Influenza Activity in the United States during the 2024–25 Season and Composition of the 2025–26 Influenza Vaccine

Executive Summary

The Centers for Disease Control and Prevention (CDC) collects, compiles, and analyzes data on influenza viruses and associated morbidity and mortality in the United States. Influenza activity in the United States during the 2024-25 season (September 29, 2024 - August 30, 2025) was classified as high severity, marking the most severe influenza season since 2017-18. High severity was reflected across multiple indicators, including influenza-associated outpatient visits, hospitalizations, and deaths, underscoring the significant public health impact of the 2024-25 season. Influenza activity began increasing in mid-November and peaked in early February 2025. Influenza activity then steadily declined to interseasonal levels by May 2025 and has remained low through August 2025.

During the 2024-25 season, influenza A viruses were predominant, with A(H1N1)pdm09 and A(H3N2) viruses detected at approximately equal levels. Influenza B activity remained low throughout the season but increased slightly later in the season. No influenza B/Yamagata lineage viruses were detected for the fifth consecutive season.

This report summarizes influenza activity in the United States during the 2024-25 season as reported to CDC through its collaborative efforts with clinical and public health laboratories, outpatient providers, emergency departments, hospitals, vital statistics offices, and state and local health departments. It also includes the recommended composition of the Northern Hemisphere 2025-26 influenza vaccines and a brief update on influenza activity occurring between April and September of 2025 in the Southern Hemisphere.

Virus Surveillance

The influenza virologic surveillance system is comprised of the U.S. Influenza Collaborating Laboratories (ICL) and National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories, which include both clinical and public health laboratories throughout the United States. Clinical laboratories primarily test for diagnostic purposes, and their data are used to understand timing and intensity of influenza activity. Public health laboratories primarily test specimens for influenza for surveillance purposes to understand what influenza virus subtypes and lineages are circulating and the ages of people that are infected 1.

During the 2024-25 influenza season, clinical laboratories reporting to CDC tested 3,978,954 respiratory specimens for influenza viruses using clinical diagnostic tests. Among these, 489,579 (12.3%) tested positive, including 434,985 (88.8%) for influenza A and 54,594 (11.2%) for influenza B viruses (Figure 1). The national weekly percentage of respiratory specimens testing positive for influenza ranged from 0.4% to 31.6%, with peak activity observed during the week ending February 1, 2025 (week 5). This represents the highest peak percentage reported in the past nine influenza seasons (since 2015-16)*. Influenza A viruses predominated overall and peaked at 30.4% in late January, while influenza B viruses circulated at lower levels, peaking at 4.3% in late March.

Based on the percentage of specimens testing positive in clinical laboratories, peak influenza activity in each of the 10 Health and Human Services (HHS) regions occurred between late January 2025 and early February 2025. Regions 4, 6, 7, and 9 (Southeast, South Central, Central, and West Coast) peaked late in January. Regions 1, 2, 3, 5, 8, and 10 (New England, New York/New Jersey/Puerto Rico, Mid-Atlantic, Midwest, Mountain, and Pacific Northwest) peaked in early February.

Figure 1. Percentage of Influenza Positive Test Results Reported by Clinical Laboratories to CDC, National Summary by Week and Influenza Season - United States, 2024-25 and Selected Previous Seasons

^{*Beginning in the 2015-16 season, reports from public health and clinical laboratories were presented separately in the weekly influenza update, FluView.}

Public health laboratories tested 155,297 specimens and reported 100,015 influenza positive specimens, with 94,078 (94.1%) positive for influenza A and 5,937 (5.9%) positive for influenza B viruses. Among 84,260 (89.6%) seasonal influenza A viruses that were subtyped, 44,733 (53.1%) were influenza A(H1N1)pdm09 viruses, and 39,527 (46.9%) were influenza A(H3N2) viruses. Influenza B lineage information was available for 3,297 (55.5%) influenza B viruses, with all of them belonging to the Victoria lineage (Figure 2). No influenza B/Yamagata lineage viruses have been identified worldwide after March 2020.

Figure 2. Seasonal Influenza Positive Tests Reported to CDC by U.S. Public Health Laboratories, National Summary, 2024-25 Season

Nationally, the 2024-25 influenza season was characterized by the co-circulation of influenza A(H1N1)pdm09 and A(H3N2) viruses, with A(H1N1)pdm09 being detected slightly more frequently at 53.1%, while A(H3N2) accounted for 46.9% of the subtyped seasonal influenza A viruses. As overall influenza activity declined beginning in late March, influenza A(H1N1)pdm09, A(H3N2), and B viruses continued to co-circulate at lower levels. In most HHS regions, influenza A(H1N1)pdm09 and A(H3N2) circulated at similar proportions and together accounted for the majority of viruses, while Influenza B viruses increased slightly in mid-May. For additional regional level data, please visit FluView Interactive.

Among influenza A viruses that were subtyped, similar proportions of A(H1N1)pdm09 and A(H3N2) viruses were reported among persons aged 0-4 years (50.2% and 49.8%, respectively). A(H1N1)pdm09 viruses were slightly more frequently reported among individuals aged 25-64 years (55.5%) and 65 years and older (59.9%), while influenza A(H3N2) viruses were slightly more frequently reported in persons aged 5-24 years (53.7%). For additional age group data, please visit FluView Interactive.

Virus Characterization & Antiviral Susceptibility

Genetic characterization of the viruses circulating during this time period was conducted using next generation sequencing, and the genomic data were analyzed and submitted to publicly accessible databases (GenBank: https://www.ncbi.nlm.nih.gov/genbank/ and EpiFlu: https://www.gisaid.org). Phylogenies of representative subsets of genetic data from CDC and other submitters can be visualized in real time at https://nextstrain.org/.

