How many studies do you have to throw at the vaccine hysterics before they quit? How much of a scientific consensus, how many unimpeachable experts and how exquisitely rational an argument must you present?
That's a trick question, of course. There's no magic number. There's no number, period. And that's because the anti-vaccine crowd (or anti-vaxxers) aren't trafficking in anything as concrete, mundane and quaint as facts. They are not really engaged in a debate about medicine. They're immersed in a world of conspiracies, in the dark shadows where no data can be trusted, nothing is what it seems and those who buy the party line are pitiable sheep.