Given that this is caused almost entirely by anti-vaccination hysteria, and that this in in turn caused mostly by a combination of ignorance and bizarre religious beliefs, it's hard to imagine an effective response. It's been said that you can't reason someone out of a position they didn't reason themselves into, and that seems apt here. Perhaps an attempt to shame people by presenting the young, innocent victims is best (e.g. little 8-month old Suzy died because XYZ Church members refused to vaccinate their children).