To evaluate whether genetic changes in the hemagglutinin (HA) of circulating viruses affected antigenicity, antigenic characterizations were conducted using hemagglutination inhibition (HI) assays (A(H1N1)pdm09, A(H3N2) and B viruses) or High-content Imaging-based micro-Neutralization Test (HINT) (A(H3N2) viruses) and post-infection ferret antisera raised to reference viruses representing the cell- and recombinant-based 2024-25 Northern Hemisphere vaccine viruses. Both assays determine how well antibodies raised to the influenza vaccine reference viruses recognize or bind to circulating viruses. HI assay measures how well antibodies inhibit the binding between the HA of the virus and the sialic acid receptors on the surface of red blood cells, whereas HINT measures how well antibodies neutralize or block virus infection of a cell. 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.

CDC genetically characterized 5,180 viruses (4,278 influenza A and 902 influenza B) collected in the United States during the 2024-25 season and antigenically characterized viruses representing the co-circulating genetic clades. Overall, the influenza vaccine antigens for A(H1N1)pdm09 and B/Victoria elicited antibodies that reacted well with most of the viruses tested during this period. Antibodies elicited by the vaccine antigens for A(H3N2) exhibited reduced reactivity but still reacted well with the majority of the circulating viruses tested. Detailed characterization data for each group are below.

Influenza A(H1N1)pdm09

Phylogenetic analysis of the HA demonstrated that viruses characterized belonged to 2 clades: 5a.2a (32.3% [n=646]) and 5a.2a.1 (67.7% [n=1,353]). The A(H1N1)pdm09 component for the 2024-25 Northern Hemisphere influenza vaccines contained HA genes from clade 5a.2a.1. Each clade was further classified to HA subclades using Nextclade (https://clades.nextstrain.org). In clade 5a.2a, subclade C.1.9.3 was the largest proportion subclade identified, representing 24.4% (n=487) of total characterized A(H1N1)pdm09 viruses. Subclade D.3.1 was the largest proportion subclade in clade 5a.2a.1 and among all characterized A(H1N1)pdm09 viruses, representing 56.4% (n=1,128) of all characterized A(H1N1)pdm09 viruses. Multiple subclades co-circulated in the beginning of the season, but subclade D.3.1 increased in proportion and has been the predominant subclade since February 2025. Nearly all (99.3%, n=573) of the A(H1N1)pdm09 viruses antigenically characterized were well recognized (reacting at titers that were within 4-fold of the homologous virus titer by HI) by ferret antisera to cell-grown A/Wisconsin/67/2022-like reference viruses which represent the A(H1N1)pdm09 component for the cell- and recombinant-based influenza vaccines.

Influenza A(H3N2)

The majority (99.7% [n=2,273]) of genetically characterized A(H3N2) viruses belonged to the 2a.3a.1 clade, while the rest (0.3% [n=6]) belonged to the 2a.3a clade. The A(H3N2) component for the 2024-25 Northern Hemisphere influenza vaccines contained HA genes from the 2a.3a.1 clade. Clade 2a.3a.1 was classified into multiple HA subclades using Nextclade (https://clades.nextstrain.org), among which subclade J.2 represented the majority (74.3% [n=1,693]) of characterized A(H3N2) viruses while another 25% of viruses belonged to further divided J.2 subclades (J.2.1 to J.2.5). Antigenic analysis showed that 429 (54.7%) of the A(H3N2) viruses antigenically characterized 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 8-fold of the homologous virus in HINT) by ferret antisera to cell-grown A/Massachusetts/18/2022-like reference viruses that represented the A(H3N2) component for the cell- and recombinant-based influenza vaccines. Viruses recognized less well by ferret antisera to cell-grown A/Massachusetts/18/2022-like reference viruses were observed in some J.2 viruses and all viruses in the J.2.3, J.2.4 and J.2.5 subclades. Many of the viruses from subclade J.2 with reduced recognition had HA genes with additional substitutions of S145N or T135K (loss of a putative glycosylation site) or V223I. Additional HA substitutions of note for the other subclades include N158K (J.2.5), N158K and K189R (J.2.3), and N135K (loss of a putative glycosylation site) and K189R (J.2.4).

Influenza B

All influenza B viruses for which characterization was possible belonged to the B/Victoria-lineage. Infections with B/Yamagata viruses have not been identified globally after March 2020. Phylogenetic analysis of the HA genes showed that all (n=902) belonged to clade V1A.3a.2. The genetic clade of the B/Victoria-lineage component for the 2024-25 Northern Hemisphere influenza vaccines was V1A.3a.2 clade. Clade V1A.3a.2 was classified into various HA subclades using Nextclade (https://clades.nextstrain.org). Among them, 9 subclades (C.3, C.3.1, C.3.2, C.5, C.5.1, C.5.5, C.5.6, C.5.6.1 and C.5.7) co-circulated during the 2024-25 season. While C.5, C.5.1, C.5.6 and C.5.7 were the major subclades in the beginning of the season, subclade C.3.1 increased in proportion and became one of the major co-circulating subclades after March 2025. Antigenic analysis showed that 264 (84.9%) of the B/Victoria viruses antigenically characterized 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. Most viruses that were recognized less well by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses belonged to subclades C.3.1 and C.3.2. Both of these subclades have an additional HA substitution D197N which adds a putative glycosylation site.

