Anti-vaccine parents do not live in a world in which they weigh the risk of infectious diseases versus the risk of vaccination. They live in a world in which their child faces only very slight risks of contracting infectious disease, due to the decisions of all the other socially responsible parents around them who vaccinate their children. This allows them — if they are narrowly self-interested — to avoid the even slighter risk of side effects from vaccination. It is only the socially responsible decision of parents who vaccinate their children that allows anti-vaccine parents to live in that world.

—Refusing vaccination is anti-social, not irrational - The Washington Post