reported to CDC during Week 2. The deaths occurred between Week 48 of 2023 (the week ending December 2, 2023) and Week 1 of 2024 (the week ending January 6, 2024). Two deaths were associated with influenza A viruses for which no subtyping was performed and five deaths were associated with influenza B viruses with no lineage determined.

A total of 47 influenza-associated pediatric deaths occurring during the 2023-2024 season have been reported to CDC.

Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Trend Indicators

Increasing:
Decreasing:
Stable: Indicators Status by System

Clinical Labs: Up or down arrows indicate a change of greater than or equal to 0.5 percentage points in the percent of specimens positive for influenza compared to the previous week.
Outpatient Respiratory Illness (ILINet): Up or down arrows indicate a change of greater than 0.1 percentage points in the percent of visits due to respiratory illness (ILI) compared to the previous week.
NHSN Hospitalizations: Up or down arrows indicate change of greater than or equal to 5% of the number of patients admitted with laboratory-confirmed influenza compared to the previous week.
NCHS Mortality: Up or down arrows indicate change of greater than 0.1 percentage points of the percent of deaths due to influenza compared to the previous week. Reference Footnotes

¹U.S. Influenza Surveillance: Purpose and Methods (2023 Oct). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/weekly/overview.htm#ILINet.

²Grohskopf LA, Blanton LH, Ferdinands JM, Chung JR, Broder KR, Talbot HK. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2023–24 Influenza Season. MMWR Recomm Rep 2023;72(No. RR-2):1–25. DOI: http://dx.doi.org/10.15585/mmwr.rr7202a1

³Influenza Antiviral Medications: Summary for Clinicians (2023 Sept). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/professional...clinicians.htm.

Additional National and International Influenza Surveillance Information

FluView Interactive: FluView includes enhanced web-based interactive applications that can provide dynamic visuals of the influenza data collected and analyzed by CDC. These FluView Interactive applications allow people to create customized, visual interpretations of influenza data, as well as make comparisons across flu seasons, regions, age groups and a variety of other demographics.

National Institute for Occupational Safety and Health: Monthly surveillance data on the prevalence of health-related workplace absenteeism among full-time workers in the United States are available from NIOSH.

U.S. State and local influenza surveillance: Select a jurisdiction below to access the latest local influenza information.
https://www.cdc.gov/flu/weekly/index.htm

**Lance** · January 26, 2024, 04:39 PM

Weekly U.S. Influenza Surveillance Report

Print
Updated January 26, 2024

Key Updates for Week 3, ending January 20, 2024

Seasonal influenza activity remains elevated in most parts of the country. Viruses

Clinical Lab 14.2%

(Trend )

positive for influenza
this week

Public Health Lab
The most frequently reported influenza viruses this week were influenza A(H1N1)pdm09.

Virus Characterization
Genetic and antigenic characterization and antiviral susceptibility are summarized in this report. Illness

Outpatient Respiratory Illness 4.3%

(Trend )

of visits to a health care provider this week were for respiratory illness
(above baseline).

Outpatient Respiratory Illness: Activity Map
This week 12 jurisdictions experienced moderate activity and 22 jurisdictions experienced high or very high activity.

FluSurv-NET 43.3 per 100,000

cumulative hospitalization rate.

NHSN Hospitalizations 12,382 (Trend )

patients admitted to hospitals with influenza this week.

NCHS Mortality 1.1%

(Trend )

of deaths attributed to influenza this week.

Pediatric Deaths 10

influenza-associated deaths were reported
this week for a total of 57 deaths this season.

All data are preliminary and may change as more reports are received.

Directional arrows indicate changes between the current week and the previous week. Additional information on the arrows can be found at the bottom of this page.

A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.

Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.

Key Points

Seasonal influenza activity remains elevated in most parts of the country.
Key flu indicators have decreased or remained stable nationally for three weeks. CDC will continue to monitor for a second period of increased influenza activity that often occurs after the winter holidays.
Outpatient respiratory illness has been above baseline¹nationally since November and is above baseline in all 10 HHS regions.
The number of weekly flu hospital admissions has decreased for three consecutive weeks.
During Week 3, of the 699 viruses reported by public health laboratories, 581 (83.1%) were influenza A and 118 (16.9%) were influenza B. Of the 370 influenza A viruses subtyped during Week 3, 264 (71.4%) were influenza A(H1N1) and 106 (28.6%) were A(H3N2).
Ten influenza-associated pediatric deaths were reported during Week 3, bringing the 2023-2024 season total to 57 pediatric deaths.
CDC estimates that there have been at least 18 million illnesses, 210,000 hospitalizations, and 13,000 deaths from flu so far this season.
CDC recommends that everyone 6 months and older get an annual flu vaccine as long as influenza viruses are spreading.² Vaccination now can still provide benefit this season.
There also are prescription flu antiviral drugs that can treat flu illness; those should be started as early as possible and are especially important for higher risk patients.³
Flu viruses are among several viruses contributing to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, flu, and RSV activity on a weekly basis.

