[kingbirdy]

Influenza isn't in a steady state, there are hundreds of strains[0] that compete with each other and recombine to form novel versions. Each year's flu vaccine targets four of them based on modeling as to which will be most prevalent in the upcoming flu season[1/2].

0: https://www.cdc.gov/flu/about/viruses/types.htm

1: https://www.cdc.gov/flu/prevent/quadrivalent.htm

2: https://www.fda.gov/vaccines-blood-biologics/lot-release/inf...