Influenza viruses mutate frequently, resulting in small changes to the structure of their antigens – the surface molecules that are recognised by the immune system. While most of these mutations do not significantly alter the viral antigens, they accumulate over time – this is called antigenic drift. In this way, viruses diverge until the previous vaccines no longer provide adequate protection against infection.
Each year, countries collect data about which influenza viruses are making people sick, the extent to which those viruses are spreading, and how well the previous season’s vaccine protects against those viruses. Vaccine composition is then updated accordingly.