U.S. Virologic Surveillance

Nationally, the percentage of respiratory specimens testing positive for influenza in clinical laboratories remained stable (change of <0.5 percentage points) compared to the previous week, but trends varied by region. Regions 1, 3, 5, 6, 9, and 10 reported an increase in percent positivity while regions 2, 4, 7, and 8 reported a decrease. The regions with the highest percent positivity were regions 6 (25.1%), 8 (20.0%), 5 (15.7%), and 3 (14.5%). Since Week 40, influenza A(H1N1)pdm09 has been the predominant virus circulating in all regions. However, the distribution of circulating viruses varies by region. For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent receipt of live attenuated influenza vaccine (LAIV) or found upon further testing to be a vaccine virus are not included, as they are not circulating influenza viruses. Clinical Laboratories

The results of tests performed by clinical laboratories nationwide are summarized below. Data from clinical laboratories (the percentage of specimens tested that are positive for influenza virus) are used to monitor whether influenza activity is increasing or decreasing.

No. of specimens tested	85,664	1,649,919
No. of positive specimens (%)	12,194 (14.2%)	156,919 (9.5%)
*Positive specimens by type*
Influenza A	9,509 (78.0%)	125,365 (79.9%)
Influenza B	2,685 (22.0%)	31,554 (20.1%)

View Chart Data | View Full Screen Public Health Laboratories

The results of tests performed by public health laboratories nationwide are summarized below. Data from public health laboratories are used to monitor the proportion of circulating influenza viruses that belong to each influenza subtype/lineage.

No. of specimens tested	2,238	58,291
No. of positive specimens	699	17,481
*Positive specimens by type/subtype*
Influenza A	581 (83.1%)	14,442 (82.6%)
Subtyping Performed	370 (63.7%)	11,623 (80.5%)
(H1N1)pdm09	264 (71.4%)	9,314 (80.1%)
H3N2	106 (28.6%)	2,309 (19.9%)
H3N2v	0 (0.0%)	0 (0.0%)
Subtyping not performed	211 (36.3%)	2,819 (19.5%)
Influenza B	118 (16.9%)	3,039 (17.4%)
Lineage testing performed	70 (59.3%)	2,450 (80.6%)
Yamagata lineage	0 (0.0%)	0 (0.0%)
Victoria lineage	70 (100%)	2,450 (100%)
Lineage not performed	48 (40.7%)	589 (19.4%)

View Chart Data | View Full Screen

Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data Influenza Virus Characterization

CDC performs genetic and antigenic characterization of U.S. viruses submitted from state and local public health laboratories according to the Right Size Roadmap submission guidance. These data are used to compare how similar the currently circulating influenza viruses are to the reference viruses representing viruses contained in the current influenza vaccines. The data are also used to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. CDC also tests susceptibility of circulating influenza viruses to antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.

CDC has genetically characterized 1,144 influenza viruses collected since October 1, 2023.

A/H1	540
		6B.1A.5a	540 (100%)	2a	158 (29.3%)
				2a.1	382 (70.7%)
A/H3	279
		3C.2a1b.2a	279 (100%)	2a.1b	1 (0.4%)
				2a.3a	1 (0.4%)
				2a.3a.1	276 (98.9%)
				2b	1 (0.4%)
B/Victoria	325
		V1A	325 (100%)	3a.2	325 (100%)
B/Yamagata	0
		Y3	0	Y3	0 (0%)

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) (H1N1pdm09, H3N2, B/Victoria, and B/Yamagata viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2023-2024 Northern Hemisphere recommended cell or recombinant-based vaccine viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to the reference (vaccine) virus. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than or equal to 8-fold. Viruses selected for antigenic characterization are a subset representing the genetic changes in the surface proteins seen in genetically characterized viruses.

Influenza A Viruses

A (H1N1)pdm09: 107 A(H1N1)pdm09 viruses were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown A/Wisconsin/67/2022-like reference viruses representing the A(H1N1)pdm09 component for the cell- and recombinant-based influenza vaccines.
A (H3N2): Ninety-one A(H3N2) viruses were antigenically characterized by HI or HINT, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer in HI or reacting at titers that were less than or equal to 8-fold of the homologous virus in HINT) by ferret antisera to cell-grown A/Darwin/6/2021-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.

Influenza B Viruses

B/Victoria: Sixty-one influenza B/Victoria-lineage virus were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines.
B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.

Assessment of Virus Susceptibility to Antiviral Medications

CDC assesses susceptibility of influenza viruses to the antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir using next generation sequence analysis supplemented by laboratory assays. Information about antiviral susceptibility test methods can be found at U.S. Influenza Surveillance: Purpose and Methods | CDC.

