Vaccines have been one of the most important public health interventions ever developed. As a new study notes, past analyses have estimated that the childhood immunization schedule prevents 42,000 deaths and 20 million cases of disease—and that's only for the kids born in a single year. The estimated savings is currently at $14 billion a year.

But, despite the amazing benefits, immunization rates have been falling, driven by a fear that vaccines cause health problems such as autism. The autism risk has been both thoroughly debunked and the paper that originally suggested it turned out to be the product of an unethical, financially motivated individual. Despite this debunking, surveys show that a quarter of US parents think that vaccines can trigger autism, and rates of vaccination have continued to fall in many states. A new study looks at incoming kindergartners in California, and finds that the lack of vaccination is threatening herd immunity in some schools, and that some measures of risk have doubled in just three years.

California, like other states, has a mandatory immunization schedule, set as a requirement for children entering school. But California is also one of 20 states that allows a personal belief exemption, where parents can file notice that they have a personal issue with vaccines, and get their kids into schools despite a lack of vaccination. The rates of people asking for these exemptions has been slowly climbing, rising from half a percent in 1996 to 1.5 percent in 2007.

But the statewide rate only tells part of the story, as the cultural factors that influence vaccine takeup may potentially cluster in certain communities. For example, a measles outbreak occurred in San Diego when an intentionally unvaccinated child picked it up in Europe, and then returned to a school with a high rate of exemptions.

How often is this the case? California health records make it possible to do this sort of analysis, as they include information about the number of exemptions as well as identifying the school the exempted child is attending. The authors of the new analysis combine data on measles vaccinations with some useful statistical tools that were first developed to measure racial segregation within a school system. These include what's called the interaction index, which indicates how often a vaccinated student would encounter someone with an exemption, and the aggregation index, which measures how often exempted students will end up in the same school.

Using data from 2008 to 2010, the authors found some worrying trends. Over just two years, the statewide interaction index increased by 25 percent in this short time. That means far more students are coming in contact with peers who are unvaccinated—a serious risk, since vaccines are not 100 percent effective. Making matters worse, there was also an increase in the clustering of these students. The average exempted student was in a school where 14.7 of 100 students also had exemptions in 2008. By 2010, that number had increased to 15.6.

Herd immunity occurs when a few unvaccinated children are protected by the fact that almost everyone around is vaccinated and therefore cannot infect them. It's important for those for whom vaccines have not worked, those who have immune problems, or those who cannot be vaccinated due to specific health risks. But it requires very high rates of vaccination, typically 80-90 percent. And, in California, it's at risk of breaking down. "The number of kindergartners attending schools in which there were more than 20 exempted kindergartners almost doubled (from 1937 in 2008 to 3675 in 2010)," the authors note.

There were definitely regional issues, as well. In one county in the northern part of the state (Trinity), the average student who started school was in an environment where 13.8 percent of his or her classmates had a vaccination exemption. In Sutter county (just north of Sacramento), the aggregation index was an astonishing 46.3, meaning that every child with an exemption was at a school where nearly half of their classmates would also be exempted.

There are some limits to this study. Some students may be granted an exemption but then ultimately receive all the required vaccinations at a later date, and its authors didn't have access to data like classroom size, which could affect the risk of exposure. But even if these limitations affect the study's precision, the trends it detects are clearly worrisome. About the only bright spot is that the authors note that the tools they have developed can help state officials target educational programs to the communities that need it the most.

American Journal of Public Health, 2012. DOI: 10.2105/AJPH.2012.300821 (About DOIs).