In the Northern hemisphere, winter is the time for flu, but the exact timing and duration of flu seasons vary. While seasonal flu outbreaks can happen as early as October, most of the time flu activity peaks between December and February, although activity can last as late as May.* The figure below shows peak flu activity for the United States by month for the 1982-83 through 2013-2014 flu seasons. The “peak month of flu activity” is the month with the highest percentage of respiratory specimens testing positive for influenza virus infection. During this 32-year period, flu activity most often peaked in February (14 seasons), followed by December (6 seasons), January and March (5 seasons each).

Peak Month of Flu Activity

*During 2008-2009, flu activity peaked twice because of the 2009 H1N1 pandemic. Activity in the United States peaked once in in February due to seasonal influenza activity and then again in the Spring (June), with the first wave of 2009 H1N1 viruses A second, larger peak of 2009 H1N1 activity occurred in October, the peak of the 2009-2010 season.

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, and clinics and emergency departments. Information in five categories is collected from eight 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 hospitalizations and deaths in the United States

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