Viruses collected in the U.S. since October 01, 2023, were tested for antiviral susceptibility as follows:

Neuraminidase Inhibitors	Oseltamivir	Viruses Tested	1139	539	280	320
Reduced Inhibition	1 (0.1%)	1 (0.2%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	1 (0.1%)	1 (0.2%)	0 (0.0%)	0 (0.0%)
Peramivir	Viruses Tested	1139	539	280	320
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	1 (0.1%)	1 (0.2%)	0 (0.0%)	0 (0.0%)
Zanamivir	Viruses Tested	1139	539	280	320
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
PA Cap-Dependent Endonuclease Inhibitor	Baloxavir	Viruses Tested	1126	529	277	320
Decreased Susceptibility	1 (0.1%)	0 (0.0%)	1 (0.4%)	0 (0.0%)

One A(H1N1)pdm09 virus had NA-H275Y amino acid substitution and showed highly reduced inhibition by oseltamivir and peramivir. One (H1N1)pdm09 virus had NA-S247N and NA-I223V amino acid substitutions and showed reduced inhibition by oseltamivir. One A(H3N2) virus had PA-I38T amino acid substitution and showed reduced susceptibility to baloxavir.

High levels of resistance to the adamantanes (amantadine and rimantadine) persist among influenza A(H1N1)pdm09 and influenza A(H3N2) viruses (the adamantanes are not effective against influenza B viruses). Therefore, use of these antivirals for treatment and prevention of influenza A virus infection is not recommended and data from adamantane resistance testing are not presented. Outpatient Respiratory Illness Surveillance

The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient visits for respiratory illness referred to as influenza-like illness [ILI (fever plus cough or sore throat)], not laboratory-confirmed influenza and will therefore capture respiratory illness visits due to infection with pathogens that can present with similar symptoms, including influenza viruses, SARS-CoV-2, and RSV. It is important to evaluate syndromic surveillance data, including that from ILINet, in the context of other sources of surveillance data to obtain a more complete and accurate picture of influenza, SARS-CoV-2, and other respiratory virus activity. CDC is providing integrated information about COVID-19, influenza, and RSV activity on a website that is updated weekly. Information about other respiratory virus activity can be found on CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS) website. Outpatient Respiratory Illness Visits

Nationwide, during Week 3, 4.3% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This is a decrease compared to Week 2, but remains above the national baseline of 2.9%. All regions, with the exception of Region 6, have decreased compared to Week 2; Region 6 has remained stable. All regions remain above their region-specific baselines this week. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

Outpatient Respiratory Illness Visits by Age Group

About 70% of ILINet participants provide both the number of patient visits for respiratory illness and the total number of patient visits for the week broken out by age group. Data from this subset of providers are used to calculate the percentages of patient visits for respiratory illness by age group.

The percentage of visits for respiratory illness reported in ILINet decreased (change of > 0.1 percentage points) in all age groups during Week 3 compared to Week 2.

Outpatient Respiratory Illness Activity Map

Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).

Week 3 (Week ending Jan. 20, 2024)	Week 2 (Week ending Jan. 13, 2024)	Week 3 (Week ending Jan. 20, 2024)	Week 2 (Week ending Jan. 13, 2024)
Very High	5	8	15	22
High	17	17	83	125
Moderate	12	14	110	101
Low	13	11	197	189
Minimal	8	5	277	259
Insufficient Data	0	0	247	233

*Data collected in ILINet may disproportionally represent certain populations within a jurisdiction or CBSA, and therefore, may not accurately depict the full picture of influenza activity for the entire jurisdiction or CBSA. Differences in the data presented here by CDC and independently by some health departments likely represent differing levels of data completeness with data presented by the health department likely being the more complete.

Additional information about medically attended visits for ILI for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or ILI Activity Map Hospitalization Surveillance

FluSurv-NET

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in select counties in 14 states and represents approximately 9% of the U.S. population. FluSurv-NET hospitalization data are preliminary. As data are received each week, prior case counts and rates are updated accordingly.

A total of 13,224 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2023, and January 20, 2024. The weekly hospitalization rate observed in Week 3 was 2.9 per 100,000 population. The weekly hospitalization rate observed during Week 52 is the fourth highest weekly rate peak observed during all seasons going back to 2010-2011 following the 2014-2015, 2017-2018, and 2022-2023 seasons. The overall cumulative hospitalization rate was 43.3 per 100,000 population. This cumulative hospitalization rate is the fourth highest cumulative hospitalization rate when compared against previous end-of-season rates for Week 3, while it is the second highest cumulative in-season hospitalization rate observed in Week 3, following the 2022-2023 season (58.1). Cumulative in-season hospitalization rates observed in Week 3 from 2010-2011 through 2021-2022 ranged from 0.5 to 41.9.

