"Influenza vaccination is the most effective method for preventing influenza virus infection and its potentially severe complications. An influenza epidemic emerges during each winter's flu season. Each year there are two flu seasons due to the occurrence of influenza at different times in the Northern and Southern Hemispheres. Tens of thousands of Americans die in a typical year, but there are notable variations from year to year. In 2010 the Centers for Disease Control and Prevention in the United States changed the way it reports the 30-year estimates for deaths from influenza.
