Weekly US Influenza Surveillance Report: Key Updates for Week 13, ending March 29, 2025
What to know
Seasonal influenza activity remains elevated nationally but has decreased for two consecutive weeks.
Summary
Viruses
Clinical Lab 9.7% (Trend
)
positive for influenza
this week. Public Health Lab Influenza A(H1N1)pdm09, A(H3N2), and B
viruses were co-circulating this week.
Illness
Outpatient Respiratory Illness 3.2% (Trend
)
of visits to a health care provider this
week were for respiratory illness
(above baseline). Activity Map 12 moderate jurisdictions 2 high or very high jurisdictions FluSurv-NET 121.9 per 100,000
cumulative hospitalization rate National Healthcare Safety Network (NHSN) Hospital Respiratory Data 9,364 (Trend
)
patients admitted to hospitals
with influenza this week. NCHS Mortality 0.9% (Trend
)
of deaths attributed to influenza this week. Pediatric Deaths 9 influenza-associated deaths
were reported this week for
a total of 168 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.[SUP]1[/SUP]
Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.
Key Points
COVID-19, flu, and RSV activity
U.S. virologic surveillance
Nationally, and in regions 1, 3, 4, 5, 7, 8, 9 and 10, the percentage of respiratory specimens testing positive for influenza virus in clinical laboratories decreased (change ≥ 0.5 percentage points), while in regions 2 and 6 the percentage remained stable (change < 0.5 percentage points) compared to the previous week. Influenza A and B viruses were co-circulating this week. 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]*This data reflects 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). It does not reflect specimens tested only at CDC and could include more than one specimen tested per person. The guidance for influenza A/H5 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 than the number of human H5 cases. For more information on the number of people infected with A/H5, 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]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.[/SUP]
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
No confirmed human infections with influenza A(H5) virus were reported to CDC this week. To date, human-to-human transmission of avian influenza A(H5) virus (H5 bird flu) has not been identified in the United States.
The CSTE position statement, which includes updated case definitions for confirmed, probable, and suspected cases is available at http://www.cste.org/resource/resmgr/position_statements_files_2023/24-ID-09_Novel_Influenza_A.pdf
An up-to-date human case summary during the 2024 outbreak by state and exposure source is available at www.cdc.gov/bird-flu/situation-summary/index.html
Information about avian influenza is available at https://www.cdc.gov/flu/avianflu/index.htm.
A(H5N1) virus 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 relative to the reference viruses representing 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 polymerase acidic protein (PA) endonuclease inhibitor baloxavir. The HA clade and subclades were assigned using Nextclade (https://clades.nextstrain.org).
CDC has genetically characterized 3,263 influenza viruses collected since September 29, 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 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 | CDC.
Viruses collected in the U.S. since September 29, 2024, were tested for antiviral susceptibility as follows:
Five A(H1N1)pdm09 viruses had NA-H275Y amino acid substitution conferring highly reduced inhibition by oseltamivir and peramivir. One A(H1N1)pdm09 virus had NA-I223V and NA-S247N amino acid substitutions and showed reduced inhibition by oseltamivir. One A(H3N2) virus had NA-E119V amino acid substitution and showed highly reduced inhibition by oseltamivir. One A(H3N2) virus had PA-I38T amino acid substitution associated with 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 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 13, 3.2% 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 12 and remains above the national baseline of 3.0% for the eighteenth consecutive week. The percentage of visits for ILI increased slightly (change of > 0.1 percentage points) in Region 6 (likely due to a reporting anomaly) and decreased (change of > 0.1 percentage points) in all other regions this week compared to last. Regions 1, 2, 3, 6, and 10 are above their respective baselines, Region 5 is at its baseline, and regions 4, 7, 8, and 9 are below their respective baselines. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infections 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 (change of > 0.1 percentage point) in all age groups (0-4 years, 5-24 years, 25-49 years, 50-64 years, and 65+ years) in Week 13 compared to Week 12.