When examining rates by age, the highest cumulative hospitalization rate per 100,000 population was among adults aged 65 years and older (115.9), followed by adults aged 50-64 years (54.0) and children aged 0-4 years (46.5). When examining age-adjusted rates by race and ethnicity, the highest rate of hospitalization per 100,000 population was among non-Hispanic Black persons (83.6), followed by Hispanic persons (39.5), non-Hispanic American Indian or Alaska Native persons (39.0), non-Hispanic White persons (30.1), and non-Hispanic Asian/Pacific Islander persons (25.6).

Among 13,224 hospitalizations, 11,691 (88.4%) were associated with influenza A virus, 1,416 (10.7%) with influenza B virus, 29 (0.2%) with influenza A virus and influenza B virus co-infection, and 88 (0.7%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 1,882 (77.8%) were A(H1N1)pdm09 and 536 (22.2%) were A(H3N2). Based on preliminary data, of the 765 laboratory-confirmed influenza-associated hospitalizations with more complete data, 3.8% (95% CI: 2.2%-5.5%) also tested positive for SARS-CoV-2.

Among 590 hospitalized adults with information on underlying medical conditions, 94.5% had at least one reported underlying medical condition, the most commonly reported were hypertension, cardiovascular disease, metabolic disorder, and obesity. Among 734 hospitalized women of childbearing age (15-49 years) with information on pregnancy status, 22.2% were pregnant. Among 301 hospitalized children with information on underlying medical conditions, 68.7% had at least one reported underlying medical condition; the most commonly reported was asthma, followed by neurologic disease and obesity.

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In this figure, cumulative rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 season, rates for recent hospitals admissions are subject to reporting delays. As hospitalization data are reviewed each week, prior case counts and rates are updated accordingly.

View Full Screen

In this figure, weekly rates for all seasons prior to the 2023-24 season reflect end-of-season rates. For the 2023-24 season, rates for recent hospital admissions are subject to reporting delays and are shown as a dashed line for the current season. As hospitalization data are received each week, prior case counts and rates are updated accordingly.

Additional FluSurv-NET hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods |FluView Interactive: Rates by Age, Sex, and Race/Ethnicity or Data on Patient Characteristics | RESP-NET Interactive National Healthcare Safety Network (NHSN) Hospitalization Surveillance

Hospitals report to NHSN the weekly number of patients admitted with laboratory-confirmed influenza. During Week 3, 12,382 patients with laboratory-confirmed influenza were admitted to a hospital. The number of patients admitted to a hospital with laboratory-confirmed influenza for Week 3 decreased compared to Week 2 (change of >5%) nationally and in all 10 HHS regions.

View Chart Data | View Full Screen

Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on January 25, 2024, 1.1% of the deaths that occurred during the week ending January 20, 2024 (Week 3), were due to influenza. This percentage decreased (≥ 0.1 percentage point change) compared to Week 2. The data presented are preliminary and may change as more data are received and processed.

The percentages of deaths due to pneumonia and influenza (P&I) and due to pneumonia, influenza, or COVID-19 (PIC) will no longer be displayed in FluView but are available in FluView Interactive.

Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Influenza-Associated Pediatric Mortality

Ten influenza-associated pediatric deaths occurring during the 2023-2024 season were reported to CDC during Week 3. The deaths occurred between Week 51 of 2023 (the week ending December 23, 2023) and Week 2 of 2024 (the week ending January 13, 2024). Six deaths were associated with influenza A viruses. Three of the influenza A viruses had subtyping performed and all were A(H1N1) viruses. Three deaths were associated with influenza B viruses with no lineage determined. One death was associated with a co-infection with influenza A(H1N1) and influenza B/Victoria viruses.

A total of 57 influenza-associated pediatric deaths occurring during the 2023-2024 season have been reported to CDC.

Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Trend Indicators

Increasing:
Decreasing:
Stable: Indicators Status by System

Clinical Labs: Up or down arrows indicate a change of greater than or equal to 0.5 percentage points in the percent of specimens positive for influenza compared to the previous week.
Outpatient Respiratory Illness (ILINet): Up or down arrows indicate a change of greater than 0.1 percentage points in the percent of visits due to respiratory illness (ILI) compared to the previous week.
NHSN Hospitalizations: Up or down arrows indicate change of greater than or equal to 5% of the number of patients admitted with laboratory-confirmed influenza compared to the previous week.
NCHS Mortality: Up or down arrows indicate change of greater than 0.1 percentage points of the percent of deaths due to influenza compared to the previous week. Reference Footnotes

¹U.S. Influenza Surveillance: Purpose and Methods (2023 Oct). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/weekly/overview.htm#ILINet.

