How is flu season defined?

Influenza activity can start as early as October and continue as late as May, but influenza activity often peaks in the United States between December and February. However, the exact season can vary from year to year.

Flu activity is measured through two main sources:

* Influenza-like illness (ILI) surveillance tracks the percentage of people seeing their doctor or going to an emergency department with an illness that includes a fever, cough, or sore throat.

* Virologic surveillance tracks the number of laboratory confirmed cases of influenza.

The Centers for Disease Control and Prevention (CDC) defines the start of the flu season as the first week of the year in which both the ILI and virologic surveillance systems indicate widespread influenza activity in at least one region of the United States. The end of the flu season is defined as the last week of the year in which both surveillance systems indicate no widespread influenza activity in any region of the United States.

The CDC also tracks the severity of the flu season by looking at the number of hospitalizations and deaths due to influenza. A severe flu season is one in which there are a high number of hospitalizations and deaths due to influenza.