Key points
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 52 compared to Week 51. Region 8 had the highest percent positivity (45.5%) and Region 10 had the lowest (16.9%). 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.
| Week 52 | Data Cumulative since September 28, 2025 (Week 40) |
|
|---|---|---|
| No. of specimens tested | 70,757 | 853,101 |
| No. of positive specimens (%) | 23,271 (32.9%) | 82,349 (9.7%) |
| Positive specimens by type | ||
| Influenza A | 21,587 (92.8%) | 77,309 (93.9%) |
| Influenza B | 1,684 (7.2%) | 5,040 (6.1%) |
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.
| Week 52 | Data Cumulative since September 28, 2025 (Week 40) |
|
|---|---|---|
| No. of specimens tested | 1,317 | 22,185 |
| No. of positive specimens | 994 | 12,057 |
| Positive specimens by type/subtype | ||
| Influenza A | 971 (97.7%) | 11,652 (96.6%) |
| Subtyping Performed | 600 (61.8%) | 10,571 (90.7%) |
| (H1N1)pdm09 | 53 (8.8%) | 1,534 (14.5%) |
| H3N2 | 547 (91.2%) | 9,035 (85.5%) |
| H3N2v† | 0 | 0 |
| H5* | 0 | 2 (<0.1%) |
| Subtyping not performed | 371 (38.2%) | 1,081 (9.3%) |
| Influenza B | 23 (2.3%) | 405 (3.4%) |
| Lineage testing performed | 8 (34.8%) | 115 (28.4%) |
| Yamagata lineage | 0 | 0 |
| Victoria lineage | 8 (100%) | 115 (100%) |
| Lineage not performed | 15 (65.2%) | 290 (71.6%) |
*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.
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 634 influenza viruses collected since September 28, 2025.
| Virus Subtype or Lineage | Genetic Characterization | ||||
|---|---|---|---|---|---|
| Total No. of Subtype/Lineage Tested |
HA Clade |
Number (% of subtype/lineage tested) |
HA Subclade |
Number (% of subtype/lineage tested) |
|
| A/H1 | 189 | ||||
| 5a.2a | 2 (1.1%) | C.1.9.3 | 2 (1.1%) | ||
| 5a.2a.1 | 187 (98.9%) | D.3.1 | 93 (49.2%) | ||
| D.3.1.1 | 94 (49.7%) | ||||
| A/H3 | 389 | ||||
| 2a.3a.1 | 389 (100%) | J.2 | 2 (0.5%) | ||
| J.2.2 | 5 (1.3%) | ||||
| J.2.3 | 20 (5.1%) | ||||
| J.2.4 | 10 (2.6%) | ||||
| K | 352 (90.5%) | ||||
| B/Victoria | 56 | ||||
| 3a.2 | 56 (100%) | C.3.1 | 33 (58.9%) | ||
| C.5.1 | 7 (12.5%) | ||||
| C.5.6 | 7 (12.5%) | ||||
| C.5.6.1 | 5 (8.9%) | ||||
| C.5.7 | 4 (7.1%) | ||||
| B/Yamagata | 0 | ||||
| Y3 | 0 | Y3 | 0 | ||
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): 59 A(H3N2) viruses collected since September 28, 2025 were antigenically characterized by HI or HINT, and 3 (5.1%) 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 United States since May 18, 2025, were tested for antiviral susceptibility are as follows:
| Antiviral Medication | Total Viruses | A/H1 | A/H3 | B/Victoria | ||
|---|---|---|---|---|---|---|
| Neuraminidase Inhibitors | Oseltamivir | Viruses Tested | 619 | 189 | 376 | 54 |
| Reduced Inhibition | 0 | 0 | 0 | 0 | ||
| Highly Reduced Inhibition | 0 | 0 | 0 | 0 | ||
| Peramivir | Viruses Tested | 619 | 189 | 376 | 54 | |
| Reduced Inhibition | 0 | 0 | 0 | 0 | ||
| Highly Reduced Inhibition | 0 | 0 | 0 | 0 | ||
| Zanamivir | Viruses Tested | 619 | 189 | 376 | 54 | |
| Reduced Inhibition | 0 | 0 | 0 | 0 | ||
| Highly Reduced Inhibition | 0 | 0 | 0 | 0 | ||
| PA Cap-Dependent Endonuclease Inhibitor | Baloxavir | Viruses Tested | 542 | 156 | 342 | 44 |
| Decreased Susceptibility | 0 | 0 | 0 | 0 | ||
High levels of resistance to the adamantanes (amantadine and rimantadine) persist among influenza A(H1N1)pdm09 and influenza A(H3N2) viruses (the adamantanes are not effective against influenza B viruses). Therefore, use of these antivirals for treatment and prevention of influenza 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 52, 8.2% 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 increased (change of > 0.1 percentage points) compared to Week 51 and is above the national baseline of 3.1% for the fourth consecutive week. The increase in the percentage of patient visits for ILI may be influenced in part by a reduction in routine healthcare visits during the winter holidays, as has occurred during previous seasons. The percentage of visits for ILI increased in all ten HHS regions this week compared to last. All ten regions are above their respective regional baselines this week. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.
