Yet the vaccine resisters and delayers are not the only parents whose kids miss out on shots. Far more children are undervaccinated for reasons unrelated to personal beliefs, according to a January 2013 study funded by the Centers for Disease Control and Prevention (CDC).
The study found that an astonishing 49 percent of toddlers born from 2004 through 2008 hadn't had all their shots by their second birthday, but only about 2 percent had parents who refused to have them vaccinated. They were missing shots for pretty mundane reasons—parents' work schedules, transportation problems, insurance hiccups. An earlier CDC study concluded that children in poor communities were more likely to miss their shots than those in wealthier neighborhoods, and while that may not be too surprising, it's still a dangerous pattern. "If you're going to delay one or two vaccines, it's not going to make a huge difference," says the new study's lead author, Jason Glanz, an epidemiologist at the Kaiser Permanente Colorado Institute for Health Research. "But you could also think of it like this: If a million kids delay their vaccines by a month, that's time during which a disease could spread."