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 overall percentage of emergency department (ED) visits with a discharge diagnosis of influenza reported in NSSP was 1.4% during Week 13, a decrease (change of > 0.1 percentage point) compared to the previous week. The percentage of ED visits with influenza discharge diagnoses decreased in all 10 HHS regions and across all age groups. RegionNationalRegion 1Region 2Region 3Region 4Region 5Region 6Region 7Region 8Region 9Region 10
Season2023-2024 & 2024-20252022-2023 Skip Over Chart Container
2.0%4.0%6.0%8.0%10.0%12.0%14.0%16.0%Percent of Emergency Department Visits for InfluenzaWeek 41 of 2023Week 45 of 2023Week 49 of 2023Week 1 of 2024Week 5 of 2024Week 9 of 2024Week 13 of 2024Week 17 of 2024Week 21 of 2024Week 25 of 2024Week 29 of 2024Week 33 of 2024Week 37 of 2024Week 41 of 2024Week 45 of 2024Week 49 of 2024Week 1 of 2025Week 5 of 2025Week 9 of 2025Week 13 of 2025
Age Group
All ages
0-4 years
5-17 years
18-64 years
65+ Skip Data Table
Data Table Download Data (CSV) Skipped data table.
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. As data are received each week, prior case counts and rates are updated accordingly.
A total of 37,358 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2024, and March 29, 2025. The weekly hospitalization rate observed during Week 13 was 1.4 per 100,000 population. The weekly hospitalization rate observed during Week 6 (13.6 per 100,000 population) was the highest peak weekly rate observed across all seasons since 2010-2011. The cumulative hospitalization rate observed in Week 13 was 121.9 per 100,000 population, which is the highest cumulative hospitalization rate for Week 13 across all seasons since 2010-2011.
Among all hospitalizations, 36,198 (96.9%) were associated with influenza A virus, 979 (2.6%) with influenza B virus, 32 (0.1%) with influenza A virus and influenza B virus co-infection, and 149 (0.4%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 5,939 (57.2%) had A(H1N1) pdm09 and 4,441 (42.8%) had A(H3N2).
When examining rates by age, the highest cumulative hospitalization rate per 100,000 population was among adults aged 65 years and older (384.0), followed by adults aged 50-64 years (142.7), children aged 0-4 years (100), adults aged 18-49 years (49.6), and children aged 5-17 years (37.6).
When examining age-adjusted rates by race and ethnicity, the highest cumulative hospitalization rate per 100,000 population was among non-Hispanic Black persons (205.2), followed by American Indian/Alaska Native persons (150.8), non-Hispanic White persons (105.3), Hispanic persons (101.6), and Asian/Pacific Islander persons (76.1).
Among 4,082 hospitalized adults with information on underlying medical conditions, 95% had at least one reported underlying medical condition; the most commonly reported were hypertension, cardiovascular disease, metabolic disease, and obesity. Among 1,824 hospitalized women of childbearing age (15-49 years) with information on pregnancy status, 28.6% were pregnant. Among 1,554 hospitalized children with information on underlying medical conditions, 53.9% had at least one reported underlying medical condition; the most commonly reported was asthma, followed by neurologic disease and obesity.
[SUB]**In this figure, weekly rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 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) Hospital Respiratory Data
Hospitals report to NHSN the weekly number of patients with laboratory-confirmed influenza who were admitted to the hospital. Nationally, during Week 13, 9,364 laboratory confirmed influenza-associated hospitalizations were reported. This week's influenza-associated hospitalizations decreased (change of > 5%) compared to Week 12.
The weekly hospital admission rate observed in Week 13 was 2.8 per 100,000. The weekly rate of hospital admissions decreased in all 10 HHS regions and ranged from 1.6 (Region 9) to 4.9 (Region 3).
When examining rates by age for Week 13, all age groups decreased compared to the previous week. The highest hospital admission rate per 100,000 population was among those 65+ years (8.3), followed by 50-64 years (2.8), and 0-4 years (2.3).
Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive
Mortality surveillance
National Center for Health Statistics (NCHS)
Based on NCHS mortality surveillance data available on April 3, 2025, 0.9% of the deaths that occurred during the week ending March 29, 2025 (Week 13), were due to influenza. This percentage decreased (≥ 0.1 percentage point change) compared to Week 12. 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
Nine influenza-associated pediatric deaths occurring during the 2024-2025 season were reported to CDC during Week 13. The deaths occurred during Week 52 of 2024 (the week ending December 28, 2024) and between weeks 9 and 12 of 2025 (the weeks ending March 1, 2025, and March 22, 2025). Seven deaths were associated with influenza A viruses. Four of the influenza A viruses had subtyping performed; one was an A(H1N1) virus and three were A(H3N2) viruses. Two deaths were associated with influenza B viruses with no lineage determined.
A total of 168 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
Additional National and International Influenza Surveillance Information
Indicators Status by System
Increasing:
Decreasing:
Stable:
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.
Additional 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/fluview/surveillance/2025-week-13.html
What to know
Seasonal influenza activity remains elevated nationally but has decreased for two consecutive weeks.
Summary
Viruses
Clinical Lab 9.7% (Trend
positive for influenza
this week. Public Health Lab Influenza A(H1N1)pdm09, A(H3N2), and B
viruses were co-circulating this week.
Illness
Outpatient Respiratory Illness 3.2% (Trend
of visits to a health care provider this
week were for respiratory illness
(above baseline). Activity Map 12 moderate jurisdictions 2 high or very high jurisdictions FluSurv-NET 121.9 per 100,000
cumulative hospitalization rate National Healthcare Safety Network (NHSN) Hospital Respiratory Data 9,364 (Trend
patients admitted to hospitals
with influenza this week. NCHS Mortality 0.9% (Trend
of deaths attributed to influenza this week. Pediatric Deaths 9 influenza-associated deaths
were reported this week for
a total of 168 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.[SUP]1[/SUP]
Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.
Key Points
- Seasonal influenza (flu) activity continues to decline; however, CDC expects several more weeks of flu activity.
- This season is classified as a high severity season overall and for all age groups (children, adults, older adults) and is the first high severity season since 2017-2018.
- During Week 13, of the 1,049 viruses reported by public health laboratories, 929 were influenza A and 120 were influenza B. Of the 806 influenza A viruses subtyped during Week 13, 448 (55.6%) were influenza A(H1N1)pdm09, 358 (44.4%) were A(H3N2), and 0 (0%) were A(H5).
- No new influenza A(H5) cases were reported to CDC this week. To date, human-to-human transmission of avian influenza A(H5) virus (H5 bird flu) has not been identified in the United States.
- Nationally, outpatient respiratory illness remained stable this week and remains above baseline for the eighteenth consecutive week. HHS regions 1, 2, 3, and 6 are above their region-specific baselines, Region 5 is at its baseline, and all other HHS regions are below their baselines in regions.
- Based on data from FluSurv-NET, the cumulative hospitalization rate for this season is the highest observed since the 2010-2011 season.
- Nine pediatric deaths associated with seasonal influenza virus infection were reported this week, bringing the 2024-2025 season total to 168 pediatric deaths.
- CDC estimates that there have been at least 45 million illnesses, 580,000 hospitalizations, and 25,000 deaths from flu so far this season.
