I live in an urban area, around many educated parents. When my children were young, I was a bit appalled to find that some of them didn't vaccinate their children. The reasons were, to my rationalist mind, odd, even weird, based on a sense of purity, and relying on free riding on others who did vaccinate, not some kind of informed risk/gain calculation.

Parents of young children often desperately want some kind of objective metric of how well they are doing.  That's why they get obsessed with things like 100% fruit juice drinks (flavored sugar water, much like any other kind of sugar water), organic food (indistinguishable from conventionally raised equivalents), bike helmets (if you're not riding in traffic, I'm not sure it matters)...and whether you let your children get a vaccine shot. They used to swaddle them, or refuse to hug them, or put mustard plasters on their chest.

Because, really, you have no way of telling how well you're doing.

Not vaccinating (or feeling guilty because you did) is a way of regaining some control over the narrative of your child's life.  But you really have almost no control over it. The results are pretty much random. The steering wheel isn't connected to anything.

Now, make no mistake. Opposition to vaccination is a dysfunctional narrative, one that leads to a vaguely creepy glaze-eyed stance. If you talk to those folks on that topic for too long you begin to wonder how many people who go on a killing spree with an automatic weapon oppose vaccination...or were not vaccinated because their parents opposed it.  Why aren't sociologists paying attention to the things that are really worth studying?