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@BowsacNoodle @Shadowman311 Influenza immunization is a particularly special case:

Nothing, wild type virus or all vaccine attempts to date provide sterilizing immunity. You get that when the adaptive immune system recognizes one or more "conserved" parts of a virus, things that can't change or the virus won't virus.

Its genome is divided into a handful of segments; you, a pig, a duck, whatever can get two strains at once, some cells will get infected by both, and hybrid viruses will result. Thus is has mutation patterns you don't see in other viruses.

Even if we were to move entirely to the faster grow in bug cells in reactors Protein Sciences method instead of some companies still using eggs (or rather, the membrane cells of chicken eggs), it take a while to make a season's worth of vaccine. So you have to guess ahead of time which strains will be prevalent every six months (northern and southern hemisphere).

For the first reason, there's no point in trying for long term immunization, so one simple dose of surface proteins is all we try. (Seniors in the US get quadruple doses because their immune systems are getting less effective, and last time I checked there was one US licenced alternative with an adjuvant for them.)