Something about original sin
Antigenic Original Sin.
The immune system trains to fight a specific strain, but if a virus is particularly prone to mutation, it develops new strains quickly.
Antibodies trained to fight one strain will be less (or in rare cases in)effective against others.
And it tends to be "stuck" that way.
How this functions with Influenza:
Each generation trains immunity to a to the first strain they encounter, and their immune response is blunted against other strains to varying degrees.
For each strain that circles the globe each year, a significant proportion of populace (representing 1 or more generations) carries significant immunity.
Because this immunity is generational, rather than uniform, the herd has greater resilience against catastrophic pandemic.
Now...
The vax has caused uniform (rather than generation-segmented) immune response
to the INITIAL strain among vast numbers of the population.
Guess what happens with each iterative mutation further away from that strain?
(the "unvaccinated" are going to save us from another massive pandemic)
This is why, with diseases with super-low mortality like WUHAN virus or the flu, you don't push vaxes on people that are below 65 years old. Better for them to get sick and fight it for the sake of stronger overall herd immunity.