Too Much Virus
By 
This may be simple and obvious, but it’s worth saying.
The more virus there is around, the more danger to all of us, including the vaqccinated. That’s why nonpharmaceutical interventions (NPIs) like masks and distancing are important.
The more virus you encounter, the more likely it is that one of those encounters will go well for the virus and badly for you. So when case numbers go up, it makes sense to take more precautions.
The reason that anti-vaxxers are often able to avoid having their children infected with measles is that in most places there is usually no measles virus circulating. An immune system that can’t handle the virus is safe as long as it doesn’t confront the virus.
This ties into speculation on the endemic endgame for SARS-CoV-2, although I haven’t seen anyone consider it explicitly. We can, through a combination of vaccines, natural immunity, and NPIs, get the virus down to the levels we’re accustomed to with measles. At current rates, however, that will take years. It might be instructive to look at how measles cases declined after vaccination became available.
Cross-posted to Nuclear Diner