²Grohskopf LA, Blanton LH, Ferdinands JM, Chung JR, Broder KR, Talbot HK. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2023–24 Influenza Season. MMWR Recomm Rep 2023;72(No. RR-2):1–25. DOI: http://dx.doi.org/10.15585/mmwr.rr7202a1

³Influenza Antiviral Medications: Summary for Clinicians (2023 Sept). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/professional...clinicians.htm.

Additional National and International Influenza Surveillance Information

FluView Interactive: FluView includes enhanced web-based interactive applications that can provide dynamic visuals of the influenza data collected and analyzed by CDC. These FluView Interactive applications allow people to create customized, visual interpretations of influenza data, as well as make comparisons across flu seasons, regions, age groups and a variety of other demographics.

National Institute for Occupational Safety and Health: Monthly surveillance data on the prevalence of health-related workplace absenteeism among full-time workers in the United States are available from NIOSH.
https://www.cdc.gov/flu/weekly/index.htm

**Lance** · February 2, 2024, 04:50 PM

Weekly U.S. Influenza Surveillance Report

Print
Updated February 2, 2024

Key Updates for Week 4, ending January 27, 2024

Seasonal influenza activity remains elevated nationally with increases in some parts of the country. Viruses

Clinical Lab 16.2%

(Trend )

positive for influenza
this week

Public Health Lab
The most frequently reported influenza viruses this week were influenza A(H1N1)pdm09.

Virus Characterization
Genetic and antigenic characterization and antiviral susceptibility are summarized in this report. Illness

Outpatient Respiratory Illness 4.3%

(Trend )

of visits to a health care provider this week were for respiratory illness
(above baseline).

Outpatient Respiratory Illness: Activity Map
This week 13 jurisdictions experienced moderate activity and 20 jurisdictions experienced high or very high activity.

FluSurv-NET 47.8 per 100,000

cumulative hospitalization rate.

NHSN Hospitalizations 12,186 (Trend )

patients admitted to hospitals with influenza this week.

NCHS Mortality 0.9%

(Trend )

of deaths attributed to influenza this week.

Pediatric Deaths 8

influenza-associated deaths were reported
this week for a total of 65 deaths this season.

All data are preliminary and may change as more reports are received.

Directional arrows indicate changes between the current week and the previous week. Additional information on the arrows can be found at the bottom of this page.

A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.

Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.

Key Points

Seasonal influenza activity remains elevated nationally with increases in some parts of the country.
Outpatient respiratory illness has been above baseline¹nationally since November and is above baseline in all 10 HHS regions.
The number of weekly flu hospital admissions remains stable compared to last week.
During Week 4, of the 937 viruses reported by public health laboratories, 739 (78.9%) were influenza A and 198 (21.1%) were influenza B. Of the 412 influenza A viruses subtyped during Week 4, 249 (60.4%) were influenza A(H1N1) and 163 (39.6%) were A(H3N2).
Eight influenza-associated pediatric deaths were reported during Week 4, bringing the 2023-2024 season total to 65 pediatric deaths.
CDC estimates that there have been at least 20 million illnesses, 230,000 hospitalizations, and 14,000 deaths from flu so far this season.
CDC recommends that everyone 6 months and older get an annual flu vaccine as long as influenza viruses are spreading.² Vaccination now can still provide benefit this season.
There also are prescription flu antiviral drugs that can treat flu illness; those should be started as early as possible and are especially important for higher risk patients.³
Flu viruses are among several viruses contributing to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, flu, and RSV activity on a weekly basis.

U.S. Virologic Surveillance

Nationally, the percentage of respiratory specimens testing positive for influenza in clinical laboratories increased (change of >0.5 percentage points) compared to the previous week, but trends varied by region. Regions 1, 4, 5, 6, 7, and 10 reported an increase in percent positivity, region 2 remained stable and regions 3, 8, and 9 reported a decrease. The regions with the highest percent positivity were regions 6 (30.2%), 8 (19.0%), and 5 (18.0%). Since Week 40, influenza A(H1N1)pdm09 has been the predominant virus circulating in all regions. However, the distribution of circulating viruses varies by region. For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent receipt of live attenuated influenza vaccine (LAIV) or found upon further testing to be a vaccine virus are not included, as they are not circulating influenza viruses. Clinical Laboratories

The results of tests performed by clinical laboratories nationwide are summarized below. Data from clinical laboratories (the percentage of specimens tested that are positive for influenza virus) are used to monitor whether influenza activity is increasing or decreasing.

No. of specimens tested	93,011	1,741,820
No. of positive specimens (%)	15,032 (16.2%)	174,738 (10.0%)
*Positive specimens by type*
Influenza A	10,989 (73.1%)	138,492 (79.3%)
Influenza B	4,043 (26.9%)	36,236 (20.7%)

View Chart Data | View Full Screen Public Health Laboratories

The results of tests performed by public health laboratories nationwide are summarized below. Data from public health laboratories are used to monitor the proportion of circulating influenza viruses that belong to each influenza subtype/lineage.

