[ethbro]

Friend of a friend stumbled across a more effective method.

Ask antivaxxers "What if anti-vaccine information is fake news planted by Russia to weaken our country?" (Not sure if the geopolitics play in your locale, salt to taste)

Only half-joking. It's surprisingly effective because it plays with their (a) propensity to believe conspiracies, (b) distrust of traditional media, (c) distrust of governments, (d) desire to feel smarter than everyone else.