In 1940s, Maurice Hilleman discovered antigenic drift, which is the most common way that influenza viruses change.
The rate of antigenic drift is dependent on two characteristics: the duration of the epidemic, and the strength of host immunity.
[7] Sites recognized on the hemagglutinin and neuraminidase proteins by host immune systems are under constant selective pressure.
[9] In human populations, immune (vaccinated) individuals exert selective pressure for single point mutations in the hemagglutinin gene that increase receptor binding avidity, while naive individuals exert selective pressure for single point mutations that decrease receptor binding avidity.
[8] These dynamic selection pressures facilitate the observed rapid evolution in the hemagglutinin gene.
[10] To meet the challenge of antigenic drift, vaccines that confer broad protection against heterovariant strains are needed against seasonal, epidemic and pandemic influenza.