The Flu Season

In the Northern hemisphere, winter is the time for flu. The timing and duration of flu seasons vary. While flu outbreaks can happen as early as October, most of the time influenza activity peaks in January or later. The figure below shows peak influenza activity for the United States by month for the 1976–77 through 2008–09 influenza seasons. The month with the highest percentage of cases (nearly 50%) was February, followed by January with 20% and March and December, with approximately 15% of all cases.

Peak Month of Influenza Activity
1976—77 through 2008—09 Seasons

Monitoring Flu Activity

CDC collects, compiles and analyzes information on influenza activity year round in the United States and produces a weekly report from October through mid-May. The U.S. influenza surveillance system is a collaborative effort between CDC and its many partners in state and local health departments, public health and clinical laboratories, vital statistics offices, physicians, clinics and emergency departments, and the Departments of Defense and Veteran"e;s Affairs. Information in five categories is collected from 10 different data sources that allow CDC to:

• Find out when and where influenza activity is occurring
• Track influenza-related illness
• Determine what influenza viruses are circulating
• Detect changes in influenza viruses
• Measure the impact influenza is having on deaths in the United States

For more information about flu surveillance or to access these reports, visit Flu Activity & Surveillance.