Figure 3. Genetic Characterization of HA genes Based on Phylogenic Analysis of Influenza Viruses Collected during the 2024-25 season

CDC also assesses susceptibility of influenza viruses to antiviral medications. Between September 29, 2024, and August 30, 2025, a total of 5,078 viruses collected in the United States were characterized for susceptibility to neuraminidase (NA) inhibitors using sequence-based analysis, and a subset of 487 viruses (9.6%) selected to represent a variety of locations, collection dates and genetic clades, were tested using a functional neuraminidase inhibition assay. Based on sequence analysis, 12 viruses had known, or suspected mutations associated with reduced inhibition by the NA inhibitors. Of these 12 viruses, 8 A(H1N1)pdm09 viruses contained NA-H275Y amino acid substitution, 1 A(H1N1)pdm09 virus had NA-S247R, and 1 A(H1N1)pdm09 virus had a combination of NA substitutions I223V + S247N. The remaining 2 viruses were A(H3N2) viruses; one had an NA-E119V substitution, and another had a mix of amino acids at two positions of the NA, 119 (E and V) and 292 (R and K). All viruses lacking NA mutations displayed normal inhibition by the NA inhibitors. Six of eight A(H1N1)pdm09 viruses containing NA-H275Y were available for testing by a functional assay and, as expected, they showed highly reduced inhibition by oseltamivir and peramivir. One A(H1N1)pdm09 virus with NA-S247R displayed reduced inhibition by oseltamivir and zanamivir and highly reduced inhibition by peramivir. One A(H1N1)pdm09 virus with substitutions I223V + S247N showed reduced inhibition by oseltamivir. One A(H3N2) virus with NA-E119V showed highly reduced inhibition by oseltamivir. To better understand the effects of NA-E119V or NA-R292K, separation of viruses containing NA-E119V or NA-R292K was done by a limiting dilution using a clinical specimen with a mix of NA-E119V/E + R292K/R. The separated virus with NA-E119V displayed highly reduced inhibition by oseltamivir. The separated virus with NA-R292K also displayed highly reduced inhibition by oseltamivir, as well as reduced inhibition by zanamivir and peramivir.

A total of 4,862 viruses were characterized for susceptibility to the polymerase acidic (PA) cap-dependent endonuclease inhibitor baloxavir using sequence-based analysis, and a subset of 486 viruses (10.0%) were tested phenotypically. Based on sequence analysis, all viruses, except 3, lacked known or suspected substitutions associated with reduced susceptibility to baloxavir. Of these 3 viruses, 1 A(H1N1)pdm09 and 1 A(H3N2) viruses contained a PA-I38T amino acid substitution, and 1 A(H3N2) virus had PA-A37T. All tested viruses were susceptible to baloxavir, except these 3 viruses with PA-I38T or PA-A37T, which showed reduced susceptibility to baloxavir.

Table 1. Assessment of Virus Susceptibility to Antiviral Medications, Viruses Collected in the U.S.
September 29, 2024 - August 30, 2025

Influenza Vaccine Strain Selection 2025-26

Viruses to be included in the 2025-26 Northern Hemisphere influenza vaccines were recommended at the World Health Organization's (WHO) Consultation on the Composition of the Influenza Vaccines in February 2025 and selected in the Food and Drug Administration (FDA) Center for Biologics Evaluation and Research (CBER) Interagency Meeting in March 2025. The recommendations were made after reviewing and evaluating data on the 2024-25 influenza season and the performance of the 2024-25 influenza vaccines23. The A(H1N1)pdm09 and B/Victoria lineage components are unchanged from the 2024-25 Northern Hemisphere influenza vaccines. Both the egg-based and cell- or recombinant-based vaccines had a change in the A(H3N2) component. In the egg-based vaccine it was changed to an A/Croatia/10136RV/2023 (H3N2)-like virus and in cell- or recombinant-based vaccines it was changed to an A/District of Columbia/27/2023 (H3N2)-like virus23. The genetic clade and subclade for the recommended vaccine viruses were 5a.2a.1 for A(H1N1)pdm09, 2a.3a.1_J.2 for A(H3N2), V1A.3a.2 for B/Victoria.

Table 2. Influenza Vaccine Viruses Northern Hemisphere 2025-26 and 2024-25* Seasons2.

Novel Influenza A Viruses

Novel influenza A viruses are influenza A virus subtypes that are different from currently circulating human seasonal influenza viruses. Early identification and investigation of human infections with novel influenza A viruses are critical so that the risk of infection can be understood, and appropriate public health measures can be taken1. From September 29, 2024, through June 21, 2025, a total of 56 confirmed and seven probable cases of novel influenza A virus infections were reported to CDC (Figure 4).

Of the 56 confirmed novel influenza A virus infections reported, 55 avian influenza A(H5) virus detections* in people were reported from nine states (37-CA, 1-IA, 1-LA, 1-NV, 1-OR, 1-OH, 11-WA, 1-WI, 1-WY). Thirty-six of the 55 cases were associated with exposure to highly pathogenic avian influenza (HPAI) A(H5N1) infected dairy cattle, 15 were associated with poultry culling operations, 2 with infected backyard flocks, and 2 cases did not have a determined source of exposure. Fifty-three of the 55 cases occurred in adults aged ≥18 years; 52 reported mild illness, 3 patients were hospitalized, and one of the hospitalized patients died. No human-to-human transmission of avian influenza A(H5) viruses was identified associated with these cases.

One additional novel influenza A virus infection was reported during the 2024-25 season, confirmed to be variant virus (i.e., a swine influenza virus identified in a human and designated with a "v"). This A(H1N2v) virus infection was detected and reported to CDC from Iowa. The patient was ≥18 years of age, was hospitalized, and had recovered from their illness. An investigation by state public health officials did not identify direct or indirect swine contact by the patent. No illness was identified among the patient's close contacts. No human-to-human transmission was identified associated with this case.

