Do they not trust the vaccines?
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwi9o4nuh_fvAhUlHzQIHWIZA2cQFjAAegQIEBAD&url=https%3A%2F%2Fwww.nejm.org%2Fdoi%2Ffull%2F10.1056%2Fnejm199711133372006&usg=AOvVaw0QgCWhwj3pLAIXrGp9vyVP