For more information, see the American Heart Association . Influenza Influenza is an annually occurring viral epidemic. The primary tool for reducing this death toll is the use of immunization against this virus. The virus changes slightly from year to year. This means that last year's immunization may not be effective against this year's viruses.
Annual immunization is required for effectiveness.
The annual outbreaks of influenza usually begin in December. The vaccine requires about two weeks to become effective. The best time for immunization is in October or November. Protection provided by the vaccine lasts about four to six months. The influenza season in the U.S. ends in March, so one properly timed immunization is usually effective.