Weekly US Influenza Surveillance Report: Key Updates for Week 51, ending December 20, 2025

Summary

Viruses

Illness

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

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

A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.¹

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

U.S. virologic surveillance

Nationally and in all ten HHS regions the percentage of respiratory specimens testing positive for influenza virus in clinical laboratories increased (change of at least 0.5 percentage points) in Week 51 compared to Week 50. Region 8 had the highest percent positivity (34.9%) and Region 9 had the lowest (10.8%). Influenza A(H3N2) viruses were the most frequently reported influenza viruses this week; however, the distribution of circulating viruses varies by region. For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent receipt of live attenuated influenza vaccine (LAIV) or found upon further testing to be a vaccine virus are not included, as they are not circulating influenza viruses.

Clinical Laboratories

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

Public Health Laboratories

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

_{*These data reflect specimens tested, and the number determined to be positive for influenza viruses at the public health labs (specimens tested is not the same as cases). The data do not reflect specimens tested only at CDC and could include more than one specimen tested per person. For more information on the number of people infected with A/H5 viruses, please visit the "How CDC is monitoring influenza data among people to better understand the current avian influenza A (H5N1) situation"}

_{^†When an influenza virus that normally circulates in swine (but not people) is detected in a person, it is called a "variant" influenza virus. Most human infections with variant influenza viruses occur following exposure to swine, but human-to-human transmission can occur. It is important to note that in most cases, variant influenza viruses have not shown the ability to spread easily and sustainably from human-to-human.}

_{*This graph reflects the number of specimens 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 avian influenza A(H5) are included.}

Novel Influenza A Virus Infections

No new confirmed human infections with avian influenza A(H5) virus were reported to CDC this week. To date, person-to-person 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 current 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.

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 457 influenza viruses collected since September 28, 2025.

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) assay (H1N1pdm09, H3N2, and B/Victoria viruses) or neutralization-based HINT (H3N2 viruses) using antisera from ferrets infected with reference viruses representing the recommended cell-based or recombinant influenza vaccines for the 2025-2026 Northern Hemisphere season. Antigenic differences between viruses are determined by comparing how well the antibodies raised against the vaccine reference viruses recognize the circulating viruses, which were 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 are deemed antigenically similar when their HI titer differences are less than or equal to 4-fold. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than 8-fold. Circulating viruses with antigenic testing results that show titer differences greater than 4-fold by HI or equal to or greater than 8-fold by HINT) are considered "low reactors" or antigenically drifted compared to the vaccine virus. From the recent genetically characterized viruses, a subset is selected for antigenic characterization based on identified genetic changes in their surface proteins. The subset tested may not be proportional to the number of such viruses circulating in the United States.

Influenza A Viruses

• A(H1N1)pdm09: 47 A(H1N1)pdm09 viruses collected since September 28, 2025 were antigenically characterized by HI, and 46 (97.9%) 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): 35 A(H3N2) viruses collected since September 28, 2025 were antigenically characterized by HI or HINT, and 3 (8.6%) 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/District Of Columbia/27/2023-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.

Influenza B Viruses

• B/Victoria: 12 influenza B/Victoria-lineage viruses collected since September 28, 2025 since were antigenically characterized by HI, and 8 (66.7%) were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines.
• B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.
  

Assessment of Virus Susceptibility to Antiviral Medications

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

Viruses collected in the U.S. since September 28, 2025, were tested for antiviral susceptibility as follows:

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 51, 6.0% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This week's percentage increased (change of > 0.1 percentage points) compared to Week 50 and is above the national baseline of 3.1% for the third consecutive week. The percentage of visits for ILI increased in all ten HHS Regions this week compared to last, and all ten HHS Regions are above their respective regional baselines in Week 51. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

Outpatient Respiratory Illness Visits by Age Group

About 70% of ILINet participants provide both the number of patient visits for respiratory illness and the total number of patient visits for the week broken out by age group. Based on these data, the percentage of visits for respiratory illness increased (change of > 0.1 percentage points) in all age groups (0-4 years, 5-24 years, 25-49 years, 50-64 years, and 65 years and older) in Week 51 compared to Week 50.

Outpatient Respiratory Illness Activity Map

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

National Syndromic Surveillance System (NSSP)

The national percentage of emergency department (ED) visits with a discharge diagnosis (DD) of influenza reported in NSSP was 5.4% during Week 51 and increased (change of > 0.1 percentage point) compared to the previous week. The percentage of ED visits with a DD of influenza increased this week compared to the previous week among all age groups (0-4 years, 5-17 years, 18-64 years, and 65 years and older) and in all ten HHS regions.

