Weekly US Influenza Surveillance Report: Key Updates for Week 40, ending October 10, 2026

Summary

All data 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. 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 HHS Regions 4, 6, 7, 8, and 9, the percentage of respiratory specimens testing positive for influenza virus in clinical laboratories increased (change of at least 0.5 percentage points) compared to the previous week. In HHS Regions 1, 2, 3, 5, and 10, the percentage remained stable. Influenza A(H1N1)pdm09 viruses were the most frequently reported by public health laboratories this week. For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent receipt of live attenuated influenza vaccine (LAIV) or found upon further testing to be a vaccine virus are not included, as they are not circulating influenza viruses.

Clinical Laboratories

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

Public Health Laboratories

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

_{^†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 person-to-person transmission can occur. It is important to note that in most cases, variant influenza viruses have not shown the ability to spread easily and sustainably from person to person.}

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

_{This graph reflects the number of specimens determined to be positive for influenza viruses at the public health laboratory (specimens tested are not the same as cases). It does not reflect specimens tested only at CDC and could include more than one specimen tested per person. Specimens tested as part of routine influenza surveillance as well as those tested as part of targeted testing for people exposed to influenza A(H5) are included.}

Novel Influenza A Virus Infections

No new confirmed human infections with a novel influenza virus were reported to CDC as of October 14, 2026.

Novel influenza A virus infections include human infections with influenza A viruses that are different from currently circulating human seasonal influenza A(H1) and A(H3) viruses or avian influenza A viruses that have sporadically infected people.

An up-to-date summary of confirmed novel influenza virus cases is available at https://gis.cdc.gov/grasp/fluview/Novel_Influenza.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 416 influenza viruses collected since May 24, 2026.

Genetic Characterization of Hemagglutinin Genes from Influenza Viruses Collected since May 24, 2026

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) assay (A(H1N1)pdm09, A(H3N2), and B/Victoria viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2026-2027 Northern Hemisphere recommended cell-, recombinant- and nucleic acid-based influenza vaccine viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to the reference (vaccine) virus. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than 8-fold. Viruses selected for antigenic characterization are a subset of the recent genetically characterized viruses and are chosen based on the genetic changes in their surface proteins and may not be proportional to the number of such viruses circulating in the United States.

Influenza A Viruses

• A (H1N1)pdm09: 74 A(H1N1)pdm09 viruses were antigenically characterized by HI, and 70 (94.6%) were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown A/Missouri/11/2025-like reference viruses representing the A(H1N1)pdm09 component for the cell-, recombinant- and nucleic acid-based influenza vaccines.
• A (H3N2): 20 A(H3N2) viruses were antigenically characterized by HI or HINT, and 20 (100%) 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/Darwin/1415/2025-like reference viruses representing the A(H3N2) component for the cell-, recombinant- and nucleic acid-based influenza vaccines.

Influenza B Viruses

• B/Victoria: 30 influenza B/Victoria-lineage viruses were antigenically characterized by HI, and 30 (100%) were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Pennsylvania/14/2025-like reference viruses representing the B/Victoria component for the cell-, recombinant- and nucleic acid-based influenza vaccines.

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 May 24, 2026, were tested for antiviral susceptibility as follows:

Eight A(H1N1)pdm09 viruses had amino acid substitutions NA-I223V and NA-S247N and showed reduced inhibition by oseltamivir.

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

ILINet

The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient and emergency department visits for respiratory illness referred to as influenza-like illness [ILI (fever plus cough or sore throat)], not laboratory-confirmed influenza, and can therefore capture respiratory illness visits due to infection with any pathogen that can present with similar symptoms. 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 picture.

Nationally, during Week 40, 2.1% of patient visits reported through ILINet were due to ILI. This week's national percentage increased (change of > 0.1 percentage points) compared to Week 39 and is below the national baseline of 2.9%. ILI activity increased in HHS regions 1, 2, 3, 4, 6, 8, and 9 and remained stable (change of ≤ 0.1 percentage points) in regions 5, 7, and 10 compared to Week 39. All regions are below their respective baselines. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

_{*Some calendar years do not include an epidemiologic Week 53. In those years, the Week 53 value shown is the average between Week 52 and Week 1.}

_{**Effective October 3, 2021 (Week 40), the respiratory illness definition (fever plus cough or sore throat) no longer includes "without a known cause other than influenza."}

The percentage of visits for respiratory illness increased (change of > 0.1 percentage points) in the 0-4 years and 5-24 years age groups and remained stable (change of ≤ 0.1 percentage points) in the 25-49 years, 50-64 years, and 65 years and older age groups this week compared to the previous week.

ILI 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 0.8% during Week 40 and increased (change of > 0.1 percentage point) compared to the previous week. Nationally, the percentage of ED visits with a DD of influenza increased among the 0-4 years and 5-17 years age groups and remained stable among the 18-64 years and 65 years and older age groups. The percentage of ED visits with a DD of influenza increased this week compared to the previous week in HHS regions 4, 6, and 9, and remained stable in all other regions (1, 2, 3, 5, 7, 8, and 10). The age group trends varied by region but most regions reported a pattern similar to the national data.

Hospitalization Surveillance

FluSurv-NET

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in select counties in 14 states and represents approximately 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 256 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 4, 2026, and October 10, 2026. The weekly hospitalization rate observed during Week 40 was 0.7 per 100,000 population. After accounting for reporting delays, the estimated rate during Week 40 was 1.2 per 100,000 population, and weekly rates have continued to increase in recent weeks.

Among all hospitalizations, 247 (96.5%) were associated with influenza A virus and 9 (3.5%) with influenza B virus. Among those with influenza A subtype information, 2 (6.1%) were A(H3N2), and 31 (93.9%) 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 (2.3), followed by adults aged 50-64 years (0.8), children aged 0-4 years (0.6), children aged 5-17 years (0.2), and adults aged 18-49 years (0.2).

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 40, 4,024 laboratory-confirmed influenza-associated hospitalizations were reported. This week's influenza-associated hospital admission rate (1.2 per 100,000 population) remained stable (difference of < 0.2) compared to Week 39.

Laboratory-confirmed influenza-associated hospital admission rates per 100,000 population increased in HHS regions 2, 5, and 6 and remained stable in all other regions this week compared to the previous week. Regional admission rates during Week 40 ranged from 0.5 (Region 7) to 1.5 (Region 4).

When examining rates by age for Week 40, the 0-4 years and 65 years and older age groups increased and all other age groups (5-17 years, 18-49 years, and 50-64 years) remained stable compared to the previous week. The highest hospital admission rate per 100,000 population was among those 65 years and older (3.8), followed by the 0-4 years age group (1.3) and the 50-64 years age group (1.1).

Mortality Surveillance

National Center for Health Statistics (NCHS)

Based on NCHS mortality surveillance data available on October 15, 2026, 0.2% of the deaths that occurred during the week ending October 10, 2026 (Week 40) were due to influenza. This percentage increased (≥ 0.1 percentage point change) compared to Week 39. The data presented are preliminary and may change as more data are received and processed.

Influenza-Associated Pediatric Mortality

No influenza-associated pediatric deaths occurring during the 2026-2027 season have been reported to CDC.

Additional National and International Influenza Surveillance Information