Figure 4. Novel Influenza A Virus Cases by Season- United States, 2010-11 to 2024-25 Season

^{*Among the 70 reported A(H5) human cases during April of 2024 to February 2025, 15 occurred between before the start date of this report.}

Outpatient Respiratory Illness Surveillance

Influenza-like Illness Surveillance Network (ILINet)

Information on outpatient visits to health care providers for respiratory illness, referred to as influenza-like illness ([ILI], fever plus cough or sore throat), is collected through the U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet)1. Nationally, the weekly percentage of outpatient visits for ILI reported through ILINet remained at or above the national baseline of 3.0% for 17 consecutive weeks from November 2024 to March 2025, peaking at 7.9% during the week ending February 8, 2025 (week 6). The 2024-25 season had the highest peak of absolute percentage of visits for ILI since the 2009 -10 influenza pandemic season, which peaked at 7.7%. However, since the national baseline can vary from season to season based on the providers enrolled, it is important to compare the degree of peak activity using the difference between the peak percent ILI and the baseline for that season. Using that metric, the 2024-2025 season had the third highest level of outpatient respiratory illness activity (4.9 percentage points between peak and baseline), following only the 2017-2018 season (5.3 percentage points) and the 2009-2010 pandemic season (5.4 percentage points). The duration of elevated ILI activity defined as the number of weeks with activity above baseline was similar to that observed in pre-pandemic seasons (Figure 5).

At the regional level, the timing of peak ILI activity varied across the 10 HHS regions during the 2024-25 influenza season. Regions 8, 9, and 10 (Mountain, West Coast, and Pacific Northwest) peaked earliest, during the week ending December 28, 2024 (week 52). Region 2 (New York/New Jersey/Puerto Rico, and the U.S. Virgin Islands) peaked with the same ILI percentage during the three weeks ending January 25, 2025, February 1, 2025, and February 15, 2025 (weeks 4, 5, and 7). Regions 4 and 6 (Southeast and South Central) reported peak ILI activity during the week ending February 1, 2025 (week 5) and regions 1, 3, 5, and 7 (New England, Mid-Atlantic, Midwest, and Central) peaked during the week ending February 8, 2025 (week 6).

The duration of elevated ILI activity also varied by region. Regions 1 and 3 (New England and Mid-Atlantic) experienced the longest span of elevated activity, lasting 22 weeks. Region 10 (Pacific Northwest) followed with 19 weeks. Regions 2, 4, 5, 6, 7, and 9 (New York/New Jersey/Puerto Rico/Virgin Islands, Southeast, Midwest, South Central, Central, and West Coast) experienced elevated activity for 16 to 18 weeks and region 8 (Mountain) had the shortest period of elevated ILI activity, lasting 12 weeks.

Multiple respiratory viruses co-circulated throughout the season, and the contribution of influenza viruses to ILI activity likely varied by week and geographic area. Overall, ILI activity peaked later in the 2024-25 season (early February) compared to the 2022-23 (mid-November) and 2023-24 seasons (late December) but during the month that was most frequently the peak during pre-COVID seasons.

Figure 5. Percentage of Outpatient Visits for Respiratory Illness as Reported by ILINet, National Summary by Season - United States, 2017-18 to 2024-25 Seasons

More than 70% of ILINet providers report total number of patient visits, as well as ILI visits, by age group, allowing the calculation of percentage of visits for ILI by age group. Throughout the 2024-25 season, children aged 0-4 years and those aged 5-24 years each had a higher percentage of visits for ILI compared to adult age groups, consistent with trends observed in previous seasons. Peak ILI activity occurred during the week ending February 8, 2025 (week 6) for the age groups less than 65 years [0-4 years (18.1%), 5-24 years (12.6%), 25-49 years (6.6%), and 50-64 years (5.1%)]. Among adults age 65 years and older, ILI activity peaked one week later, during the week ending February 15, 2025 (week 7), at 3.7% (Figure 6).

Figure 6. Percentage of Outpatient Visits by Age Group for Respiratory Illness as Reported by ILINet - United States, 2024-25 Season

ILINet data are used to produce weekly jurisdiction and core-based statistical area (CBSA)-level measures of ILI activity. From the 2008-09 through the 2018-19 seasons, ILI activity categories ranged from minimal to high; the very high category was introduced beginning with the 2019-20 season1. For the 11-week period from the week ending December 21, 2024, through the week ending March 1, 2025 (weeks 51-9), more than 50% of the 55 jurisdictions reported high or very high ILI activity each week. This represents 11 consecutive weeks of widespread elevated ILI activity. In comparison, during the 2023-24 season, only four weeks (including three consecutive weeks) had 50% or more of jurisdictions reporting high or very high activity. The highest proportion of jurisdictions reporting high or very high ILI activity during the 2024-25 season occurred in the week ending February 8, 2025 (week 6), when 87.3% of jurisdictions met that threshold. This is the highest weekly percentage observed since the 2019-20 season, when 87.0% of jurisdictions reported high or very high activity in a single week. For a weekly snapshot of geographic ILI activity levels, please see FluView Interactive | FluView | CDC.

National Syndromic Surveillance System (NSSP)

CDC's BioSense Platform allows emergency departments (EDs) to upload patient data as early as 24-hours after a patient's visit from a participating facility. The National Syndromic Surveillance System (NSSP) includes data from 83% of U.S. EDs, totaling more than 7,200 healthcare facilities in all 50 states and the District of Columbia4. NSSP's BioSense Platform hosts the Electronic Surveillance System for the Early Notification of Community-based Epidemics (ESSENCE)-a system that allows public health professionals to access and interpret data that flow into the BioSense Platform. On April 17, 2020, CDC created the "CDC Influenza DD v1" definition in ESSENCE which identifies visits of patients with a discharge diagnosis of influenza (Table 3). The weekly percentage of all ED visits with a discharge diagnosis of influenza is calculated by age group nationally and for each of the 10 HHS regions.

Table 3. CDC Influenza DD v1 inclusion codes in the NSSP Electronic Surveillance System for the Early Notification of Community-Based Epidemics (ESSENCE) system5.

