Surveillance of Seasonal Influenza (Flu)

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Influenza surveillance

  • Influenza-like illness activity in Wisconsin is based on information received from clinicians throughout the state of Wisconsin by the Division of Public Health.
  • This data includes information on patients with influenza-like illness and the total number of patients seen in a given week.
  • For surveillance purposes, influenza-like illness is defined as the presence of fever (>100 degrees) with a fever or a cough, and does not reflect laboratory confirmed cases of influenza. 
  • Listed below is information about the Wisconsin Sentinel Clinician Program, as well as additional resources on national and international influenza surveillance. 

Influenza-associated hospitalizations

Wisconsin monitors influenza activity in several ways:

  • Voluntary submission of isolates by clinical virology laboratories to the Wisconsin State Laboratory of Hygiene.
  • Voluntary reporting by virology laboratories that participate in the Wisconsin Laboratory Information Network (LIN) of positive test results and total number of respiratory virus specimens tested.
  • Voluntary reporting by a statewide network of sentinel clinicians of the number of patients presenting with influenza-like illness and the total number of patient visits by age group each week. This reporting occurs year-round.
  • Voluntary reporting of influenza outbreaks in long-term care facilities, schools and other congregate settings.

Many cases are never reported because influenza is not a reportable disease in Wisconsin. We do not attempt to track or get reports on all cases. Most cases are never reported to anyone since most people with influenza never see a doctor about their illness, and many of those who do are never tested.

Even if it were possible to track all cases of influenza in the state, it wouldn't be useful to do so. Influenza is so common during the winter months that we could never actively investigate all of the cases reported to us. We would simply be counting cases, and that wouldn't help us protect the health of the public. Because some providers actively test for influenza and others do not, counting the number of reported cases would not be a reliable way to track influenza.

Although confirmed cases may provide a rough indication of influenza activity, that's not the primary reason we keep track of them. Confirmed cases allow us to:

  • Determine when we first started to see influenza activity each year
    (the "first influenza case of the season") AND

  • Determine what strains of influenza are circulating in any given year.

The main reason we confirm cases in the lab is to determine what kind of influenza is around, and whether the current vaccine protects against it. Only a tiny fraction of all cases are ever confirmed in our lab.

Additional resources


Thomas Haupt, Influenza Surveillance Coordinator
Wisconsin Division of Public Health 
Bureau of Communicable Diseases
Phone 608-266-5326
Fax 608-261-4976

Wisconsin Local Health Departments - Regional offices - Tribal agencies

Last Revised: September 20, 2018