Influenza Surveillance Report

Have questions? Here are answers to the most frequently asked questions about our data and methodology.

Frequently Asked Questions

Weekly flu report - details about the metrics

Definitions and Acronyms

  • 2024-2025 influenza season:
    The 2024 – 2025 influenza season begins CDC Week 40 (week ending October 5, 2024) and ends CDC week 39 (week ending September 27, 2025). The Missouri Weekly Influenza Surveillance Report is updated each Friday from October 11, 2024 – May 23, 2024.
  • CDC
    The Centers for Disease Control and Prevention
  • CMS
    The Centers for Medicare & Medicaid Services
  • DHSS
    The Missouri Department of Health and Senior Services
  • ED
    Emergency Department
  • ESSENCE
    The Electronic Surveillance System for the Early Notification of Community-based Epidemics, ESSENCE, is a software system that inputs electronic emergency department data for participating hospitals.
  • HHS
    The United States Department of Health and Human Services
  • Influenza Case
    For this report, a case of influenza is equal to a laboratory confirmation of influenza. Laboratory-confirmed influenza is reportable in aggregate in Missouri (for more information see https://www.sos.mo.gov/cmsimages/adrules/csr/current/19csr/19c20-20.pdf) and includes positive results on any of the following test methods: rapid influenza diagnostic tests (antigen); reverse transcriptase polymerase chain reaction (RT-PCR) and other molecular assays; immunofluorescence antibody staining (Direct (DFA) or Indirect (IFA)); and viral culture.  
  • ILI
    Influenza-like illness. Defined as fever (temperature of 100°F [37.8°C] or greater) and a cough and/or a sore throat.
  • ILINet
    The U.S. Outpatient Influenza-like Illness Surveillance Network. ILINet consists of outpatient health care providers from around the country that report to CDC on a weekly basis.
  • NHSN
    The National Healthcare Safety Network (NHSN) is a CDC tracking system for healthcare-associated infections. For more information see https://www.cdc.gov/nhsn/index.html.
  • NREVSS
    The National Respiratory and Enteric Virus Surveillance System. NREVSS is a laboratory-based system that monitors temporal and geographic circulation patterns of various viruses. On a weekly basis, participating U.S. laboratories voluntarily report the total number of tests performed, the method used for detection, and the number of those tests with positive results.
  • WebSurv
    The Missouri Health Surveillance Information System, a centralized and integrated database that allows the DHSS and local public health agency staff the ability to enter and update case report information, including aggregate reporting of laboratory-confirmed influenza cases.
  • WHO
    The World Health Organization

Trend Indicators

Indicator symbol key - increasing, decreasing, stable

Data and Methods

  • Outpatient Data
    • ILI Activity Level
      Data is sourced from ILINet. Levels of ILI activity are classified as minimal, low, moderate, high, and very high. Activity levels are based on the percent of outpatient visits due to ILI in a jurisdiction compared to the average percent of ILI visits that occur during weeks with little or no influenza virus circulation in that jurisdiction. For more information see https://gis.cdc.gov/grasp/fluview/main.html. 
    • Percent of Emergency (ED) Visits due to ILI
      Data is sourced from the hospital emergency room visit chief complaint data reported through Missouri’s ESSENCE. ILI is defined by ESSENCE as Emergency Department chief complaints for Influenza or (Fever Plus and (Cough or Sore Throat) and not Non-ILI Fevers). The ESSENCE ILI Baseline is the mean percent of ILI visits for each week during the previous three influenza seasons when percentage of ILI visits were less than 2% of total visits, plus two standard deviations.
  • Laboratory Data
    • Weekly Percent Positivity
      Data is sourced from NREVSS. The positivity rate is based on laboratory data submitted by participating clinical hospitals for the CDC week of report.
    • Influenza A Subtype
      Data is sourced from NREVSS. The data is based on which influenza A subtype had the highest proportion positive for the CDC week of report. Data may be limited on weeks with low influenza activity.
  • Case Data, Influenza Cases by Jurisdiction, and Influenza Cases by Week
    Data is sourced from WebSurv.  Influenza case data is entered by local public health agencies weekly.  
  • New Influenza Hospital Admissions by Week
    Effective May 1, 2024, hospitals are no longer required to report hospital admissions, hospital capacity, or hospital occupancy data to HHS through the CDC's NHSN. Since May 3, 2024, the NHSN flu hospitalization data has not be included in the Missouri Influenza Weekly Surveillance Interactive Report, however, beginning on November 1, 2024, the CMS will require the reporting of this information via NHSN. This data will be included in future reports once available. For more information see https://healthdata.gov/Hospital/COVID-19-Reported-Patient-Impact-and-Hospital-Capa/anag-cw7u and https://www.cdc.gov/fluview/?CDC_AAref_Val=https://www.cdc.gov/flu/weekly/overview.htm#ILINet.
  • Regional Data
    Data is sourced from WebSurv. Rates are calculated using 2020 population data from the Missouri Information for Community Assessment (MICA), an interactive system that was developed to make health data accessible at the local level through an easy-to-use format. For more information see https://health.mo.gov/data/documentation/population/population-general-info.php and https://healthapps.dhss.mo.gov/MoPhims/MICAHome.
  • Influenza Deaths
    Data is sourced from Missouri’s death certificate data. Decedents with influenza listed as a cause or contributor to death are classified as an influenza-associated mortality. Death certificate data are generally available two weeks following the current CDC week. Death totals are cumulative from the start of the influenza season.
  • Laboratory Data (Expanded View)
    Data is sourced from ILINet. Laboratory surveillance data on influenza specimens from Missouri residents are reported by WHO/NREVSS collaborating laboratories. Data from both public health and clinical laboratories are included in the total number of respiratory specimens tested for influenza and the number positive for influenza viruses. ILINet laboratory data is generally available one week following the current CDC week. The 2020-2021 influenza season was unusually low due in part to the ongoing COVID-19 pandemic. Therefore, the influenza-associated data for the 2020-2021 influenza season are substantially different from previous seasons.