- CDC continues to recommend that everyone ages 6 months and older get an annual flu vaccine as long as influenza viruses are circulating.[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 patients at higher risk for severe illness.[SUP]2[/SUP]
- 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 regions 1, 3, 4, 5, 7, 8, 9 and 10, the percentage of respiratory specimens testing positive for influenza virus in clinical laboratories decreased (change ≥ 0.5 percentage points), while in regions 2 and 6 the percentage remained stable (change < 0.5 percentage points) compared to the previous week. Influenza A and B viruses were co-circulating this week. 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 | 69,263 | 2,876,143 |
| No. of positive specimens (%) | 6,725 (9.7%) | 457,645 (15.9%) |
| Positive specimens by type | ||
| Influenza A | 3,444 (51.2%) | 421,653 (92.1%) |
| Influenza B | 3,281 (48.8%) | 35,992 (7.9%) |
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 | 1,753 | 122,424 |
| No. of positive specimens | 1,049 | 83,336 |
| Positive specimens by type/subtype | ||
| Influenza A | 929 (88.6%) | 80,414 (96.5%) |
| Subtyping Performed | 806 (86.8%) | 70,797 (88.0%) |
| (H1N1)pdm09 | 448 (55.6%) | 37,185 (52.5%) |
| H3N2 | 358 (44.4%) | 33,532 (47.4%) |
| H3N2v | 0 | 0 |
| H5* | 0 | 80 (0.1%) |
| Subtyping not performed | 123 (13.2%) | 9,617 (12.0%) |
| Influenza B | 120 (11.4%) | 2,922 (3.5%) |
| Lineage testing performed | 48 (40.0%) | 1,357 (46.4%) |
| Yamagata lineage | 0 | 0 |
| Victoria lineage | 48 (100%) | 1,357 (100%) |
| Lineage not performed | 72 (60.0%) | 1,565 (53.6%) |
[SUP]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.[/SUP]
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
No confirmed human infections with influenza A(H5) virus were reported to CDC this week. To date, human-to-human transmission of avian influenza A(H5) virus (H5 bird flu) has not been identified in the United States.
The CSTE position statement, which includes updated case definitions for confirmed, probable, and suspected cases is available at http://www.cste.org/resource/resmgr/position_statements_files_2023/24-ID-09_Novel_Influenza_A.pdf
An up-to-date human case summary during the 2024 outbreak by state and exposure source is available at www.cdc.gov/bird-flu/situation-summary/index.html
Information about avian influenza is available at https://www.cdc.gov/flu/avianflu/index.htm.
A(H5N1) virus 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 relative to the reference viruses representing 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 polymerase acidic protein (PA) endonuclease inhibitor baloxavir. The HA clade and subclades were assigned using Nextclade (https://clades.nextstrain.org).
CDC has genetically characterized 3,263 influenza viruses collected since September 29, 2024.
| A/H1 | 1,218 | ||||
| 5a.2a | 587 (48.2%) | C.1.9 | 75 (6.2%) | ||
| C.1.9.1 | 68 (5.6%) | ||||
| C.1.9.2 | 5 (0.4%) | ||||
| C.1.9.3 | 433 (35.6%) | ||||
| C.1.9.4 | 6 (0.5%) | ||||
| 5a.2a.1 | 631 (51.8%) | D | 30 (2.5%) | ||
| D.1 | 11 (0.9%) | ||||
| D.3 | 412 (33.8%) | ||||
| D.5 | 178 (14.6%) | ||||
| A/H3 | 1,706 | ||||
| 2a.3a | 1,935 (100%) | G.1.3.1 | 6 (0.4%) | ||
| 2a.3a.1 | 1,700 (99.6%) | J.1 | 1 (0.1%) | ||
| J.1.1 | 7 (0.4%) | ||||
| J.2 | 1,563 (91.6%) | ||||
| J.2.1 | 45 (2.6%) | ||||
| J.2.2 | 84 (4.9%) | ||||
| B/Victoria | 339 | ||||
| 3a.2 | 339 (100%) | C.3 | 4 (1.2%) | ||
| C.5 | 37 (10.9%) | ||||
| C.5.1 | 175 (51.6%) | ||||
| C.5.5 | 1 (0.3%) | ||||
| C.5.6 | 45 (13.3%) | ||||
| C.5.7 | 77 (22.7%) | ||||
| B/Yamagata | 0 | ||||
| Y3 | 0 | Y3 | 0 (0%) |
Influenza A Viruses
- A (H1N1)pdm09: 280 A(H1N1)pdm09 viruses were antigenically characterized by HI, and 278 (99.3%) 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): 362 A(H3N2) viruses were antigenically characterized by HI or HINT, and 235 (64.9%) 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: 112 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 | CDC.
