On June 5, the number of measles cases in the U.S. this year passed 1,000, a milestone the country last reached in 1992. There’s little doubt, in most circles, about the source of this resurgence: It’s the anti-vaxxers’ fault. “Federal health officials attribute this year’s outbreak to U.S. parents who refuse to vaccinate their children,” noted Reuters in a recent update on the crisis.
That much seems self-evident: Measles spreads most readily through undervaccinated populations, so if the disease is newly spreading, then there must also be a major outbreak of vaccine refusal. The numbers tell a different story, though. As I’ve noted here in Slate, U.S. vaccination rates for measles aren’t really plummeting. In fact, they’ve been very stable over many years, at around 91 or 92 percent of the population. While it’s possible that local hotspots of refusal have gotten slightly bigger over time, or that more of these communities are cropping up, hard evidence in support of this idea has been rather modest.