Weekly U.S. Influenza Surveillance Report

FluView a weekly influenza surveillance report prepared by the Influenza Division" />

Influenza A(H1N1)pdm09 and A(H3N2) were co-circulating this week.

Genetic and antigenic characterization and antiviral susceptibility are summarized in this report.

Illness

1.9%

(Trend )

of visits to a health care provider this week were for respiratory illness
(below baseline).

1 Moderate

0 High or Very High

0 per 100,000

weekly hospitalization rate.

Because of the ongoing COVID-19 pandemic, this system will suspend data collection for the 2020-21 influenza season.

No data this week.

2

influenza-associated deaths were reported this week for a total of 197 deaths this season.

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

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

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

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

Key Points

• Seasonal influenza activity remains low nationally.
• CDC estimates that there have been at least 35 million illnesses, 400,000 hospitalizations, and 25,000 deaths from flu so far this season.
• There are prescription flu antiviral drugs that can treat flu illness; those should be started as early as possible and are especially important for higher risk patients. 1
• Seasonal flu viruses are among several viruses contributing to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, flu, and RSV activity on a weekly basis.

U.S. Virologic Surveillance

Nationally, the percentage of respiratory specimens testing positive for influenza in clinical laboratories remained stable (change of ≤0.5 percentage points) compared to the previous week. Nationally, influenza A(H1N1)pdm09, A(H3N2), and B/Victoria viruses are all co-circulating. However, the distribution of circulating viruses varies by region. For regional and state level data and age group distribution, please visit FluView Interactive.

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.

* This data reflects specimens tested and the number determined to be positive for influenza viruses at the public health labs (specimens tested is not the same as cases). It does not reflect specimens tested only at CDC and could include more than one specimen tested per person. The guidance for influenza A/H5 testing recommends testing both a conjunctival and respiratory swab for people with conjunctivitis which has resulted in more specimens testing positive for influenza A/H5 than the number of human H5 cases. For more information on the number of people infected with A/H5, please visit the “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 tested and the number determined to be positive for influenza viruses at the public health lab (specimens tested is not the same as cases). It does not reflect specimens tested only at CDC and could include more than one specimen tested per person. Specimens tested as part of routine influenza surveillance as well as those tested as part of targeted testing for people exposed to influenza A(H5) are included.

Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data

Novel Influenza A Virus:

No new human infections with novel influenza A viruses were reported during Week 35.

During the 2023-2024 influenza season, a total of 13 cases of human infections with influenza A (H5) virus have been reported in the United States. Four of these occurred in individuals working with dairy cows and nine in individuals associated with poultry depopulation and disposal. An ongoing outbreak of H5N1 continues in domestic dairy cows and poultry, and monitoring for additional human cases is ongoing.

Seven variant influenza virus cases were also reported during the 2023-2024 season (four A(H1N2)v, two A(H3N2)v, and one A(H1N1)v virus), for a total of 20 novel influenza A virus cases reported this season.

Information about avian influenza is available at https://www.cdc.gov/flu/avianflu/index.htm.

Interim recommendations for Prevention, Monitoring, and Public Health Investigations are available at https://www.cdc.gov/bird-flu/prevention/hpai-interim-recommendations.html.

The latest case reports on avian influenza outbreaks in wild birds, commercial poultry, backyard or hobbyist flocks, and mammals in the United States are available from the USDA at https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/animal-disease-information/avian/avian-influenza/2022-hpai .

Additional information on influenza in swine, variant influenza virus infection in humans, and guidance to interact safely with swine can be found at www.cdc.gov/flu/swineflu/index.htm.

Additional information regarding human infections with novel influenza A viruses:

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 to the reference viruses representing viruses contained in 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 PA endonuclease inhibitor baloxavir.

CDC has genetically characterized 5,348 influenza viruses collected since October 1, 2023.

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) (H1N1pdm09, H3N2, B/Victoria, and B/Yamagata viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2023-2024 Northern Hemisphere recommended cell or recombinant-based vaccine viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to the reference (vaccine) virus. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than or equal to 8-fold. Viruses selected for antigenic characterization are a subset representing the genetic changes in the surface proteins seen in genetically characterized viruses.

Influenza A Viruses

• A (H1N1)pdm09: 574 A(H1N1)pdm09 viruses were antigenically characterized by HI, and 573 (99.8%) 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): 665 A(H3N2) viruses were antigenically characterized by HI or HINT, and 629 (94.6%) 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 or equal to 8-fold of the homologous virus in HINT) by ferret antisera to cell-grown A/Darwin/6/2021-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.

Influenza B Viruses

• B/Victoria: 436 influenza B/Victoria-lineage virus were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines.
• B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.

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 October 1, 2023, were tested for antiviral susceptibility as follows:

Four A(H1N1)pdm09 viruses had NA-H275Y amino acid substitution and one A(H1N1)pdm09 virus had NA-H275Y/H, conferring highly reduced inhibition by oseltamivir and peramivir. One (H1N1)pdm09 virus had NA-I223V and NA-S247N amino acid substitutions and showed reduced inhibition by oseltamivir. Two B viruses had NA-A245G amino acid substitution and showed reduced inhibition by peramivir. One B virus had NA-D197N amino acid substitution and showed reduced inhibition by zanamivir and peramivir. One B virus had NA-H273Y amino acid substitution and showed highly reduced inhibition by peramivir.One A(H3N2) virus had PA-I38T amino acid substitution and showed reduced susceptibility to baloxavir.

High levels of resistance to the adamantanes (amantadine and rimantadine) persist among influenza A(H1N1)pdm09 and influenza A(H3N2) viruses (the adamantanes are not effective against influenza B viruses). Therefore, use of these antivirals for treatment and prevention of influenza A virus infection is not recommended and data from adamantane resistance testing are not presented.

Outpatient Respiratory Illness Surveillance

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 pathogens that can present with similar symptoms, including influenza viruses, 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 more complete and accurate picture of influenza, SARS-CoV-2, and other respiratory virus activity. CDC is providing integrated information about COVID-19, influenza, and RSV activity on a website that is updated weekly. Information about other respiratory virus activity can be found on CDC’s National Respiratory and Enteric Virus Surveillance System (NREVSS) website.

Outpatient Respiratory Illness Visits

Nationally, the percentage of visits for respiratory illness that were reported through ILINet remained stable (change of ≤ 0.1 percentage points) compared to the previous week but has been trending upward for the past two weeks and remains below the national baseline. All 10 regions are below their region-specific baselines. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

* Effective October 3, 2021 (week 40), the ILI definition (fever plus cough or sore throat) no longer includes “without a known cause other than influenza.”