Lots of schools will require it upon matriculation because you'll have sim lab, patient contact in your first year (MANY schools do this now) or just a general requirement to begin with. If you're anti-vax you shouldn't be in healthcare, let alone become a doctor. Go find another profession.
As a medical student and future physician I refuse to work with any med student/resident/attending who isn't vaccinated.
I'm being VERY nice about this. If you asked this question to my dean you'd get ripped a new one