For previous season please see:
US FluView - Weekly Surveillance Flu report 2023/2024 season - for trend analysis
-----------------------------------------
Weekly US Influenza Surveillance Report: Key Updates for Week 40, ending October 5, 2024
What to know
Seasonal influenza activity remains low nationally.
Summary
Viruses
Clinical Lab 0.7% (Trend
)
positive for influenza
this week. Public Health Lab Influenza A(H1N1)pdm09 and A(H3N2)
viruses were co-circulating this week.
Illness
Outpatient Respiratory Illness
1.9% (Trend
)
of visits to a health care provider this
week were for respiratory illness
(below baseline).
Activity Map
0 moderate jurisdictions 0 high or very high jurisdictions
FluSurv-NET
0 per 100,000
weekly hospitalization rate
NCHS Mortality
0.06% (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.1
Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.
Key Points
• Seasonal influenza activity is low nationally.
• Percent positivity for influenza and the percentage of emergency department visits for influenza are stable at low levels.
• During Week 40, all 56 viruses reported by public health laboratories were influenza A. Of the 43 influenza A viruses subtyped during Week 40, 24 (55.8%) were influenza A(H1N1)pdm09 and 19 (44.2%) were A(H3N2).
• Two human infections with influenza A(H5) virus were reported by the California Department of Public Health.
• No influenza-associated pediatric deaths occurring during the 2024-2025 season were reported to CDC during Week 40.
• CDC recommends that everyone ages 6 months and older get an annual influenza (flu) vaccine, ideally by the end of October.[SUP]1[/SUP]
• There 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.2
• Influenza viruses are among several viruses contributing to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, flu, and respiratory syncytial virus (RSV) activity on a weekly basis.
COVID-19, flu, and RSV activity
U.S. virologic surveillance
Nationally and in HHS regions 1 through 9, the percentage of respiratory specimens testing positive for influenza virus in clinical laboratories remained stable (change of <0.5 percentage points) compared to the previous week. In Region 10, the percent positive increased slightly compared to the previous week. Influenza A(H1N1)pdm09 and A(H3N2) viruses are co-circulating; 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.
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.
[SUB]*These data reflect specimens tested, and the number determined to be positive for influenza viruses at the public health labs (specimens tested is not the same as cases). The data do not reflect specimens tested only at CDC and could include more than one specimen tested per person. The guidance for influenza A/H5 virus testing recommends testing both a conjunctival and respiratory swab for people with conjunctivitis which has resulted in more specimens testing positive for influenza A/H5 virus than the number of human H5 cases. For more information on the number of people infected with A/H5 viruses, please visit the [/SUB][SUB]"How CDC is monitoring influenza data among people to better understand the current avian influenza A (H5N1) situation"[/SUB]
[SUP]†[/SUP][SUB]When an influenza virus that normally circulates in swine (but not people) is detected in a person, it is called a "variant" influenza virus. Most human infections with variant influenza viruses occur following exposure to swine, but human-to-human transmission can occur. It is important to note that in most cases, variant influenza viruses have not shown the ability to spread easily and sustainably from person to person.[/SUB]
[SUB]*This graph reflects the number of specimens tested and the number determined to be positive for influenza viruses at the public health lab (specimens tested is not the same as cases). It does not reflect specimens tested only at CDC and could include more than one specimen tested per person. Specimens tested as part of routine influenza surveillance as well as those tested as part of targeted testing for people exposed to influenza A(H5) are included.[/SUB]
Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data
Novel Influenza A Virus Infections
Two human infections with influenza A(H5) viruses were reported to CDC this week by the California Department of Public Health. These cases are associated with an ongoing outbreak of HPAI A(H5N1) virus in dairy cows.
Both individuals are aged >18 years and work at commercial dairy cattle farms where highly pathogenic avian influenza (HPAI) A(H5N1) virus had been detected in cows. These individuals work on different farms and there is no known link or contact between them or between the two cases reported from California last week. These individuals had mild symptoms which they reported to the local health department. Specimens were collected from both persons and were initially tested at the local public health laboratory using the Centers for Disease Control and Prevention (CDC) influenza A(H5) assay before being sent to CDC for further testing. Specimens from both individuals were positive for influenza A(H5) virus using diagnostic rRT-PCR at CDC. Additional analysis including genetic sequencing is underway.
