Exactly when that shift would occur was anyone’s guess ... known in 1918 – but they had also developed a vaccine. Antigenic drift rendered this vaccine ineffective in the 1946 flu season.

Catani and colleagues provide data on antigenic properties of neuraminidase proteins of pandemic H1N1 and show that antigenic diversity of the neuraminidase from 2009 to 2020 largely falls into two ...

In 1947, the vaccine was unexpectedly less efficient; it was discovered that the antigenic composition of the circulating virus had changed and antigenic drift ... and antigenic shift was ...