How do flu vaccines change from year to year?

Influenza viruses change through antigenic drift, a gradual process of mutations in the viral genes that affect immune recognition. Because of this, protection from last year’s flu strain may be weaker against this year’s viruses. Each year, global surveillance networks monitor circulating flu viruses and the World Health Organization recommends strains for seasonal vaccines. In some years, both the Northern and Southern Hemisphere vaccines are adjusted separately because the seasons differ. This is why annual flu vaccination is recommended for many people.