Weekly US Influenza Surveillance Report: Key Updates for Week 11, ending March 15, 2025

Summary

Viruses

Clinical Lab

13.3% (Trend )
positive for influenza
this week.

Public Health Lab

Influenza A(H1N1)pdm09 and A(H3N2)
were the predominant viruses reported this week.

Illness

Outpatient Respiratory Illness

3.9% (Trend )
of visits to a health care provider this
week were for respiratory illness
(above baseline).

Activity Map

13 moderate jurisdictions 20 high or very high jurisdictions

FluSurv-NET

116.5 per 100,000
cumulative hospitalization rate

National Healthcare Safety Network (NHSN) Hospital Respiratory Data

17,722 (Trend )
patients admitted to hospitals
with influenza this week.

NCHS Mortality

1.5% (Trend )
of deaths attributed to influenza this week.

Pediatric Deaths

17 influenza-associated deaths
were reported this week for
a total of 151 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.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 regions 1, 2, 5, 6, 7, 8, 9 and 10, the percentage of respiratory specimens testing positive for influenza virus in clinical laboratories decreased (change ≥ 0.5 percentage points), while in regions 3 and 4 the percentage remained stable compared to the previous week. Influenza A(H1N1)pdm09 and A(H3N2) were the predominant viruses reported 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/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.}

Novel Influenza A Virus

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

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.

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 2,816 influenza viruses collected since September 29, 2024.

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) assay (H1N1pdm09, H3N2, and B/Victoria viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2024-2025 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 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: 197 A(H1N1)pdm09 viruses were antigenically characterized by HI, and 196 (99.5%) 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): 263 A(H3N2) viruses were antigenically characterized by HI or HINT, and 153 (58.2%) 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/Massachusetts/18/2022-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.

Influenza B Viruses

• B/Victoria: 96 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 United States since September 29, 2024, were tested for antiviral susceptibility as follows:

Five A(H1N1)pdm09 viruses had NA-H275Y amino acid substitution conferring highly reduced inhibition by oseltamivir and peramivir. One A(H1N1)pdm09 virus had NA-I223V and NA-S247N amino acid substitutions and showed reduced inhibition by oseltamivir. One A(H3N2) virus had PA-I38T amino acid substitution associated with 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 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 11, 3.9% 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 decreased (change of > 0.1 percentage points) compared to Week 10 but remains above the national baseline of 3.0% for the sixteenth consecutive week. The percentage of visits for ILI remained stable (change of ≤ 0.1 percentage points) in Region 9 and decreased (change of > 0.1 percentage points) in all other regions (1, 2, 3, 4, 5, 6, 7, 8, and 10) this week compared to last. Region 8 is below its baseline while all other regions (1, 2, 3, 4, 5, 6, 7, 9, and 10) are above their respective baselines. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infections 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 decreased (change of > 0.1 percentage point) in all age groups (0-4 years, 5-24 years, 25-49 years, 50-64 years, and 65+ years) in Week 11 compared to Week 10.

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). The state of Vermont is working with CDC to ensure that appropriate data are being used to calculate the state's activity level. Vermont's activity level will be reported again after the issue is resolved.

National Syndromic Surveillance System (NSSP)

The overall percentage of emergency department (ED) visits with a discharge diagnosis of influenza reported in NSSP was 2.4% during Week 11, a decrease (change of > 0.1 percentage point) compared to the previous week. The percentage of ED visits with influenza discharge diagnoses decreased in all 10 HHS regions and across all age groups.

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 9% 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 35,689 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2024, and March 15, 2025. The weekly hospitalization rate observed during Week 11 was 2.6 per 100,000 population. The weekly hospitalization rate observed during Week 6 (13.6 per 100,000 population) was the highest peak weekly rate observed across all seasons since 2010-2011. The cumulative hospitalization rate observed in Week 11 was 116.5 per 100,000 population, which is the highest cumulative hospitalization rate for Week 11 across all seasons since 2010-2011.

Among all hospitalizations 34,745 (97.4%) were associated with influenza A virus, 765 (2.1%) with influenza B virus, 29 (0.1%) with influenza A virus and influenza B virus co-infection, and 150 (0.4%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 5,455 (56.7%) had A(H1N1) pdm09 and 4,167 (43.3%) had A(H3N2).

When examining rates by age, the highest cumulative hospitalization rate per 100,000 population was among adults aged 65 years and older (365.6), followed by adults aged 50-64 years (137.0), children aged 0-4 years (96.2), adults aged 18-49 years (47.4), and children aged 5-17 years (36.1).

When examining age-adjusted rates by race and ethnicity, the highest cumulative hospitalization rate per 100,000 population was among non-Hispanic Black persons (197.6), followed by American Indian/Alaska Native persons (142.6), non-Hispanic White persons (100.4), Hispanic persons (95.1), and Asian/Pacific Islander persons (72.5).

Among 3,631 hospitalized adults with information on underlying medical conditions, 95.1% had at least one reported underlying medical condition; the most commonly reported were hypertension, cardiovascular disease, metabolic disease, and obesity. Among 1,709 hospitalized women of childbearing age (15-49 years) with information on pregnancy status, 28.3% were pregnant. Among 1,237 hospitalized children with information on underlying medical conditions, 53.0% had at least one reported underlying medical condition; the most commonly reported was asthma, followed by neurologic disease and obesity.

_{**In this figure, weekly rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 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 11, 17,722 laboratory confirmed influenza-associated hospitalizations were reported. This week's influenza-associated hospitalizations decreased (change of > 5%) compared to Week 10.

The weekly hospital admission rate observed in Week 11 was 5.3 per 100,000. The weekly rate of hospital admissions in all 10 HHS regions ranged from 2.8 (Region 9) to 9.2 (Region 3). The weekly rate of hospital admissions decreased in all 10 HHS regions.

When examining rates by age for Week 11, all age groups decreased compared to the previous week. The highest hospital admission rate per 100,000 population was among those 65+ years (16.8), followed by 50-64 years (5.6), and 0-4 years (4.0).

Mortality surveillance

National Center for Health Statistics (NCHS)

Based on NCHS mortality surveillance data available on March 20, 2025, 1.5% of the deaths that occurred during the week ending March 15, 2025 (Week 11), were due to influenza. This percentage decreased (≥ 0.1 percentage point change) compared to Week 10. The data presented are preliminary and may change as more data are received and processed.

Influenza-Associated Pediatric Mortality

Seventeen influenza-associated pediatric deaths occurring during the 2024-2025 season were reported to CDC during Week 11. The deaths occurred between Week 51 of 2024 (the week ending December 21, 2024) and Week 10 of 2025 (the week March 8, 2025). Fifteen deaths were associated with influenza A viruses. Ten of the influenza A viruses had subtyping performed; five were A(H1N1) viruses and five were A(H3N2) viruses. Two deaths were associated with influenza B viruses with no lineage determined.

A total of 151 influenza-associated pediatric deaths occurring during the 2024-2025 season have been reported to CDC.

Additional National and International Influenza Surveillance Information

Indicators Status by System

Additional surveillance information