In response to these detections, additional case investigations and surveillance activities are being conducted by public health officials.
Eighteen cases of human infections with influenza A(H5) viruses have been reported in the United States during 2024. Eight of these occurred in individuals working with dairy cows, nine in individuals associated with poultry depopulation and disposal, and one in an individual with an unknown source of exposure. An ongoing outbreak of avian influenza A(H5N1) continues in domestic dairy cows and poultry, and monitoring for additional human cases is ongoing.
Information about avian influenza is available at https://www.cdc.gov/flu/avianflu/index.htm.
Interim recommendations for Prevention, Monitoring, and Public Health Investigations are available at https://www.cdc.gov/bird-flu/prevention/hpai-interim-recommendations.html.
The latest case reports on avian influenza outbreaks in wild birds, commercial poultry, backyard or hobbyist flocks, and mammals in the United States are available from the USDA at https://www.aphis.usda.gov/aphis/ou...e-information/avian/avian-influenza/2022-hpai.
Additional information regarding human infections with novel influenza A viruses:
Surveillance Methods | FluView Interactive
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 the current season's 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 polymerase acidic protein (PA) endonuclease inhibitor baloxavir. The HA clade and subclades were assigned using Nextclade (Nextclade (nextstrain.org).
CDC has genetically characterized 571 influenza viruses collected since May 19, 2024.
CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) assay (H1N1pdm09, H3N2, and B/Victoria viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2024-2025 Northern Hemisphere recommended cell or recombinant-based influenza 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 of the recent genetically characterized viruses and are chosen based on the genetic changes in their surface proteins and may not be proportional to the number of such viruses circulating in the United States. Influenza A Viruses
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.
Viruses collected in the United States since May 19, 2024, were tested for antiviral susceptibility are as follows:
Two A(H1N1)pdm09 viruses had NA-H275Y amino acid substitution conferring 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 virus infection is not recommended and data from adamantane resistance testing are not presented.
Outpatient and Emergency Department Illness Surveillance
Outpatient respiratory illness visits
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.
Nationally, during Week 40, 1.9% 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 week's percentage remained stable (change of ≤ 0.1 percentage points) compared to Week 39 and is below the national baseline of 3.0%. All 10 HHS regions are below their respective baselines. The percentage of visits for ILI remained stable in regions 1, 3, 4, 5, 7, 8, and 9, increased slightly in regions 6 and 10, and decreased slightly in region 2 in Week 40 compared to Week 39. 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. Based on these data, the percentage of visits for respiratory illness decreased for two age groups (0-4 years, 50-64 years) and remained stable (change of ≤ 0.1 percentage point) for three age groups (5-24, 25-49 years, and 65+ years) in Week 40 compared to Week 39.
ILINet Age Rates Week 40 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).
*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 National Syndromic Surveillance System (NSSP)
The percentage of emergency department (ED) visits with a discharge diagnosis of influenza reported in NSSP was 0.2% overall during Week 40. This week's percentage remained stable overall and among all age groups compared to the previous week.
NSSP Age Rates Week 40 Additional information about emergency department visits for flu for current and past seasons:
Surveillance Methods | Emergency Department Visits for COVID-19, flu, and RSV
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.
A total of 15 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1 (the start of FluSurv-NET enrollment for the 2024-2025 season) and October 5, 2024. The weekly hospitalization rate observed in week 40 was 0.0 per 100,000 population.
[SUB]**In this figure, weekly rates for all seasons prior to the 2024-2025 season reflect end-of-season rates. For the 2024-2025 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.[/SUB]
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
Effective November 1, 2024, all acute care and critical access hospitals will be required to report the number of patients admitted with laboratory-confirmed influenza to NHSN. These data will be updated weekly in FluView and on FluView Interactive once sufficient data has been collected. Public datasets, including data reported on a voluntary basis prior to November 1, can be found here: https://data.cdc.gov/Public-Health-...talization-Metrics-by-Ju/aemt-mg7g/about_data.
Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive
Mortality surveillance
Based on NCHS mortality surveillance data available on October 10, 2024, 0.06% of the deaths that occurred during the week ending October 5, 2024 (Week 40), were due to influenza. This percentage remained stable (≤ 0.1 percentage point change) compared to week 39. 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
No influenza-associated pediatric deaths occurring during the 2024-2025 season have been reported to CDC.
Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive
https://www.cdc.gov/fluview/surveillance/2024-week-40.html
US FluView - Weekly Surveillance Flu report 2023/2024 season - for trend analysis
-----------------------------------------
Weekly US Influenza Surveillance Report: Key Updates for Week 40, ending October 5, 2024
What to know
Seasonal influenza activity remains low nationally.
Summary
Viruses
Clinical Lab 0.7% (Trend
positive for influenza
this week. Public Health Lab Influenza A(H1N1)pdm09 and A(H3N2)
viruses were co-circulating this week.
Illness
Outpatient Respiratory Illness
1.9% (Trend
of visits to a health care provider this
week were for respiratory illness
(below baseline).
Activity Map
0 moderate jurisdictions 0 high or very high jurisdictions
FluSurv-NET
0 per 100,000
weekly hospitalization rate
NCHS Mortality
0.06% (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.1
Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.
Key Points
• Seasonal influenza activity is low nationally.
• Percent positivity for influenza and the percentage of emergency department visits for influenza are stable at low levels.
• During Week 40, all 56 viruses reported by public health laboratories were influenza A. Of the 43 influenza A viruses subtyped during Week 40, 24 (55.8%) were influenza A(H1N1)pdm09 and 19 (44.2%) were A(H3N2).
• Two human infections with influenza A(H5) virus were reported by the California Department of Public Health.
• No influenza-associated pediatric deaths occurring during the 2024-2025 season were reported to CDC during Week 40.
• CDC recommends that everyone ages 6 months and older get an annual influenza (flu) vaccine, ideally by the end of October.[SUP]1[/SUP]
• There 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.2
• Influenza viruses are among several viruses contributing to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, flu, and respiratory syncytial virus (RSV) activity on a weekly basis.
COVID-19, flu, and RSV activity
U.S. virologic surveillance
Nationally and in HHS regions 1 through 9, the percentage of respiratory specimens testing positive for influenza virus in clinical laboratories remained stable (change of <0.5 percentage points) compared to the previous week. In Region 10, the percent positive increased slightly compared to the previous week. Influenza A(H1N1)pdm09 and A(H3N2) viruses are co-circulating; 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 | 53,699 | 53,424 |
| No. of positive specimens (%) | 380 (0.7%) | 264 (0.5%) |
| Positive specimens by type | ||
| Influenza A | 333 (87.6%) | 208 (78.8%) |
| Influenza B | 47 (12.4%) | 56 (21.2%) |
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 | 638 | 1,113 |
| No. of positive specimens | 56 | 154 |
| Positive specimens by type/subtype | ||
| Influenza A | 56 (100.0%) | 147 (95.5%) |
| Subtyping Performed | 43 (76.8%) | 135 (91.8%) |
| (H1N1)pdm09 | 24 (55.8%) | 69 (51.1%) |
| H3N2 | 19 (44.2%) | 66 (48.9%) |
| H3N2v[SUP]†[/SUP] | 0 | 0 |
| H5* | 0 | 0 |
| Subtyping not performed | 13 (23.2%) | 12 (8.2%) |
| Influenza B | 0 | 7 (4.5%) |
| Lineage testing performed | 0 | 7 (100.0%) |
| Yamagata lineage | 0 | 0 |
| Victoria lineage | 0 | 7 (100.0%) |
| Lineage testing not performed | 0 | 0 |
[SUP]†[/SUP][SUB]When an influenza virus that normally circulates in swine (but not people) is detected in a person, it is called a "variant" influenza virus. Most human infections with variant influenza viruses occur following exposure to swine, but human-to-human transmission can occur. It is important to note that in most cases, variant influenza viruses have not shown the ability to spread easily and sustainably from person to person.[/SUB]
[SUB]*This graph reflects the number of specimens tested and the number determined to be positive for influenza viruses at the public health lab (specimens tested is not the same as cases). It does not reflect specimens tested only at CDC and could include more than one specimen tested per person. Specimens tested as part of routine influenza surveillance as well as those tested as part of targeted testing for people exposed to influenza A(H5) are included.[/SUB]
Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data
Novel Influenza A Virus Infections
Two human infections with influenza A(H5) viruses were reported to CDC this week by the California Department of Public Health. These cases are associated with an ongoing outbreak of HPAI A(H5N1) virus in dairy cows.
