Why do flu vaccines change every year?

Influenza A and B viruses change through mutation and reassortment, which can alter the proteins the immune system recognizes. Because of this, immunity from last year’s vaccine may not be a perfect match this year. Public health experts track flu strains worldwide through systems coordinated by the World Health Organization and update vaccine recommendations for each season. The seasonal flu vaccine is therefore reformulated regularly, unlike some vaccines that target more stable germs. This yearly update is one reason flu vaccination remains important even if you were vaccinated the year before.