Even if they cant, for the period when there are lots of people vaccinated and also lots of people infected, that's lots of mutations happening in infected peop

Even if they cant, for the period when there are lots of people vaccinated and also lots of people infected, that's lots of mutations happening in infected people and then trying to jump into vaccinated people, giving it more chances that one of those mutations would then spread from vaccinations people even if current mutations don't (which isn't known yet). If it spreads from vaccinated people already then that would be even worse.