The vast majority of those infected with measles during an outbreak at Disneyland in December were unvaccinated against the disease, including kids who were too young for the shots and anti-vaxxers who chose against them.

But six of the 52 cases got their measles-mumps-rubella vaccine—the MMR shot—and still managed to get infected. And all but two of them had gotten at least two doses, the standard recommendation.

So what happened?