Viruses collected in the U.S. since September 29, 2024, were tested for antiviral susceptibility as follows:
| Neuraminidase Inhibitors | Oseltamivir | Viruses Tested | 3,184 | 1,207 | 1,676 | 301 |
| Reduced Inhibition | 1 (<0.1%) | 1 (0.1%) | 0 | 0 | ||
| Highly Reduced Inhibition | 6 (0.2%) | 5 (0.4%) | 1 (0.1%) | 0 | ||
| Peramivir | Viruses Tested | 3,184 | 1,207 | 1,676 | 301 | |
| Reduced Inhibition | 0 | 0 | 0 | 0 | ||
| Highly Reduced Inhibition | 5 (0.2%) | 5 (0.4%) | 0 | 0 | ||
| Zanamivir | Viruses Tested | 3,184 | 1,207 | 1,676 | 301 | |
| Reduced Inhibition | 0 | 0 | 0 | 0 | ||
| Highly Reduced Inhibition | 0 | 0 | 0 | 0 | ||
| PA Cap-Dependent Endonuclease Inhibitor | Baloxavir | Viruses Tested | 3,066 | 1,098 | 1,662 | 306 |
| Decreased Susceptibility | 1 (<0.1%) | 0 | 1 (0.1%) | 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 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 13, 3.2% 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 12 and remains above the national baseline of 3.0% for the eighteenth consecutive week. The percentage of visits for ILI increased slightly (change of > 0.1 percentage points) in Region 6 (likely due to a reporting anomaly) and decreased (change of > 0.1 percentage points) in all other regions this week compared to last. Regions 1, 2, 3, 6, and 10 are above their respective baselines, Region 5 is at its baseline, and regions 4, 7, 8, and 9 are below their respective baselines. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infections 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 (change of > 0.1 percentage point) in all age groups (0-4 years, 5-24 years, 25-49 years, 50-64 years, and 65+ years) in Week 13 compared to Week 12.
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 13 (Week ending Mar. 29, 2025) | Week 12 (Week ending Mar. 22, 2025) | Week 13 (Week ending Mar. 29, 2025) | Week 12 (Week ending Mar. 22, 2025) | |
| Very High | 0 | 0 | 1 | 2 |
| High | 2 | 7 | 18 | 45 |
| Moderate | 12 | 11 | 48 | 77 |
| Low | 9 | 19 | 170 | 193 |
| Minimal | 31 | 17 | 455 | 384 |
| Insufficient Data | 1 | 1 | 237 | 228 |
*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 overall percentage of emergency department (ED) visits with a discharge diagnosis of influenza reported in NSSP was 1.4% during Week 13, a decrease (change of > 0.1 percentage point) compared to the previous week. The percentage of ED visits with influenza discharge diagnoses decreased in all 10 HHS regions and across all age groups. RegionNationalRegion 1Region 2Region 3Region 4Region 5Region 6Region 7Region 8Region 9Region 10
Season2023-2024 & 2024-20252022-2023 Skip Over Chart Container
2.0%4.0%6.0%8.0%10.0%12.0%14.0%16.0%Percent of Emergency Department Visits for InfluenzaWeek 41 of 2023Week 45 of 2023Week 49 of 2023Week 1 of 2024Week 5 of 2024Week 9 of 2024Week 13 of 2024Week 17 of 2024Week 21 of 2024Week 25 of 2024Week 29 of 2024Week 33 of 2024Week 37 of 2024Week 41 of 2024Week 45 of 2024Week 49 of 2024Week 1 of 2025Week 5 of 2025Week 9 of 2025Week 13 of 2025
Age Group
All ages
0-4 years
5-17 years
18-64 years
65+ Skip Data Table
Data Table Download Data (CSV) Skipped data table.