Both individuals are aged >18 years and work at commercial dairy cattle farms where highly pathogenic avian influenza (HPAI) A(H5N1) virus had been detected in cows. These individuals work on different farms and there is no known link or contact between them or between the two cases reported from California last week. These individuals had mild symptoms which they reported to the local health department. Specimens were collected from both persons and were initially tested at the local public health laboratory using the Centers for Disease Control and Prevention (CDC) influenza A(H5) assay before being sent to CDC for further testing. Specimens from both individuals were positive for influenza A(H5) virus using diagnostic rRT-PCR at CDC. Additional analysis including genetic sequencing is underway.
In response to these detections, additional case investigations and surveillance activities are being conducted by public health officials.
Eighteen cases of human infections with influenza A(H5) viruses have been reported in the United States during 2024. Eight of these occurred in individuals working with dairy cows, nine in individuals associated with poultry depopulation and disposal, and one in an individual with an unknown source of exposure. An ongoing outbreak of avian influenza A(H5N1) continues in domestic dairy cows and poultry, and monitoring for additional human cases is ongoing.
Information about avian influenza is available at https://www.cdc.gov/flu/avianflu/index.htm.
Interim recommendations for Prevention, Monitoring, and Public Health Investigations are available at https://www.cdc.gov/bird-flu/prevention/hpai-interim-recommendations.html.
The latest case reports on avian influenza outbreaks in wild birds, commercial poultry, backyard or hobbyist flocks, and mammals in the United States are available from the USDA at https://www.aphis.usda.gov/aphis/ou...e-information/avian/avian-influenza/2022-hpai.
Additional information regarding human infections with novel influenza A viruses:
Surveillance Methods | FluView Interactive
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 the current season's 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 polymerase acidic protein (PA) endonuclease inhibitor baloxavir. The HA clade and subclades were assigned using Nextclade (Nextclade (nextstrain.org).
CDC has genetically characterized 571 influenza viruses collected since May 19, 2024.
| A/H1 | 197 | ||||
| 5a.2a | 93 (47.2%) | C.1 | 1 (0.5%) | ||
| C.1.9 | 92 (46.7%) | ||||
| 5a.2a.1 | 104 (52.8%) | C.1.1 | 2 (1.0%) | ||
| D | 92 (46.7%) | ||||
| D.2 | 1 (0.5%) | ||||
| D.3 | 5 (2.5%) | ||||
| D.4 | 4 (2.0%) | ||||
| A/H3 | 310 | ||||
| 2a.3a | 1 (0.3%) | G.1.3.1 | 1 (0.3%) | ||
| 2a.3a.1 | 309 (99.7%) | J.1 | 10 (3.2%) | ||
| J.2 | 299 (96.5%) | ||||
| B/Victoria | 64 | ||||
| 3a.2 | 64 (100.0%) | C.5 | 1 (1.6%) | ||
| C.5.1 | 51 (79.7%) | ||||
| C.5.6 | 8 (12.5%) | ||||
| C.5.7 | 4 (6.3%) | ||||
| B/Yamagata | 0 | ||||
| Y3 | 0 | Y3 | 0 |
- A (H1N1)pdm09: 54 A(H1N1)pdm09 viruses were antigenically characterized by HI, and 53 (98.1%) 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): 118 A(H3N2) viruses were antigenically characterized by HI or HINT, and 95 (80.5%) 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/Massachusetts/18/2022-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.
- B/Victoria: 24 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.
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.
