> If I crash without a seatbelt on and die, my going through the windshield harms only myself.

Mostly, yes. Whereas if you fail to get vaccinated and therefore spread a disease, you are harming others.

That's a very deep rabbit hole to go down, to deep for this conversation. Suffice it to say that if a pathogen has a vaccine that is proven safe and effective there's a reasonable case to be made for requiring it. It gets very murky when we try to define "safe" and "effective" though.