Nationally, the percentage of ED visits with a discharge diagnosis of influenza reported to NSSP peaked at 8.4% during the week ending February 1, 2025 (week 5). From the week ending December 7, 2024 (week 49) through this peak, the weekly percentage of influenza-related ED visits increased by 7.3 percentage points although there were two weeks of stable or declining activity (depending on the age group) during late December and early January. This lessening of activity is frequently seen around this time of year and is in part associated with changes in healthcare seeking behavior around the holidays. Activity began a steady decline in the week ending February 8, 2025 (week 6) and continued into the summer. In comparison, peak ED visit activity for influenza in the previous two seasons occurred earlier and at lower levels, with national peaks of 6.7% during the week ending November 26, 2022 (week 47), and 5.8% during the week ending December 30, 2023 (week 52) (Figure 7).

Figure 7. Percentage of Emergency Department Visits for Influenza Reported by the National Syndromic Surveillance System (NSSP), National Summary-United States, 2022-23 to 2024-25 Seasons

^{*Data for 2024-25 season are through Week 35}

Regionally, percentages of ED visits reported to NSSP with a discharge diagnosis of influenza peaked between 5.5% and 10.9% during the 2024-25 season. Region 9 (West Coast) peaked the earliest of all HHS regions at 7.1% during the week ending December 28, 2024 (week 52), while all other regions reached their seasonal peaks during the week ending February 1, 2025 (week 5) or February 8, 2025 (week 6). Regions 7 and 8 (Central, Mountain) recorded the lowest peak percentages, each reaching 5.5% during the week ending February 8, 2025 (week 6). In comparison, Region 6 (South Central) experienced the highest regional peak, with 10.9% of ED visits attributed to influenza during the week ending February 1, 2025 (week 5).

The pediatric age groups (0-4 years and 5-17 years) experienced the highest percentages of ED visits for influenza during the 2024-25 season. Both groups reached their seasonal peaks during the week ending February 8, 2025 (week 6), at 17.8% and 17.0%, respectively. In comparison, among the adult age groups, the 18-64 years age group peaked earliest at 6.6% during the week ending February 1, 2025 (week 5), while the 65 years and older age group peaked at 5.7% one week later. By the week ending May 17, 2025 (week 20), influenza-related ED visits for all age groups had declined to 1% or less, comparable to levels observed during the first five weeks of the season (September 29 to November 2, 2024) (Figure 8).

Figure 8. Percentage of Emergency Department Visits for Influenza by Age Group Reported by the National Syndromic Surveillance System (NSSP), National Summary, 2024-25 Season

Hospitalization Surveillance

CDC monitors hospitalizations associated with laboratory-confirmed influenza virus infections through two surveillance systems: the Influenza Hospitalization Surveillance Network (FluSurv-NET), which conducts population-based surveillance covering approximately 9% of the U.S. population, and National Healthcare Safety Network (NHSN) Hospital Respiratory Data (HRD) Module, which consists of reports from all hospitals across the country1.

Influenza Hospitalization Surveillance Network (FluSurv-NET)

From October 1, 2024, to April 30, 2025 (the last day of surveillance in FluSurv-NET1), a total of 39,319 laboratory-confirmed influenza-related hospitalizations were reported by FluSurv-NET sites. The peak weekly hospitalization rate for the 2024-2025 season were observed during week 6 (13.7 per 100,000 population) and were tied for the highest peak weekly rate observed across all seasons since 2010-11 (Figure 9).

Figure 9. Weekly Rates of Laboratory-Confirmed Influenza Hospitalizations - United States, 2014-15 to 2024-25* influenza seasons

^{*The most recent 11 seasons (2014-15 through 2024-25) are depicted in the figure. See https://gis.cdc.gov/GRASP/Fluview/FluHospRates.html for data from seasons going back to the 2010-11 season.}

The overall cumulative hospitalization rate through April 30, 2025, was 128.3 per 100,000 population, which is the highest cumulative hospitalization rate for all seasons since 2010-11. (Figure 10).

Figure 10. Cumulative Rates of Laboratory-Confirmed Influenza Hospitalizations - United States, 2014-15 to 2024-25* influenza seasons

^{*The most recent 11 seasons (2014-15 through 2024-25) are depicted in the figure. See https://gis.cdc.gov/GRASP/Fluview/FluHospRates.html for data from seasons going back to the 2010-11 season.}

Among 39,319 influenza-associated hospitalizations, most (95.9%) were associated with influenza A virus, with 3.8% associated with influenza B virus. Among those with influenza A subtype information, 58.9% were A(H1N1)pdm09 and 41.1% were A(H3N2).

When examining rates by age, the highest cumulative hospitalization rate per 100,000 population was among adults aged 65 years and older (407.6), followed by adults aged 50-64 years (150.1), children aged 0-4 years (104.7), adults aged 18-49 years (50.7), and was lowest among children and adolescents aged 5-17 years (40.4) (Figure 11). When examining age-adjusted rates by race and ethnicity, the highest cumulative hospitalization rate per 100,000 population was among non-Hispanic Black persons (211.9), followed by non-Hispanic American Indian or Alaska Native persons (169.2), Hispanic/Latino persons (113.4), non-Hispanic White persons (112.00.), and non-Hispanic Asian/Pacific Islander persons (79.2).

Figure 11. Cumulative Rate of Laboratory-Confirmed Influenza Hospitalizations by Age - United States, 2024-25 influenza season

Among 8,876 adults with information on underlying medical conditions who were hospitalized with influenza between October 1, 2024, and April 30, 2025, 94.1% had at least one reported underlying medical condition; the most commonly reported were hypertension, cardiovascular disease, metabolic disorder, and obesity. Among 4,135 hospitalized women of childbearing age (15-49 years) with information on pregnancy status, 22.6% were pregnant. Among 3,109 hospitalized children and adolescents with information about underlying conditions, 53.5% reported at least one underlying medical condition; the most commonly reported were asthma, neurologic disease, and obesity.

