That’s why the vaccine needs two applications. With only a single application, you’d also only get 90 days.
Additionally, a lot of the additives in the vaccine are intended to make your immune system perceive it as much more of a threat (while a virus will try to be perceived as not a threat), increasing the priority your immune system gives the virus (and the duration it will be able to stay immune)
Additionally, a lot of the additives in the vaccine are intended to make your immune system perceive it as much more of a threat (while a virus will try to be perceived as not a threat), increasing the priority your immune system gives the virus (and the duration it will be able to stay immune)