Hospitalization surveillance

FluSurv-Net

Influenza-Associated Hospitalizations: 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 10% of the U.S. population. FluSurv-NET hospitalization data are preliminary. As data are received each week, prior case counts and rates are updated accordingly.

A total of 6,349 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2025, and December 20, 2025. The weekly hospitalization rate observed during Week 51 was 6.2 per 100,000 population, which increased from a rate of 4.4 per 100,000 population last week. The cumulative hospitalization rate observed in Week 51 was 18.2 per 100,000 population. This is the third highest cumulative rate at Week 51 since the 2010-11 season following the 2022-23 and 2023-24 seasons, with rates of 48.0 and 20.3 respectively

Among all hospitalizations, 6,112 (96.3%) were associated with influenza A virus, 191 (3.0%) with influenza B virus, 7 (0.1%) with influenza A virus and influenza B virus co-infection, and 39 (0.6%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 1,155 (87.4%) were A(H3N2), and 167 (12.6%) were A(H1N1)pdm09.

When examining rates by age, the highest rate of hospitalization per 100,000 population was among adults aged 65 and older (53.4), followed by children aged 0-4 years (21.5), adults aged 50-64 years (16.4), children aged 5-17 years (11.8), and adults aged 18-49 years (7.7).

When examining age-adjusted rates by race and ethnicity, the highest rate of hospitalization per 100,000 population was among non-Hispanic Black persons (35.9), followed by American Indian or Alaska Native persons (20.9), Hispanic persons (16.5), non-Hispanic White persons (14.4), and Asian and/or Pacific Islander persons (8.5).

Additional FluSurv-NET data are available on FluView Interactive including hospitalization rates for the current and past seasons by age, sex, and race/ethnicity (http://gis.cdc.gov/GRASP/Fluview/FluHospRates.html) as well as data on patient characteristics at: (http://gis.cdc.gov/grasp/fluview/FluHospChars.html.)

FluSurv-NET data are used to generate national estimates of the total numbers of influenza cases, medical visits, hospitalizations and deaths. This season, CDC is reporting preliminary cumulative in-season estimates, which are available at https://www.cdc.gov/flu/about/burden/preliminary-in-season-estimates.htm.

_{**In this figure, weekly rates for all seasons prior to the 2025-26 season reflect end-of-season rates. For the 2025-26 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.}

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 51, 19,053 laboratory-confirmed influenza-associated hospitalizations were reported. This week's influenza-associated hospital admission rate (5.6 per 100,000 population) increased (difference of ≥ 0.2) compared to Week 50.

Laboratory confirmed influenza-associated hospital admission rates per 100,000 population increased in all HHS regions. Admission rates ranged from 1.9 (Region 10) to 11.6 (Region 2) during Week 51.

When examining rates by age for Week 51, the hospitalization rate among all age groups increased (difference of ≥ 0.2). The highest hospital admission rate per 100,000 population was among those 65 years and older (16.6), followed by children aged 0-4 years (6.7), and adults aged 50-64 years age groups (4.4).

National Healthcare Safety Network (NHSN) Long-Term Care Respiratory Pathogens & Vaccination Module

Long-term care facilities (LTCFs [e.g., Nursing homes/skilled nursing facilities]) report respiratory pathogen (e.g., COVID-19, influenza and RSV) data, including vaccination, cases, and hospitalizations among residents, to the NHSN Long-Term Care Respiratory Pathogens & Vaccination Module.

Nationally, during Week 51, the hospitalization rate for residents with a positive influenza test in the prior 10 days was 22.9 per 100,000 residents. The national rate and rate for HHS regions 1, 2, 3, 4, 5, 9, and 10 have been increasing over the past several weeks. In regions 6, 7 and 8, the rate does not show a consistent trend.

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Mortality surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on December 29, 2025, 0.5% of the deaths that occurred during the week ending December 20, 2025 (Week 51), were due to influenza. This percentage increased (≥ 0.1 percentage point change) compared to Week 50. The data presented are preliminary and may change as more data are received and processed.

Influenza-Associated Pediatric Mortality

Five influenza-associated pediatric deaths occurring during the 2025-2026 season were reported to CDC during Week 51. The deaths occurred during weeks 49, 50 and 51 (the weeks ending December 6, December 13, and December 20 of 2025). All five deaths were associated with influenza A viruses. All of the influenza A viruses had subtyping performed; one was an A(H1N1) virus and four were A(H3N2) viruses.

A total of eight influenza-associated pediatric deaths occurring during the 2025-2026 season have been reported to CDC.

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

A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component, is available on the surveillance methods page.¹

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

Additional National and International Influenza Surveillance Information

Indicators Status by System

Additional surveillance information