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. As data are received each week, prior case counts and rates are updated accordingly.
A total of 37,358 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2024, and March 29, 2025. The weekly hospitalization rate observed during Week 13 was 1.4 per 100,000 population. The weekly hospitalization rate observed during Week 6 (13.6 per 100,000 population) was the highest peak weekly rate observed across all seasons since 2010-2011. The cumulative hospitalization rate observed in Week 13 was 121.9 per 100,000 population, which is the highest cumulative hospitalization rate for Week 13 across all seasons since 2010-2011.
Among all hospitalizations, 36,198 (96.9%) were associated with influenza A virus, 979 (2.6%) with influenza B virus, 32 (0.1%) with influenza A virus and influenza B virus co-infection, and 149 (0.4%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 5,939 (57.2%) had A(H1N1) pdm09 and 4,441 (42.8%) had A(H3N2).
When examining rates by age, the highest cumulative hospitalization rate per 100,000 population was among adults aged 65 years and older (384.0), followed by adults aged 50-64 years (142.7), children aged 0-4 years (100), adults aged 18-49 years (49.6), and children aged 5-17 years (37.6).
When examining age-adjusted rates by race and ethnicity, the highest cumulative hospitalization rate per 100,000 population was among non-Hispanic Black persons (205.2), followed by American Indian/Alaska Native persons (150.8), non-Hispanic White persons (105.3), Hispanic persons (101.6), and Asian/Pacific Islander persons (76.1).
Among 4,082 hospitalized adults with information on underlying medical conditions, 95% had at least one reported underlying medical condition; the most commonly reported were hypertension, cardiovascular disease, metabolic disease, and obesity. Among 1,824 hospitalized women of childbearing age (15-49 years) with information on pregnancy status, 28.6% were pregnant. Among 1,554 hospitalized children with information on underlying medical conditions, 53.9% had at least one reported underlying medical condition; the most commonly reported was asthma, followed by neurologic disease and obesity.
[SUB]**In this figure, weekly rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 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) Hospital Respiratory Data
Hospitals report to NHSN the weekly number of patients with laboratory-confirmed influenza who were admitted to the hospital. Nationally, during Week 13, 9,364 laboratory confirmed influenza-associated hospitalizations were reported. This week's influenza-associated hospitalizations decreased (change of > 5%) compared to Week 12.
The weekly hospital admission rate observed in Week 13 was 2.8 per 100,000. The weekly rate of hospital admissions decreased in all 10 HHS regions and ranged from 1.6 (Region 9) to 4.9 (Region 3).
When examining rates by age for Week 13, all age groups decreased compared to the previous week. The highest hospital admission rate per 100,000 population was among those 65+ years (8.3), followed by 50-64 years (2.8), and 0-4 years (2.3).
Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive
Mortality surveillance
National Center for Health Statistics (NCHS)
Based on NCHS mortality surveillance data available on April 3, 2025, 0.9% of the deaths that occurred during the week ending March 29, 2025 (Week 13), were due to influenza. This percentage decreased (≥ 0.1 percentage point change) compared to Week 12. 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
Nine influenza-associated pediatric deaths occurring during the 2024-2025 season were reported to CDC during Week 13. The deaths occurred during Week 52 of 2024 (the week ending December 28, 2024) and between weeks 9 and 12 of 2025 (the weeks ending March 1, 2025, and March 22, 2025). Seven deaths were associated with influenza A viruses. Four of the influenza A viruses had subtyping performed; one was an A(H1N1) virus and three were A(H3N2) viruses. Two deaths were associated with influenza B viruses with no lineage determined.
A total of 168 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
Additional National and International Influenza Surveillance Information
Indicators Status by System
Increasing:
Decreasing:
Stable:
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.
Additional 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/fluview/surveillance/2025-week-13.html
