Seasonal Influenza (Flu)
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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 (ILI) 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 regarding the Wisconsin Sentinel Clinician Program as well as
additional resources on national and international influenza surveillance.
Letter (PDF, 36 KB)
Surveillance Definitions (PDF, 21 KB)
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 (ILI) and the
total number of patient visits by age group each week. This reporting
- 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 cases would not be a reliable way to
Although confirmed cases may provide a rough indication of
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
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.
Influenza Surveillance Coordinator
Wisconsin Division of Public Health
Bureau of Communicable Diseases and Emergency Response
(Phone 608-266-5326) (Fax 608-261-4976)
Local Health Departments - Regional offices - Tribal agencies
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December 09, 2013