No. of specimens tested	2,899	63,718
No. of positive specimens	937	19,426
*Positive specimens by type/subtype*
Influenza A	739 (78.9%)	16,059 (82.7%)
Subtyping Performed	412 (55.8%)	12,874 (80.2%)
(H1N1)pdm09	249 (60.4%)	10,197 (79.2%)
H3N2	163 (39.6%)	2,677 (20.8%)
H3N2v	0 (0.0%)	0 (0.0%)
Subtyping not performed	327 (44.2%)	3,185 (19.8%)
Influenza B	198 (21.1%)	3,367 (17.3%)
Lineage testing performed	119 (60.1%)	2,704 (80.3%)
Yamagata lineage	0 (0.0%)	0 (0.0%)
Victoria lineage	119 (100%)	2,704 (100%)
Lineage not performed	79 (39.9%)	663 (19.7%)

View Chart Data | View Full Screen

Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data Influenza Virus Characterization

CDC performs genetic and antigenic characterization of U.S. viruses submitted from state and local public health laboratories according to the Right Size Roadmap submission guidance. These data are used to compare how similar the currently circulating influenza viruses are to the reference viruses representing viruses contained in the current influenza vaccines. The data are also used to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. CDC also tests susceptibility of circulating influenza viruses to antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.

CDC has genetically characterized 1,144 influenza viruses collected since October 1, 2023.

A/H1	540
		6B.1A.5a	540 (100%)	2a	158 (29.3%)
				2a.1	382 (70.7%)
A/H3	279
		3C.2a1b.2a	279 (100%)	2a.1b	1 (0.4%)
				2a.3a	1 (0.4%)
				2a.3a.1	276 (98.9%)
				2b	1 (0.4%)
B/Victoria	325
		V1A	325 (100%)	3a.2	325 (100%)
B/Yamagata	0
		Y3	0	Y3	0 (0%)

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) (H1N1pdm09, H3N2, B/Victoria, and B/Yamagata viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2023-2024 Northern Hemisphere recommended cell or recombinant-based vaccine viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to the reference (vaccine) virus. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than or equal to 8-fold. Viruses selected for antigenic characterization are a subset representing the genetic changes in the surface proteins seen in genetically characterized viruses.

Influenza A Viruses

A (H1N1)pdm09: 107 A(H1N1)pdm09 viruses were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown A/Wisconsin/67/2022-like reference viruses representing the A(H1N1)pdm09 component for the cell- and recombinant-based influenza vaccines.
A (H3N2): Ninety-one A(H3N2) viruses were antigenically characterized by HI or HINT, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer in HI or reacting at titers that were less than or equal to 8-fold of the homologous virus in HINT) by ferret antisera to cell-grown A/Darwin/6/2021-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.

Influenza B Viruses

B/Victoria: Sixty-one influenza B/Victoria-lineage virus were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines.
B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.

Assessment of Virus Susceptibility to Antiviral Medications

CDC assesses susceptibility of influenza viruses to the antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir using next generation sequence analysis supplemented by laboratory assays. Information about antiviral susceptibility test methods can be found at U.S. Influenza Surveillance: Purpose and Methods | CDC.

Viruses collected in the U.S. since October 01, 2023, were tested for antiviral susceptibility as follows:

Neuraminidase Inhibitors	Oseltamivir	Viruses Tested	1140	539	280	321
Reduced Inhibition	1 (0.1%)	1 (0.2%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	1 (0.1%)	1 (0.2%)	0 (0.0%)	0 (0.0%)
Peramivir	Viruses Tested	1140	539	280	321
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	1 (0.1%)	1 (0.2%)	0 (0.0%)	0 (0.0%)
Zanamivir	Viruses Tested	1140	539	280	321
Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
Highly Reduced Inhibition	0 (0.0%)	0 (0.0%)	0 (0.0%)	0 (0.0%)
PA Cap-Dependent Endonuclease Inhibitor	Baloxavir	Viruses Tested	1126	529	277	320
Decreased Susceptibility	1 (0.1%)	0 (0.0%)	1 (0.4%)	0 (0.0%)

One A(H1N1)pdm09 virus had NA-H275Y amino acid substitution and showed highly reduced inhibition by oseltamivir and peramivir. One (H1N1)pdm09 virus had NA-S247N and NA-I223V amino acid substitutions and showed reduced inhibition by oseltamivir. One A(H3N2) virus had PA-I38T amino acid substitution and showed reduced susceptibility to baloxavir.

