This is a good question I've been also trying to get an answer to, and I've never had a reason to ask before now: with the seasonal flu drive are we often reusing a prior season's flu vaccine? I always assumed they developed new ones all the time, is the answer "it depends"?
Seasonal flu vaccines are reformulated every year based on the circulating virus strains. Leftover doses from the previous season are discarded, not reused.
