Although Marianne Williamson (maybe the distant #3 alternative candidate) is generally antivax since she's from the older group of naturalistic-fallacy hippie women where the idea originally caught on.

RFK and Desantis are doing it because the vaccine was a major policy accomplishment of both the last two presidents, ie the party leaders they're running against. It may seem like a natural political opportunity but it's a bad move since normal voters don't like crazy people.