High levels of resistance to the adamantanes (amantadine and rimantadine) persist among influenza A(H1N1)pdm09 and influenza A(H3N2) viruses (the adamantanes are not effective against influenza B viruses). Therefore, use of these antivirals for treatment and prevention of influenza A virus infection is not recommended and data from adamantane resistance testing are not presented. Outpatient Respiratory Illness Surveillance

The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient visits for respiratory illness referred to as influenza-like illness [ILI (fever plus cough or sore throat)], not laboratory-confirmed influenza and will therefore capture respiratory illness visits due to infection with pathogens that can present with similar symptoms, including influenza viruses, SARS-CoV-2, and RSV. It is important to evaluate syndromic surveillance data, including that from ILINet, in the context of other sources of surveillance data to obtain a more complete and accurate picture of influenza, SARS-CoV-2, and other respiratory virus activity. CDC is providing integrated information about COVID-19, influenza, and RSV activity on a website that is updated weekly. Information about other respiratory virus activity can be found on CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS) website. Outpatient Respiratory Illness Visits

Nationwide, during Week 4, 4.3% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This has remained stable (change of ≤ 0.1 percentage points) compared to Week 3 and is above the national baseline of 2.9% for the thirteenth consecutive week. The percentage of visits for ILI increased in Region 6, decreased in regions 2, 4, 9 and 10, and remained stable in regions 1, 3, 5, 7, and 8 compared to Week 3. All regions remain above their region-specific baselines this week. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

Outpatient Respiratory Illness Visits by Age Group

About 70% of ILINet participants provide both the number of patient visits for respiratory illness and the total number of patient visits for the week broken out by age group. Data from this subset of providers are used to calculate the percentages of patient visits for respiratory illness by age group.

The percentage of visits for respiratory illness reported in ILINet increased in the 5-24 years age group (change of > 0.1 percentage points), decreased in the 0-4 years, 50-64 years, and 65+ years age groups, and remained stable in the 25-49 years age group during Week 4 compared to Week 3.

Outpatient Respiratory Illness Activity Map

Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).

Week 4 (Week ending Jan. 27, 2024)	Week 3 (Week ending Jan. 20, 2024)	Week 4 (Week ending Jan. 27, 2024)	Week 3 (Week ending Jan. 20, 2024)
Very High	4	5	17	15
High	16	16	92	86
Moderate	13	13	113	111
Low	8	13	179	196
Minimal	14	8	306	284
Insufficient Data	0	0	222	237

*Data collected in ILINet may disproportionally represent certain populations within a jurisdiction or CBSA, and therefore, may not accurately depict the full picture of influenza activity for the entire jurisdiction or CBSA. Differences in the data presented here by CDC and independently by some health departments likely represent differing levels of data completeness with data presented by the health department likely being the more complete.

Additional information about medically attended visits for ILI for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or ILI Activity Map Hospitalization Surveillance

FluSurv-NET

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in select counties in 14 states and represents approximately 9% of the U.S. population. FluSurv-NET hospitalization data are preliminary. As data are received each week, prior case counts and rates are updated accordingly.

A total of 14,596 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2023, and January 27, 2024. The weekly hospitalization rate observed in Week 4 was 3.0 per 100,000 population. The weekly hospitalization rate observed during Week 52 is the third highest peak weekly rate observed during all seasons going back to 2010-2011 following the 2014-2015 and 2017-2018 seasons. The overall cumulative hospitalization rate was 47.8 per 100,000 population. This cumulative hospitalization rate is the fourth highest cumulative hospitalization rate when compared against previous end-of-season rates for Week 4, while it is the second highest cumulative in-season hospitalization rate observed in Week 4, following the 2022-2023 season (58.6). Cumulative in-season hospitalization rates observed in Week 4 from 2010-2011 through 2021-2022 ranged from 0.5 to 43.5.

When examining rates by age, the highest cumulative hospitalization rate per 100,000 population was among adults aged 65 years and older (128.6), followed by adults aged 50-64 years (59.5) and children aged 0-4 years (50.9). When examining age-adjusted rates by race and ethnicity, the highest rate of hospitalization per 100,000 population was among non-Hispanic Black persons (93.3), followed by Hispanic persons (45.5), non-Hispanic American Indian or Alaska Native persons (62.7), non-Hispanic White persons (34.3), and non-Hispanic Asian/Pacific Islander persons (27.6).

Among 14,596 hospitalizations, 12,882 (88.3%) were associated with influenza A virus, 1,577 (10.8%) with influenza B virus, 27 (0.2%) with influenza A virus and influenza B virus co-infection, and 110 (0.8%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 2,121 (76.7%) were A(H1N1)pdm09 and 643 (23.3%) were A(H3N2). Based on preliminary data, of the 782 laboratory-confirmed influenza-associated hospitalizations with more complete data, 4.0% (95% CI: 2.3%-5.7%) also tested positive for SARS-CoV-2.