National Healthcare Safety Network (NHSN) Hospital Respiratory Data (HRD) Module

Between September 29, 2024, and August 30, 2025, there were 545,026 influenza-associated hospitalizations nationwide, resulting in a cumulative hospitalization rate of 161.5 per 100,000 population for the season. Hospitalizations began to increase in mid-November, had a slight decline around the winter holidays and peaked during the week ending February 8, 2025 (week 6), with 54,272 hospitalizations (16.1 per 100,000 population). This peak occurred later and was substantially higher compared to the previous three seasons (Figure 12).

Figure 12. Number of New Influenza Hospital Admissions and Rates, Reported to CDC's NHSN HRD Module, National Summary, 2021-22 to 2024-25 Seasons

At a regional level, influenza activity had a slight decline around the winter holidays and peaked between the weeks ending February 1, 2025 (week 5) and February 15, 2025 (week 7). Regions 2 (New York/New Jersey/Puerto Rico), 6 (South Central), and 9 (West Coast) peaked during week 5; regions 1 (New England), 3 (Mid-Atlantic), 4 (Southeast), 8 (Mountain), and 10 (Pacific Northwest) peaked during the week ending February 8, 2025 (week 6); and Regions 5 (Midwest) and 7 (Central) peaked during the week ending February 15, 2025 (week 7). On a regional level, cumulative influenza-associated hospitalization rates ranged from 117.8 to 218.3 per 100,000 population. For additional details, see FluView Interactive.

When examining influenza-associated hospitalization rates nationally by age, peak weekly hospitalization rates ranged from 4.7 to 49.9 per 100,000 population, with all age groups peaking during the week ending February 8, 2025 (week 6). Persons aged 65 years and older experienced the highest peak rate (49.9 per 100,000 population). Adults aged 50-64 years had the second highest peak rate (17.5 per 100,000 population), slightly exceeding the overall national rate. Hospitalization rates among children aged 0-4 years (12.1 per 100,000 population), children aged 5-17 years (4.7 per 100,000 population), and adults aged 18-49 years (6.1 per 100,000 population) were below the overall national rate (Figure 13).

Figure 13. Rate of New Influenza Hospital Admissions, by Age Group, Reported to CDC's NHSN HRD Module, National Summary, 2024-25 Season

Mortality Surveillance

According to the National Center for Health Statistics (NCHS) Mortality Surveillance System1, during September 29, 2024 - August 30, 2025, a total of 2,784,349 deaths from all causes were reported nationally. Among these, 18,399 (0.7%) had influenza listed as an underlying or contributing cause of death. The weekly percentage of deaths due to influenza ranged from 0.02% to 2.8% and peaked during the week ending February 15, 2025 (week 7) (Figure 14). Influenza deaths began to increase in late November 2024, peaking in mid-February 2025 (week 7), and then declined. This is the highest peak since 2015-16 when NCHS influenza mortality data were incorporated into influenza surveillance. This peak was higher and occurred later compared to post-COVID influenza seasons, including the 2023-24 influenza season which peaked at 1.4% during early January, the 2022-23 influenza season which peaked at 1.6% during mid-December 2022, and the 2021-22 influenza season which peaked at 0.2% in early January. However, the timing of peak activity was similar to recent pre-COVID influenza seasons, which peaked between late January 2018 (2017-18 season) and mid-March 2016 (2015-16 season). Additionally, the cumulative percentage of deaths due to influenza this season is 0.7%, which is also the highest since 2015-16 where the seasonal cumulative percentage of deaths due to influenza ranged from 0.1% during 2015-16 and 2021-22 to 0.6% during 2017-18.

Figure 14. Influenza Mortality from the National Center for Health Statistics Mortality Surveillance System, 2020-21 to 2024-25 Seasons*

*Data presented are current as of September 9,2025.

CDC monitors influenza-associated deaths in children younger than 18 years of age through the Influenza-Associated Pediatric Mortality Surveillance System1. During September 29, 2024 - August 30, 2025, a total of 279 laboratory-confirmed influenza-associated pediatric deaths were reported to CDC (Figure 15); this is the highest number of deaths reported during a seasonal influenza epidemic. Since influenza-associated deaths in children became a nationally notifiable condition in 2004, the total number of influenza-associated deaths among children during one typical influenza season, excluding the 2020-21 season which was characterized by very low influenza virus circulation, has ranged from 37 during the 2011-12 season to 207 during the 2023-2024 season. This season, 239 of the reported deaths were associated with influenza A viruses, 38 were associated with influenza B viruses, 1 was associated with influenza A and B virus co-infection, and 1 was associated with an influenza virus with type not distinguished. Of 169 influenza A virus infections with a reported subtype, 95 (56.2%) were influenza A(H1N1), 73 (43.2%) were influenza A(H3N2), and 1 (0.6%) was an influenza A(H1N1) and A(H3N2) co-infection. Four deaths associated with influenza B viruses had a lineage reported, and all were B/Victoria lineage viruses. No pediatric deaths associated with influenza B/Yamagata have been reported since the 2017-18 season and the last detection of a B/Yamagata virus reported globally occurred in March 2020. The mean age at death for these children was 7 years (range = 2 weeks-17 years). Location of death was known for 277 (99.3%) children and adolescents; approximately half (143 [51.6%]) died after hospital admission; 73 (26.4%) died in an emergency room; and 61 (22.0%) died outside of a hospital setting. Among the 260 children and adolescents with a known medical history, 147 (56.5%) had at least one underlying medical condition associated with higher risk for developing serious influenza-related complications. Among the 207 children and adolescents who were eligible for influenza vaccination (aged ≥6 months at date of illness onset) and for whom vaccination status was known, 185 (89.4%) were not fully vaccinated against influenza.

