It's population resistance in general. But population resistance also slowly defangs a virus as we move out of the pandemic phase.

(Which I wish people would understand is a long slow process, we'll probably keep thinking it's over and then look back and go "Oh, it wasn't").

I also think that higher infection rates amongst vaccinated people may be in part behavioral. There are probably, for example, quite a few people living in tiny little towns that haven't bothered getting vaccinated because they're exposed to so few other humans and don't travel much if at all...