Among 609 hospitalized adults with information on underlying medical conditions, 94.4% had at least one reported underlying medical condition, the most commonly reported were hypertension, cardiovascular disease, metabolic disorder, and obesity. Among 827 hospitalized women of childbearing age (15-49 years) with information on pregnancy status, 18.8% were pregnant. Among 354 hospitalized children with information on underlying medical conditions, 70.4% had at least one reported underlying medical condition; the most commonly reported was asthma, followed by obesity and neurologic disease.

View Full Screen

In this figure, cumulative rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 season, rates for recent hospitals admissions are subject to reporting delays. As hospitalization data are reviewed each week, prior case counts and rates are updated accordingly.

View Full Screen

In this figure, weekly rates for all seasons prior to the 2023-24 season reflect end-of-season rates. For the 2023-24 season, rates for recent hospital admissions are subject to reporting delays and are shown as a dashed line for the current season. As hospitalization data are received each week, prior case counts and rates are updated accordingly.

Additional FluSurv-NET hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods |FluView Interactive: Rates by Age, Sex, and Race/Ethnicity or Data on Patient Characteristics | RESP-NET Interactive National Healthcare Safety Network (NHSN) Hospitalization Surveillance

Hospitals report to NHSN the weekly number of patients admitted with laboratory-confirmed influenza. During Week 4, 12,186 patients with laboratory-confirmed influenza were admitted to a hospital. The number of patients admitted to a hospital with laboratory-confirmed influenza for week 4 remained stable compared to Week 3 (change of <5%) nationally. The number of hospitalizations increased in regions 6 and 8, remained stable in regions 1, 2, 4, 5, and 7, and decreased in regions 3, 9, and 10 this week compared to Week 3.

View Chart Data | View Full Screen

Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on February 1, 2024, 0.9% of the deaths that occurred during the week ending January 27, 2024 (Week 4), were due to influenza. This percentage decreased (≥ 0.1 percentage point change) compared to Week 3. The data presented are preliminary and may change as more data are received and processed.

Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Influenza-Associated Pediatric Mortality

Eight influenza-associated pediatric deaths occurring during the 2023-2024 season were reported to CDC during Week 4. The deaths occurred between Week 1 (the week ending January 6, 2024) and Week 4 (the week ending January 27, 2024). Four deaths were associated with influenza A viruses. One of the influenza A viruses had subtyping performed and it was an A(H1N1) virus. Four deaths were associated with influenza B viruses with no lineage determined.

A total of 65 influenza-associated pediatric deaths occurring during the 2023-2024 season have been reported to CDC.

Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive Trend Indicators

Increasing:
Decreasing:
Stable: Indicators Status by System

Clinical Labs: Up or down arrows indicate a change of greater than or equal to 0.5 percentage points in the percent of specimens positive for influenza compared to the previous week.
Outpatient Respiratory Illness (ILINet): Up or down arrows indicate a change of greater than 0.1 percentage points in the percent of visits due to respiratory illness (ILI) compared to the previous week.
NHSN Hospitalizations: Up or down arrows indicate change of greater than or equal to 5% of the number of patients admitted with laboratory-confirmed influenza compared to the previous week.
NCHS Mortality: Up or down arrows indicate change of greater than 0.1 percentage points of the percent of deaths due to influenza compared to the previous week. Reference Footnotes

¹U.S. Influenza Surveillance: Purpose and Methods (2023 Oct). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/weekly/overview.htm#ILINet.

²Grohskopf LA, Blanton LH, Ferdinands JM, Chung JR, Broder KR, Talbot HK. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2023–24 Influenza Season. MMWR Recomm Rep 2023;72(No. RR-2):1–25. DOI: http://dx.doi.org/10.15585/mmwr.rr7202a1

³Influenza Antiviral Medications: Summary for Clinicians (2023 Sept). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/professional...clinicians.htm.

Additional National and International Influenza Surveillance Information

FluView Interactive: FluView includes enhanced web-based interactive applications that can provide dynamic visuals of the influenza data collected and analyzed by CDC. These FluView Interactive applications allow people to create customized, visual interpretations of influenza data, as well as make comparisons across flu seasons, regions, age groups and a variety of other demographics.

National Institute for Occupational Safety and Health: Monthly surveillance data on the prevalence of health-related workplace absenteeism among full-time workers in the United States are available from NIOSH.
https://www.cdc.gov/flu/weekly/index.htm

No. of specimens tested	47,296	53,556
No. of positive specimens (%)	530 (1.1%)	492 (0.9%)
*Positive specimens by type*
Influenza A	320 (60.4%)	315 (64.0%)
Influenza B	210 (39.6%)	177 (36.0%)

Announcement

US FluView - Weekly Surveillance Flu report 2023/2024 season - for trend analysis

US FluView - Weekly Surveillance Flu report 2023/2024 season - for trend analysis

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