Influenza Activity in the U.S. Key Points:
• Due to the level of influenza activity at the peak of the season, this season is classified as a high severity season overall and for all age groups (children, adults, older adults) and is the first high severity season since 2017-2018.
• During Week 16, of the 337 viruses reported by public health laboratories, 210 were influenza A and 127 were influenza B. Of the 187 influenza A viruses subtyped during Week 16, 128 (68.4%) were influenza A(H1N1)pdm09, 59 (31.6%) were A(H3N2), and 0 were A(H5).
• No new influenza A(H5) cases 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.
• CDC estimates that there have been at least 47 million illnesses, 610,000 hospitalizations, and 26,000 deaths from flu so far this season.
• CDC continues to recommend that everyone ages 6 months and older get an annual flu vaccine as long as influenza viruses are circulating.
• There are prescription flu antiviral drugs that can treat flu illness; those should be started as early as possible and are especially important for patients at higher risk for severe illness.
• Influenza viruses are among several viruses contributing to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, flu, and respiratory syncytial virus (RSV) activity on a weekly basis.
Below is a summary of the key influenza indicators for the week ending April 19, 2025:
Viral Surveillance - Clinical Lab: The percentage of respiratory specimens testing positive for influenza at clinical laboratories is 5.6% this week.
Viral Surveillance - Public Health Lab: Influenza A(H1N1)pdm09, A(H3N2) and B viruses were co-circulating this week.
Virus Characterization: Influenza virus characterization information can be found here:
Influenza Virus Genome Sequencing and Genetic Characterization
Outpatient Illness: The percentage of patient visits to a health care provider for respiratory illness in week 16 was 2.3% which is below baseline.
Outpatient Respiratory Illness: ILINet Activity Map: One jurisdiction experienced moderate ILI activity and 1 jurisdiction experienced high or very high ILI activity in week 16.
Laboratory-Confirmed influenza Hospitalizations can be found here: Laboratory-Confirmed Influenza Hospitalizations (cdc.gov) The weekly hospitalization rate observed in Week 16 was 0.6 per 100,000 population. The cumulative hospitalization rate was 126.6 per 100,000 population which is the highest cumulative hospitalization rate for all seasons since 2010-2011.
NCHS Mortality: 0.3% of deaths were due to influenza in week 16. This percentage decreased (> 0.1 percentage point change) compared to week 15. (NCHS:National Center for Health Statistics)
Influenza-Associated Pediatric Deaths: Six influenza-associated deaths were reported this week. Two hundred and four pediatric deaths have been reported so far this season. For more information: https://gis.cdc.gov/GRASP/Fluview/PedFluDeath.html
CDC FluView Site
https://www.cdc.gov/fluview/
Influenza Activity in Massachusetts
For more information about flu, please go to our Viral Respiratory Disease Dashboard which includes information not only on influenza but also other acute respiratory diseases, COVID-19, and respiratory syncytial virus (RSV), wastewater data and immunizations for respiratory diseases.
The figure below shows that for MMWR week 16, the percent of ILI visits in the current week is above the regional baseline of 1.9% but continues to decrease. (The CDC uses trends from past years to determine a region-specific baseline rate of ILI visits, which for Massachusetts is 1.9%. A rate above this regional baseline indicates higher than normal levels of ILI in the state.) For more information on how regional baselines are calculated see CDC’s influenza surveillance website at U.S. Influenza Surveillance: Purpose and Methods | FluView | CDC) For more information on weekly flu activity, see CDC’s influenza surveillance website at https://www.cdc.gov/fluview/surveillance/index.html.
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ILI Activity in Massachusetts by Region
- The map below shows the relative intensity of reported ILI activity in Massachusetts by region. Although regions may not all experience the same intensity of ILI at similar times, infections due to influenza can be found throughout Massachusetts during flu season. At this time, the Southeast and Outer Metro Boston regions are experiencing moderate ILI activity. The Northeast region is experiencing low ILI activity and the West, Central, Inner Metro Boston and Boston regions are experiencing minimal ILI activity.
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