You just need a good reason not to be vaccined, preferably attested by a doctor.

It's not such a rare case I think, and for any vaccine there is be a small percent of children with allergies to one component or are known to badly react to the vaccine.

In many places I know of, it is as simple as a parent writing a letter claiming to be opposed to vaccination on philosophical and/or religious grounds.

How do you know it? Do you know parents who did it, or people who have accepted such letters?

My parents were very anti-vaccination and are part of a 'religion' that exempts us from it... all we needed was a note from a 'religious leader' and its done... I'm not vaccinated for anything. The religion is 'The Universal Congregation of Wisdom' (its very chiropractic, my dads a chiropractor... the 'religious doctrine' is pretty entertaining...)

you can join with a letter and like a 200$ donation or soemthing

Homeschool?

It's always a possibility, sure, but I doubt that everyone who skipped vaccinations wants to homeschool. The sets of people could even have significant overlap, but I imagine there'd still be a decent number of who skipped vaccinations who don't want to homeschool.