Why Do I Need a Flu Vaccine Every Year?
A flu vaccine is needed every year because flu viruses are constantly changing. It's not unusual for new flu viruses to appear each year. The flu vaccine is formulated each year to keep up with the flu viruses as they change.
Also, multiple studies conducted over different seasons and across vaccine types and influenza virus subtypes have shown that the body's immunity to influenza viruses (acquired either through natural infection or vaccination) declines over time.
Getting vaccinated each year provides the best protection against influenza throughout flu season.
Content source: Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases (NCIRD).