Key points
U.S. virologic surveillance
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) are used to monitor whether influenza activity is increasing or decreasing. Nationally, during Week 21, the percentage of respiratory specimens testing positive for influenza virus in clinical laboratories was 1.9%.
| Week 21 | Data Cumulative since September 29, 2024 (Week 40) |
|
|---|---|---|
| No. of specimens tested | 37,851 | 3,416,973 |
| No. of positive specimens (%) | 727 (1.9%) | 484,581 (14.2%) |
| Positive specimens by type | ||
| Influenza A | 192 (26.4%) | 432,558 (89.3%) |
| Influenza B | 534 (73.5%) | 52,022 (10.7%) |
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 viruses that belong to each influenza virus type/subtype/lineage. Viruses known to be associated with recent live attenuated influenza vaccine (LAIV) receipt or found upon further testing to be a vaccine virus are not included as they are not circulating influenza viruses.
| Week 21 |
Data Cumulative since September 29, 2024 (Week 40) |
|
|---|---|---|
| No. of specimens tested | 393 | 140,525 |
| No. of positive specimens | 60 | 95,026 |
| Positive specimens by type/subtype | ||
| Influenza A | 26 (43.3%) | 89,845 (94.5%) |
| Subtyping Performed | 19 (73.1%) | 80,204 (89.3%) |
| (H1N1)pdm09 | 16 (84.2%) | 42,269 (52.7%) |
| H3N2 | 3 (15.8%) | 37,856 (47.2%) |
| H3N2v | 0 (0%) | 0 (0%) |
| H5 | 0 (0%) | 79 (0.1%) |
| Subtyping not performed | 7 (26.9%) | 9,641 (10.7%) |
| Influenza B | 34 (56.7%) | 5,181 (5.5%) |
| Lineage testing performed | 17 (50.0%) | 2,807 (54.2%) |
| Yamagata lineage | 0 (0%) | 0 (0%) |
| Victoria lineage | 17 (100%) | 2,807 (100%) |
| Lineage not performed | 17 (50.0%) | 2,374 (45.8%) |
Novel Influenza A Virus Infections
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.
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 may capture respiratory illness visits due to infection with any pathogen that can present with similar symptoms, including influenza, SARS-CoV-2, and RSV. Therefore, 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.
Nationwide during Week 21, 1.8% 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. 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
More than 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. Data from this subset of providers are used to calculate the percentages of patient visits for respiratory illness by age group.
During Week 21, the percentage of visits for respiratory illness reported in ILINet was 6.0% among those 0-4 years, 2.8% among those 5-24 years, 1.4% among those 25-49 years, 1.0% among those 50-64 years, and 0.7% among those 65 years and older.
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 21 (Week ending May. 24, 2025) |
Week 20 (Week ending May. 17, 2025) |
Week 21 (Week ending May. 24, 2025) |
Week 20 (Week ending May. 17, 2025) |
|
| Very High | 0 | 0 | 0 | 0 |
| High | 0 | 0 | 0 | 1 |
| Moderate | 0 | 0 | 1 | 1 |
| Low | 1 | 0 | 19 | 14 |
| Minimal | 54 | 55 | 665 | 682 |
| Insufficient Data | 0 | 0 | 244 | 231 |
*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 overall percentage of emergency department (ED) visits with a discharge diagnosis of influenza reported in NSSP was 0.3% during Week 21. The percentage of visits was 0.6% among those 0-4 years, 0.9% among those 5-17 years, 0.2% among those 18-64 years, and 0.1% among those 65+ years.
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. Patients admitted for laboratory-confirmed influenza-related hospitalization after April 30, 2025, will not be included in FluSurv-NET for the 2024-2025 season. Data on patients admitted through April 30, 2025, will continue to be updated on FluView Interactive as additional information is received.
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 21, 1,577 (0.5 per 100,000 population) laboratory-confirmed influenza-associated hospitalizations were reported. When examining rates by age, the highest hospital admission rate per 100,000 population was among those 65+ years (1.5), followed by 0-4 years (0.6), and 50-64 years age group (0.4).
Mortality Surveillance
National Center for Health Statistics (NCHS) Mortality Surveillance
Based on NCHS mortality surveillance data available on May 29, 2025, 0.1% of the deaths that occurred during the week ending May 24, 2025 (Week 21) were due to influenza. The data presented are preliminary and may change as more data are received and processed.
Influenza-Associated Pediatric Mortality
Four influenza-associated pediatric deaths occurring during the 2024-2025 season were reported to CDC during Week 21. The deaths occurred during weeks 52, 2, 5 and 7 (the weeks ending December 21, 2024, January 11, 2025, February 1, 2025, and February 15, 2025, respectively). A total of 235 influenza-associated pediatric deaths occurring during the 2024-2025 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
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.