Viruses collected in the United States since May 19, 2024, were tested for antiviral susceptibility are as follows:
| Neuraminidase Inhibitors | Oseltamivir | Viruses Tested | 559 | 194 | 305 | 60 |
| Reduced Inhibition | 0 | 0 | 0 | 0 | ||
| Highly Reduced Inhibition | 2 (0.4%) | 2 (1.0%) | 0 | 0 | ||
| Peramivir | Viruses Tested | 559 | 194 | 305 | 60 | |
| Reduced Inhibition | 0 | 0 | 0 | 0 | ||
| Highly Reduced Inhibition | 2 (0.4%) | 2 (1.0%) | 0 | 0 | ||
| Zanamivir | Viruses Tested | 559 | 194 | 305 | 60 | |
| Reduced Inhibition | 0 | 0 | 0 | 0 | ||
| Highly Reduced Inhibition | 0 | 0 | 0 | 0 | ||
| PA Cap-Dependent Endonuclease Inhibitor | Baloxavir | Viruses Tested | 545 | 177 | 304 | 64 |
| Decreased Susceptibility | 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 virus infection is not recommended and data from adamantane resistance testing are not presented.
Outpatient and Emergency Department Illness Surveillance
Outpatient respiratory illness visits
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.
Nationally, during Week 40, 1.9% 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 week's percentage remained stable (change of ≤ 0.1 percentage points) compared to Week 39 and is below the national baseline of 3.0%. All 10 HHS regions are below their respective baselines. The percentage of visits for ILI remained stable in regions 1, 3, 4, 5, 7, 8, and 9, increased slightly in regions 6 and 10, and decreased slightly in region 2 in Week 40 compared to Week 39. 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. Based on these data, the percentage of visits for respiratory illness decreased for two age groups (0-4 years, 50-64 years) and remained stable (change of ≤ 0.1 percentage point) for three age groups (5-24, 25-49 years, and 65+ years) in Week 40 compared to Week 39.
Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).
| Week 40 (Week ending Oct. 5, 2024) | Week 39 (Week ending Sep. 28, 2024) | Week 40 (Week ending Oct. 5, 2024) | Week 39 (Week ending Sep. 28, 2024) | |
| Very High | 0 | 0 | 0 | 0 |
| High | 0 | 0 | 0 | 2 |
| Moderate | 0 | 0 | 3 | 2 |
| Low | 0 | 1 | 29 | 29 |
| Minimal | 55 | 54 | 672 | 646 |
| Insufficient Data | 0 | 0 | 225 | 250 |
Surveillance Methods | FluView Interactive: National, Regional, and State Data or ILI Activity Map National Syndromic Surveillance System (NSSP)
The percentage of emergency department (ED) visits with a discharge diagnosis of influenza reported in NSSP was 0.2% overall during Week 40. This week's percentage remained stable overall and among all age groups compared to the previous week.
Surveillance Methods | Emergency Department Visits for COVID-19, flu, and RSV
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.
A total of 15 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1 (the start of FluSurv-NET enrollment for the 2024-2025 season) and October 5, 2024. The weekly hospitalization rate observed in week 40 was 0.0 per 100,000 population.
[SUB]**In this figure, weekly rates for all seasons prior to the 2024-2025 season reflect end-of-season rates. For the 2024-2025 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.[/SUB]
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
Effective November 1, 2024, all acute care and critical access hospitals will be required to report the number of patients admitted with laboratory-confirmed influenza to NHSN. These data will be updated weekly in FluView and on FluView Interactive once sufficient data has been collected. Public datasets, including data reported on a voluntary basis prior to November 1, can be found here: https://data.cdc.gov/Public-Health-...talization-Metrics-by-Ju/aemt-mg7g/about_data.
Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive
Mortality surveillance
Based on NCHS mortality surveillance data available on October 10, 2024, 0.06% of the deaths that occurred during the week ending October 5, 2024 (Week 40), were due to influenza. This percentage remained stable (≤ 0.1 percentage point change) compared to week 39. 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
No influenza-associated pediatric deaths occurring during the 2024-2025 season have been reported to CDC.
Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive
https://www.cdc.gov/fluview/surveillance/2024-week-40.html
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