About
CDC classifies severity of important flu indicators to understand the impact of flu virus activity throughout the season on the population and health care system. These data can help guide public health actions, such as targeting prevention and treatment messages to appropriate audiences and make recommendations designed to prevent flu illnesses and death.
In-Season Severity Assessment
CDC assesses the severity of weekly influenza activity using information on influenza-like illness, flu-related hospitalizations, and flu-coded deaths. Using the highest weekly values observed from October 1, 2024, through November 23, 2024, for the three indicators, CDC assesses the 2024-2025 season's severity as:
How CDC determines the severity of a flu season
Three flu surveillance indicators (influenza-like illness (ILI) outpatient visits, flu-related hospitalizations, and flu deaths) are used to assess the severity of a flu season each week. The preliminary severity for a season is classified by assessing these indicators across all age groups at the highest week of activity for each indicator during the season. The highest values are compared to historic flu seasons and classified as being low, moderate, high, or very high severity based on which intensity threshold the highest week value observed to date crosses. The highest week assessment can change as the season progresses, depending on activity levels of the flu indicators.
How CDC determines weekly flu severity
Each week, CDC assesses the flu severity in the United States using the three indicators across all age groups (percent of ILI, flu-related hospitalization rate, and percent of flu deaths) and compares the weekly values to intensity thresholds (IT) for each indicator calculated from past seasons. Each indicator is assessed separately. If the weekly indicator value falls below all thresholds, the week is classified as being low severity. If the weekly indicator value is equal to or above the IT50 (~50th percentile) and below the IT90 (~90th percentile), the classification would be moderate. If the weekly indicator value is equal to or above the IT90 and below the IT98 (~98th percentile), the classification would be high. If the weekly indicator value is above the IT98, the classification would be very high. This process is repeated for the next indicator. For each week, the overall weekly assessment of severity is classified based on all three indicators combined. If two or three indicators have the same assessment, then the overall classification is matched to those indicators. If all three of the indicators have different classifications, then the classification in the middle is used for the overall classification. Weekly severity assessments may change because indicator data may not be complete at the time of assessment because of reporting delays and backfill of information. To assign a preliminary severity classification during the season, the highest week values observed to date for each indicator are assessed. The preliminary in-season severity classification as the season progresses, depending on the activity levels of each indicator.
Data Sources
U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet): Percent of outpatient visits for flu-like illness out of all outpatient visits by week. ILINet sites that collect age-specific data on the number of influenza-like illness visits and total number of outpatient visits are used for the severity assessment.
Influenza Hospitalization Surveillance Network (FluSurv-NET): Adjusted rate of flu-related hospitalizations per 100,000 people by week. Hospitalization rates are adjusted using data from past seasons on testing practices and the sensitivity of diagnostic tests.
National Center for Health Statistics (NCHS) Mortality Surveillance Data: Percent of flu-coded deaths out of all reported deaths by week.
The data used to assess severity are available for download through the weekly FluView report.
Limitations
- In-season severity assessments are preliminary because they are made while the season is ongoing and influenza viruses are continuing to circulate. As the season progresses, severity continues to be assessed and the classification is updated as needed as additional data become available.
- The data used to assess severity are updated weekly because initial data submissions may be incomplete because of reporting delays and backfill of information. Severity assessments for earlier weeks may change as additional data become available.