So how does that explain what happened in Disneyland? If you have a group of 1,000 people concentrated in a small space—like oh, say, hypothetically, an amusement park—about 90 percent of them will be vaccinated (hopefully). One person, maybe someone who contracted measles on a recent trip to the Philippines, moves around, spreading the virus. Measles is crazy contagious, so of the 100 people who aren’t vaccinated, about 90 will get infected. Then, of the 900 people who are vaccinated, 3 percent—27 people—get infected because they don’t have full immunity.
Now the Disneyland numbers—six vaccinated infections out of the 34 cases with known records—start to make more sense. (And considering the 16 million or so visitors the park gets every year, we might reasonably expect that number to go up.) Once vaccination levels dip below 90 or 95 percent, there aren’t enough protected people to keep the disease in check—the herd immunity that epidemiologists like to talk about so much. In the US, we’ve been doing pretty well keeping those numbers up. “But there are some fluctuations,” says Cristina Cassetti, program officer at the National Institute of Allergy and Infectious Diseases, “and if vaccination levels dip down a little, you get a situation like Disneyland.”