Maybe those eligible for vaccinations at the time, and chose to get vaccinated, were more cautious overall about their health than anti-vaxxers who were also eligible but didn’t take up the vaccine?
In my view if you're skeptical of government (a problem not unique to USA), you're more likely to distrust the vaccination push, grasping at any conspiracy theory about why there's more to it than the government acting in your interest. So cause and effect are really flipped, for most people - they think cause: govt pushing the evil vaccine, effect: they distrust the govt, but in (my perceived) reality, cause: they distrust govt, effect: they think the vaccine push is evil and has ulterior motives.
And sure if you're vax-skeptical you can cite all the bad side effects and deaths, but I think the numbers speak for themselves, hundreds of millions vaccinated and there hasn't been tens of thousands of deaths; the vaccines are safe!
Of course those who distrust governments can claim the numbers are being fudged...