Figure 15. Number of Influenza-Associated Pediatric Deaths by Virus Type and Week of Death, 2021-22 to 2024-25 Seasons*

^{*Data shown are current as of September 8, 2025, and include deaths occurring until August 30, 2025. Additional pediatric deaths that occurred during this time period may continue to be reported as they are identified. The total number of deaths will be updated in FluView and on FluView Interactive as they are received.}

Influenza Disease Burden and Prevented Burden from Vaccination

CDC does not know the exact number of people who have been sick from influenza viruses because not everyone who gets sick with influenza will seek medical care or be tested for influenza virus and even if they do, it is not a requirement that the healthcare visit or laboratory result be reported in most areas of the United States. Therefore, CDC uses a mathematical model to estimate the number of influenza illnesses, medical visits, hospitalizations, and deaths that occur each season6. Preliminary end of season estimates of influenza disease burden in the United States from September 29, 2024 through April 26, 2025, show that influenza virus infection likely resulted in between 43 million-73 million symptomatic illnesses, 19 million-32 million medical visits, 560,000-1,100,000 million hospitalizations, and 38,000-99,000 deaths. Older adults (≥65 years) accounted for 57% of hospitalizations.

In addition to estimating the disease burden, CDC also estimates the benefit of influenza vaccination by calculating the number of illnesses, medical visits, hospitalizations, and deaths that are prevented by vaccination. CDC uses a compartmental model to estimate the burden that would have occurred without vaccination and the burden that occurred with vaccination6. The difference between these two estimates is the vaccine prevented burden. During the 2024-25 season (September 29, 2024 - April 26, 2025), influenza vaccine coverage ranged from 35% among those 18-49 years to 71% among older adults ≥65 years7. Influenza vaccine effectiveness in outpatient settings ranged from 37% to 56%, and in inpatient settings, it ranged from 39% to 62%, depending on age group8. For the 2024-25 season, CDC preliminarily estimates that influenza vaccination prevented 9.4 million-16 million symptomatic illnesses, 4.4 million-7.1 million medical visits, 170,000-360,000 hospitalizations, and 12,000-39,000 deaths associated with influenza.

Both disease and prevented burden will be updated later in the calendar year and updated estimates will be posted to the CDC websites for disease burden and prevented burden910.

Seasonal Severity Assessment

CDC classifies influenza season severity at the end of each season by comparing influenza virus activity data, including influenza-like illness, influenza hospitalization rates, and influenza deaths, from past seasons11. Severity is classified across all age groups and for children, adults, and older adults. The 2024-25 U.S. season was classified as a high severity season across all age groups combined and for children, adults, and older adults. This was the first high severity U.S. season since the 2017-18 season.

Influenza Activity in the Southern Hemisphere

Influenza activity in the Southern Hemisphere follows a seasonal pattern which typically occurs between April and September and can sometimes persist through October or November. Based on data collected and submitted by countries to the Global Influenza Programme's FluNet system12, as of September 2025, influenza activity is low and, in most Southern Hemisphere countries, the influenza season has ended. During the 2025 influenza season, influenza A viruses predominated with the majority of subtyped specimens found to be A(H1N1)pdm09. However, of note, the most frequently detected viral subtype among African countries in the Southern Hemisphere had been A(H3N2). This finding was in contrast with the 2024 Southern Hemisphere season where most virus detections in Southern Hemisphere countries were influenza A(H3N2) and detection of A(H1N1)pdm09 predominated Southern African countries.

Reported data from four Southern Hemisphere countries, including Argentina and Paraguay in South America and South Africa and Mauritius in Africa, showed an early start to their 2025 influenza season as compared to prior season (2022-2024) trends. However, reported data from Australia (which provides most of the data for the Oceania region) and Brazil indicated a slightly later start to their 2025 influenza seasons. As of September 2025, moderate (Argentina, Bolivia, Brazil, Chile, Peru, and Uruguay) and high (Paraguay) influenza activity had been observed during the Southern Hemisphere influenza season in South America while most reporting Southern Hemisphere countries in Africa and Oceania experienced low (South Africa and Australia) to moderate levels of activity.

In the 2025 Southern Hemisphere influenza season, influenza virus detections among individuals hospitalized with severe acute respiratory infections (SARI) remained low in South Africa13. Among South American countries, the number of SARI cases reached high (Ecuador and Brazil) to very high (Peru, Paraguay and Uruguay) levels14. In Australia, the number of patients admitted with SARI reached a peak in late June 2025 and has steadily declined through September 202515; the number of influenza virus detections among individuals hospitalized with SARI in the 2025 influenza season was higher than in the 2024 season16.

Discussion

The 2024-25 influenza season in the United States was a high severity season marked by elevated activity across multiple surveillance systems. National preliminary burden estimates suggest that influenza virus infections resulted in at least 43 million symptomatic illnesses, 19 million medical visits, 560,000 hospitalizations, and 38,000 deaths. These figures surpassed all recent influenza seasons, except for the number of deaths during the high severity 2017-18 season17.

Influenza A viruses predominated throughout the season, with nearly equal co-circulation of A(H1N1)pdm09 and A(H3N2) viruses overall, 53.1% and 46.9% of subtyped influenza A viruses, respectively. Until mid-January, A(H3N2) viruses were reported more frequently than A(H1N1)pdm09 viruses each week while the reverse was true from that point on. Influenza B viruses increased slightly later in the season but remained at lower overall levels compared to influenza A. Historically, A(H1N1) viruses have been the predominant virus in 8 of the past 26 seasons (1997-98 through 2023-24, excluding the 2020-21 season due to low influenza virus circulation, A(H3N2) in 16 seasons, and equal proportions of A(H1N1) and B Victoria in one season.

Genetic and antigenic characterization of circulating viruses showed that most A(H1N1)pdm09 and B/Victoria lineage viruses were similar to the 2024-25 Northern Hemisphere vaccine reference viruses. However, some A(H3N2) viruses showed antigenic drift due to hemagglutinin (HA) substitutions, potentially contributing to somewhat lower vaccine effectiveness (VE) against this subtype. Overall, VE was moderate to low, with higher protection against A(H1N1)pdm09 and B viruses18. Almost all (>99%) influenza viruses tested were susceptible to the neuraminidase inhibitors (e.g., oseltamivir, peramivir, zanamivir) and the PA endonuclease inhibitor baloxavir.

