That's when they pick the right strain which happens about a third of the time. It takes some time and an awful lot of eggs to make 100 million doses of anything. They start well before the flu season and have to pick between competing strains and predict which one will prevail in the wild. They don't usually get it right.
IIRC
The covid "vaccines" are "leaky" vaccines.
The measles vaccines is slightly leaky at about 97%.
Some of the flu vaccines are as low as the 40% range.
That's when they pick the right strain which happens about a third of the time. It takes some time and an awful lot of eggs to make 100 million doses of anything. They start well before the flu season and have to pick between competing strains and predict which one will prevail in the wild. They don't usually get it right.