[reissbaker]

From this [1] Wikipedia article it looks like there aren't new strains every year, but generally every few years there will be a new variant that replaces an older variant in the mix. The oldest variant currently in flu vaccines is from 2013, and the newest is from 2019.

That being said, the process generally doesn't change — you still grow and kill (or grow and weaken) viruses the same way, you just start from a different base strain. For already-approved inactivated-virus flu vaccines, the FDA does not require new clinical trials for strain updates, and for already-approved attenuated virus vaccines, they only require very minimal trials (300 adults to prove adequate attenuation) [2]. So they're not novel in the sense of the COVID vaccines, which needed large scale clinical trials to prove efficacy — the flu vaccines have generally already proved efficacy, so for strain updates the most they need to prove is safety.

1: https://en.m.wikipedia.org/wiki/Influenza_vaccine

2: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4947948/

[Spare_account]

Thanks for this