This season was marked by high severity; with levels of activity surpassing those observed in both recent and most pre-COVID-19 pandemic seasons. Clinical laboratories tested over 3.9 million respiratory specimens, with 12.3% testing positive for influenza viruses overall and peak weekly test positivity reaching 31.6%, the highest weekly percentage reported since the 2015-16 season. Influenza A viruses predominated, with the percent of specimens testing positive for influenza A peaking at 30.4% in late January, while influenza B viruses co-circulated at lower levels and later in the season with peak influenza B percent positivity (4.3%) occurring in late March. ILI activity peaked at 7.9% (4.9 percentage points above the baseline) and remained above baseline for 17 consecutive weeks. The number of weeks of elevated activity is similar to previous seasons while the peak percent of visits for ILI and the difference between the peak and baseline is the highest and third highest, respectively, since the 2009-2010 season. Additionally, the timing of activity peaks for ILINet across the HHS regions ranged from late December through early February while peak percent of specimens testing positive for influenza occurred during the two weeks in late January and early February. This difference may be due to the circulation of other respiratory viruses causing ILI occurring at slightly different times across the country. The overall cumulative influenza-associated hospitalization rate reached recorded in FluSurv-Net reached the highest level since the 2010-11 season, and the weekly peak rate reported to NHSN HRD was the highest since reporting began in the 2020-21 season.

Mortality surveillance further illustrated the season's severity. The weekly percentage of all deaths attributed to influenza peaked at 2.8%, which is the highest peak since the 2015-16 season and substantially higher than the previous two seasons (1.4% in 2023-24 and 1.6% in 2022-23). Pediatric mortality was particularly notable, with 279 influenza-associated pediatric deaths reported. This is the highest number of deaths reported during a seasonal influenza epidemic since the system began in 2004. While over half of pediatric deaths occurred in children and adolescents with underlying conditions, 44% occurred among previously healthy children. Among the deaths in children aged ≥6 months with known vaccination status approximately 90% were not fully vaccinated against influenza.

Globally, influenza activity during the Southern Hemisphere season showed regional variability in virus subtype predominance and timing. Early and intense influenza activity in some countries further underscores the unpredictable nature of seasonal influenza; however, activity in the Southern hemisphere cannot be used to definitively predict the timing or intensity of influenza activity in the United States this fall and winter.

Though influenza activity has remained low in the summer, maintaining influenza surveillance year-round is important. Sporadic human infections with novel influenza A virus subtypes distinct from currently circulating seasonal strains were reported during the 2024-25 season. These included 56 confirmed avian influenza A(H5) virus infections, most linked to exposure to infected dairy cattle or poultry, and one swine origin variant influenza A(H1N2v) virus infection. While the majority of cases were mild and no human-to-human transmission was identified, these detections underscore the continued threat posed by zoonotic influenza viruses. As outbreaks of highly pathogenic avian influenza A(H5N1) persist among animals, including poultry and dairy cows, healthcare providers and individuals exposed to potentially infected livestock and other animals should remain alert to symptoms such as shortness of breath, fever, cough, or conjunctivitis19. Prompt identification of suspected novel influenza A virus infections, patient isolation, specimen collection for public health testing, and immediate initiation of antiviral treatment remain critical to preventing further spread and mitigating pandemic risk2021.

The 2024-25 influenza season was among the most severe in recent years, characterized by widespread co-circulation of multiple influenza A virus subtypes and substantial morbidity and mortality. These findings reinforce the critical importance of annual influenza vaccination, timely antiviral treatment, and robust surveillance systems to mitigate the public health impact of influenza, particularly for high-risk populations. CDC continues to recommend that everyone aged 6 months and older receive a seasonal influenza vaccine each year, as annual vaccination remains the most effective way to protect against influenza and its potentially serious complications2223. CDC also recommends initiating influenza antiviral drug treatment as soon as possible for patients with confirmed or suspected influenza virus infection who have severe, complicated, or progressive illness; who require hospitalization; or who are at increased risk for influenza-associated complications24. Four influenza antiviral drugs approved by the Food and Drug Administration are currently recommended for use in the United States25.

Contributors: S Moon, PhD, MPH. A Busbee, MPH. A Merced-Morales, MPH. R Smith, MPH. Y. Zhou, MPH. K Reinhart, PhD, MPH. A Colón, MPH. D Hawkins, MPH. N Dempster, MPH. A Howa, PhD, MPH. S Huang, MPH. YC Pun, MPHP. C Zhang, MPH. AD Iuliano, PhD, MPH. R Threlkel, MPH. C Bozio, PhD, MPH. S Garg, MD. J Habeck, MPH. A O'Halloran, MSPH. A Kardorff, MPH. C Cummings, MPH. D Ujamaa, MS. C Castagna-McLeod, MPH. L Gubareva, MD, PhD. S Russ, PhD. L Duca, PhD. E Azziz-Baumgartner, MD. R Kondor, PhD. X Zheng, PhD. D Sundaresan, MS, MPH. P Daly, MPH. K Kniss, MPH. A Budd, MPH; Influenza Division.

Acknowledgements:

State, county, city, and territorial health departments and public health laboratories; U.S. World Health Organization collaborating laboratories; National Respiratory and Enteric Virus Surveillance System laboratories; U.S. Outpatient Influenza-Like Illness Surveillance (ILI) Network sites; Influenza Hospitalization Surveillance Network (FluSurv-NET) sites; Division of Healthcare Quality Promotion, CDC; National Center for Health Statistics, CDC; Matthew Gilmer, Sarabeth Mathis, Young M Yoo, T Davis, PhD, A Abd Elal; National Center for Immunization and Respiratory Diseases, CDC.