Wisconsin and many of the US are experiencing a "second wave" of flu cases in the late season, the Department of Health said Monday.
An increasing number of these cases also involve an influenza virus strain against which the vaccine is ineffective this season, said the Wisconsin Department of Health Services. The efficacy of the vaccines is expected to continue to decline.
A nationwide disease control and prevention program for influenza shows that the virus was common in most US states until March 1
The state said the flu caused 390 people to be admitted to hospital in the week that ended on 16 March.
Symptoms of the virus, which is spread by infected people or objects they touch, include chills, fever, headaches, body aches, sore throats, fatigue, dry cough, and a runny or stuffy nose, according to the health department.
Although most recover with the virus within two weeks, it is particularly dangerous for over 65s, pregnant or suffering from a disease such as diabetes or kidney disease.
In addition to vaccination frequent hand washing, avoiding contact with infected persons and disinfecting touched surfaces are often means to avoid flu.
Those infected should stay home from work or at school and not go out for at least 24 hours after the disappearance of a fever without taking medication.