Interestingly, your comparison is relevant, but you are responding to an argument which I don't think is being made. The OP is _not_ arguing that vaccines shouldn't be mandatory because they aren't 100% effective.
On the contrary, the OP's argument is that since vaccines are relatively effective, one person's choice to not get vaccinated will not effect the others, and it should be up to them.
Your seatbelt example is relevant, in that the same "no-harm-to-others" argument can be made in that case too.