One of the excuses for vaccine refusal that I've heard, is "I don't want to put unknown and unapproved substances into my body".
This often comes from someone who is using unknown and unapproved substances for recreation.
And when people worry about possible side effects of a vaccine, I'd suggest that they read the paper that comes with aspirin, paracetamol, or any other over-the-counter drugs. It will scare the hair off them. Or read the list of ingredients in processed food.
And, by the way, anyone who thinks that anything natural must be good for you, should be discouraged from eating poisonous berries and toadstools.