Is it under your control? Say you do have the vaccines but get to the airport and find out you are on a no fly list. Is that issue trivially easy to fix? From some of the post I've read here, it's a black box system with very little standard process to help you get out of it. Filing law suite isn't easy[0].
It certainly may be, but it doesn't inherently have to be. Kids need vaccinations to go to school (I mean pre-Wakefield and that blond lady from Playboy who married a Backstreet Boy whose name escapes me), the way you proved it was to show your documents that you got from your doctor.
[0] https://www.aclu.org/press-releases/michigan-father-wrongly-...