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Original site: www.cdc.gov/flu/php/surveillance/in-season-severity.html | RestoredCDC.org is an independent project, not affiliated with CDC or any federal entity. Visit CDC.gov for free official information. Due to archival on January 6, 2025, recent outbreak data is unavailable. Videos are not restored. Access data.restoredcdc.org for restored data. Use of this site implies acceptance of this disclaimer.[More]About Us Report Bug Compare ContentSkip directly to site content Skip directly to search Skip directly to On This PageAn official website of the United States governmentHere's how you knowOfficial websites use .govA .gov website belongs to an official government organization in the United States.Secure .gov websites use HTTPSA lock ( ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.Influenza (Flu)Explore TopicsSearchSearchClear InputFor Everyone* About* Signs and Symptoms* Causes and Spread* Flu Complications and People at Higher Risk* Prevention* Vaccine Basics* Diagnosis* Flu Treatment* View allHealth Care Providers* Clinical Guidance* Clinical Signs and Symptoms of Influenza* Guidance for Influenza Vaccination* Vaccine Product Information* Clinical Testing and Diagnosis* About Influenza Antiviral Medications* Infection Control Guidance* Laboratory Testing* View allPublic Health* How CDC Classifies Flu Severity each Season in the United States* Information for Laboratories* Understanding Influenza Viruses* WHO Collaborating Center for Surveillance, Epidemiology and Control of Influenza* Viral Genomic Sequencing Infrastructure* View allRelated Topics:Avian Flu | Swine Flu | Flu in Animals | Flu Vaccines Work | Flu Forecasting | Flu BurdenView Allsearch close searchsearchInfluenza (Flu) Menu CloseInfluenza (Flu) MenusearchFor Everyone* About* Signs and Symptoms* Causes and Spread* Flu Complications and People at Higher Risk* Prevention* Vaccine Basics* Diagnosis* Flu Treatment* View All HomeHealth Care Providers* Clinical Guidance* Clinical Signs and Symptoms of Influenza* Guidance for Influenza Vaccination* Vaccine Product Information* Clinical Testing and Diagnosis* About Influenza Antiviral Medications* Infection Control Guidance* Laboratory Testing* View All HomePublic Health* How CDC Classifies Flu Severity each Season in the United States* Information for Laboratories* Understanding Influenza Viruses* WHO Collaborating Center for Surveillance, Epidemiology and Control of Influenza* Viral Genomic Sequencing Infrastructure* View AllRelated Topics* Avian Flu* Swine Flu* Flu in Animals* Flu Vaccines Work* Flu Forecasting* Flu BurdenView All Influenza (Flu)How CDC Classifies Flu Severity each Season in the United States Information for Laboratories Understanding Influenza Viruses WHO Collaborating Center for Surveillance, Epidemiology and Control of Influenza Viral Genomic Sequencing Infrastructure View AllDecember 27, 2024May 2, 2025Español2024-2025 United States Flu Season: Preliminary In-Season Severity AssessmentAboutCDC 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 AssessmentCDC 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 December 21, 2024, for the three indicators, CDC assesses the 2024-2025 season's severity as: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 April 26, 2025, for the three indicators, CDC assesses the 2024-2025 season's severity as:Text DescriptionThis dashboard visually displays the preliminary in-season flu severity assessment for the 2024-2025 flu season, which is updated every week. The assessment is considered preliminary since the season is ongoing. As the season progresses, the severity will continue to be assessed weekly and the classification for the season may change. The first page identifies the overall in-season flu severity assessment for the United States across all ages through the latest available date. The second page gives the peak weekly value for the three measured outcomes used to assess the severity of flu activity: influenza-like illnesses, flu-related hospitalizations and flu-coded deaths. The third page of the dashboard shows a graph for each weekly value of the outcome and compares those weekly values to past-season thresholds, which are represented by dashed lines.How CDC determines the severity of a flu seasonThree 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.Keep Reading: Preliminary Estimated Flu Disease BurdenHow CDC determines weekly flu severityEach 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.Keep Reading: How CDC Classifies Flu Severity each Season in the United StatesData SourcesU.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.Limitations1. 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.2. 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.On This Page* In-Season Severity Assessment* How CDC determines weekly flu severity* Data Sources* LimitationsRelated PagesHow CDC Classifies Flu Severity each Season in the United StatesRelated Pages** How CDC Classifies Flu Severity each Season in the United States* Past Flu Season Severity Assessments* Information for Laboratories** Understanding Influenza Viruses* WHO Collaborating Center for Surveillance, Epidemiology and Control of Influenza*View All Influenza (Flu)Past Flu Season Severity AssessmentsBack to TopMay 2, 2025December 26, 2024Sources Print ShareFacebook LinkedIn Twitter SyndicateContent Source:National Center for Immunization and Respiratory Diseases (NCIRD)Related PagesHow CDC Classifies Flu Severity each Season in the United StatesRelated Pages** How CDC Classifies Flu Severity each Season in the United States* Past Flu Season Severity Assessments* Information for Laboratories** Understanding Influenza Viruses* WHO Collaborating Center for Surveillance, Epidemiology and Control of Influenza*View All Influenza (Flu)Back to TopInfluenza (Flu)Influenza (flu) is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs.View AllFor Everyone* About* Signs and Symptoms* Causes and Spread* Flu Complications and People at Higher Risk* Prevention* Vaccine Basics* Diagnosis* Flu Treatment* View allHealth Care Providers* Clinical Guidance* Clinical Signs and Symptoms of Influenza* Guidance for Influenza Vaccination* View AllPublic Health* How CDC Classifies Flu Severity each Season in the United States* Information for Laboratories* Understanding Influenza Viruses* View AllSign up for Email UpdatesContact UsContact Us* Call 800-232-4636* Contact CDCAbout CDCAbout CDC* Pressroom* Organization* Budget & Funding* Careers & JobsPolicies* Accessibility* External Links* Privacy* Web Policies* FOIA* OIG* No Fear Act* Nondiscrimination* Vulnerability Disclosure PolicyLanguagesLanguages* EspañolLanguage Assistance* Español* 繁體中文* Tiếng Việt* 한국어* Tagalog* Русский* العربية* Kreyòl Ayisyen* Français* Polski* Português* Italiano* Deutsch* 日本語* فارسی* EnglishArchive* CDC Archive* Public Health PublicationsContact UsContact Us* Call 800-232-4636* Contact CDCAbout CDC* Pressroom* Organization* Budget & Funding* Careers & Jobs* About CDCPolicies* Accessibility* External Links* Privacy* Web Policies* FOIA* OIG* No Fear Act* Nondiscrimination* Vulnerability Disclosure PolicyLanguagesLanguages* EspañolLanguage Assistance* Español* 繁體中文* Tiếng Việt* 한국어* Tagalog* Русский* العربية* Kreyòl Ayisyen* Français* Polski* Português* Italiano* Deutsch* 日本語* فارسی* EnglishArchive* CDC Archive* Public Health PublicationsHHS.gov USA.gov
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