Outpatient Respiratory Illness Visits by Age Group
About 70% of ILINet participants provide both the number of patient visits for respiratory illness and the total number of patient visits for the week broken out by age group. 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 52 compared to Week 51.
Outpatient Respiratory Illness Activity Map
Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).
| Activity Level | Number of Jurisdictions | Number of CBSAs | ||
|---|---|---|---|---|
| Week 52 (Week ending Dec. 27, 2025) |
Week 51 (Week ending Dec. 20, 2025) |
Week 52 (Week ending Dec. 27, 2025) |
Week 51 (Week ending Dec. 20, 2025) |
|
| Very High | 32 | 22 | 134 | 64 |
| High | 16 | 11 | 217 | 136 |
| Moderate | 2 | 8 | 102 | 121 |
| Low | 3 | 10 | 137 | 173 |
| Minimal | 1 | 4 | 95 | 216 |
| Insufficient Data | 1 | 0 | 244 | 219 |
*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.
National Syndromic Surveillance System (NSSP)
The national percentage of emergency department (ED) visits with a discharge diagnosis (DD) of influenza reported in NSSP was 8.3% during Week 52 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, 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 9,809 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2025, and December 27, 2025. The weekly hospitalization rate observed during Week 52 was 8.4 per 100,000 population, which increased from a rate of 7.6 per 100,000 population last week. The cumulative hospitalization rate observed in Week 52 was 28.1 per 100,000 population. This is the third highest cumulative rate at Week 52 since the 2010-11 season following the 2022-23 and 2023-24 seasons, with rates of 53.2 and 29.2 respectively.
Among all hospitalizations, 9,506 (96.9%) were associated with influenza A virus, 252 (2.6%) with influenza B virus, 9 (0.1%) with influenza A virus and influenza B virus co-infection, and 42 (0.4%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 1729 (88.9%) were A(H3N2), and 216 (11.1%) 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 (85.0), followed by children aged 0-4 years (32.6), adults aged 50-64 years (24.9), children aged 5-17 years (16.5), and adults aged 18-49 years (11.9).
When examining age-adjusted rates by race and ethnicity, the highest rate of hospitalization per 100,000 population was among non-Hispanic Black persons (57.0), followed by American Indian or Alaska Native persons (27.0), Hispanic persons (24.2), non-Hispanic White persons (22.6), and Asian and/or Pacific Islander persons (12.8).
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.
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 52, 33,301 laboratory-confirmed influenza-associated hospitalizations were reported. This week's influenza-associated hospital admission rate (9.9 per 100,000 population) increased (difference of ≥ 0.2) compared to Week 51.
Laboratory confirmed influenza-associated hospital admission rates per 100,000 population increased in all HHS regions. Admission rates ranged from 3.4 (Region 9) to 15.0 (Region 2) during Week 52.
When examining rates by age for Week 52, 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 (31.6), followed by children aged 0-4 years (10.8), and adults aged 50-64 years age groups (7.7).
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 52, the hospitalization rate for residents with a positive influenza test in the prior 10 days was 31.3 per 100,000 residents. The national rate and rate for HHS regions 1, 2, 3, 4, 5, 9, and 10 have been trending upward over the past several weeks. In regions 6, 7, and 8 the rate does not show a consistent trend.
Mortality surveillance
National Center for Health Statistics (NCHS) Mortality Surveillance
Based on NCHS mortality surveillance data available on December 31, 2025, 0.9% of the deaths that occurred during the week ending December 27, 2025 (Week 52), were due to influenza. This percentage increased (≥ 0.1 percentage point change) compared to Week 51. The data presented are preliminary and may change as more data are received and processed.
Influenza-Associated Pediatric Mortality
Two influenza-associated pediatric deaths were reported to CDC during Week 52.
One death occurred during Week 51 (the week ending December 20, 2025) and was associated with an influenza A(H3) virus. A total of nine influenza-associated pediatric deaths occurring during the 2025-2026 season have been reported to CDC.
One death occurring during the 2024–2025 season was also reported, bringing the total number of pediatric deaths for last season to 289. This death was associated with an influenza A virus for which no subtyping was performed and occurred during Week 11 of 2025 (the week ending March 15, 2025).
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
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 0.2 in the rate of hospital admissions or greater than or equal to 691 patients admitted with laboratory-confirmed influenza compared to the previous week.
NHSN Long- Term Care (LTC): Up or down arrows indicate change of greater than or equal to 5% in hospitalization rates for residents in LTC facilities who were hospitalized 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.
U.S. State and local influenza surveillance: Select a jurisdiction below to access the latest local influenza information.
Public Health Agency of Canada:
The most up-to-date influenza information from Canada is available in Canada's weekly FluWatch report.
Public Health England:
The most up-to-date influenza information from the United Kingdom is available from Public Health England.
Any links provided to non-Federal organizations are provided solely as a service to our users. These links do not constitute an endorsement of these organizations or their programs by CDC or the Federal Government, and none should be inferred. CDC is not responsible for the content of the individual organization web pages found at these links.
A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.