California kids have to be vaccinated before they enter kindergarten — unless their parents file a personal belief exemption (PBE) arguing that the shots violate their principles. As more and more parents grow suspicious of vaccines, the number of these PBEs is growing. And now researchers have found that families who get PBEs for their kids tend to cluster together, making outbreaks of diseases like measles, whooping cough, and mumps even more likely.
The percentage of students getting PBEs tripled between 1996 and 2007, from 0.5% to 1.5%. In a study published in the American Journal of Public Health, nursing professor Alison Buttenheim and her coauthors found that by 2010, it had risen to 2.3%. What's more, kids with the exemptions tend to live near each other, because parents often share beliefs about vaccines with their neighbors. That means some schools have much higher exemption rates than the state as a whole. And the number of those schools is growing.
In 2008, California had 1,937 schools where more than 20 kindergartners had a PBE. By 2010, that number had risen to 3,675. Enrollment at high-PBE schools also increased. In 2010, 1.4% of California kindergartners went to schools where 20% or more of their classmates had PBEs.
In order to be protected from a disease, Buttenheim and her coauthors write, a population needs to be 80% to 95% vaccinated (the exact percentage depends on the disease). As a whole, the California school system is close to that standard, with kindergarten vaccination rates for all vaccine-required diseases above 93%. But high-PBE schools may be dipping well below that, putting them at risk of an outbreak.
The study authors note that just such an outbreak occurred in 2008, when a child whose parents had chosen to forgo vaccines got measles in Europe. Eleven percent of the students at the child's San Diego school had PBEs that exempted them from the measles vaccine, and two more students got sick. As the number of high-PBE schools increases, this scenario could become more common.
Outbreaks like this aren't just a California problem. In 2008, a measles outbreak sickened at least 14, including an unvaccinated 2-year-old boy who had to be hospitalized for seizures. Thirteen people who contracted measles in an Indiana outbreak after this year's Super Bowl said they'd previously refused the vaccine. And much of the country is currently in the midst of a whooping cough epidemic, which officials say could be the worst in 50 years — only 85% of children and adults who are supposed to be vaccinated for it actually are.
The growth of high-PBE schools increases the likelihood of an outbreak even if the percentage of unvaccinated kids in the country as a whole remains low. And high-PBE schools could start to have even more exemptions as parents see that their peers are eschewing vaccines and start to follow suit. The study authors stop short of advocating that the state force parents to vaccinate their kids. But they do call for better public education about vaccines so that parents don't buy into misinformation about vaccines and put their kids — and others — at risk.
Update: Study author Alison Buttenheim recommends that states make PBEs harder to get: "make opting in easier than opting out." She also advocates training healthcare providers in the best ways to counsel parents that vaccines can protect their children and others. She adds, "another option is to just wait for a major outbreak that is clearly linked to vaccine exemptions — but I hope it doesn’t come to that!"