Why should influenza vaccination be repeated every year?
Influenza viruses are highly variable and change from year to year. Therefore, it is possible to get influenza several times during a lifetime and become infected with the same virus (sub)type. The immunity provided by natural infection or vaccination with a given influenza virus strain does not necessarily provide protection against a newer virus strain. That is why the influenza vaccine composition is updated every year to include currently circulating virus strains.
