They hang out in pockets," says Richard Pan, a Sacramento pediatrician and member of California's Legislature.
He is referring to parents who, invoking a "philosophical exemption," opt not to give their children the state-recommended vaccinations.
In some pockets, such as the rural foothills of the Sierra Nevada, they may belong to the conservative don't-tread-on-me crowd that distrusts all government recommendations simply because they come from the government. In others, such as the liberal organic-food-and-yoga belt along the coast, parents may forswear vaccines because they see the shots as dangerous and the diseases they protect against as mild.
These local concentrations of unvaccinated children pose a growing risk to public health. For the most common shots, vaccination rates for America overall, and even California, are still above 90 percent, at or near the levels considered necessary to provide "herd immunity" for a population. In some places, however, the rates have been falling for almost a decade. In many counties, towns and nursery schools — within California, Oregon, Vermont and Washington state, especially — vaccination rates are now far below the herd-immunity level.
This trend, predictably, is leading to the resurgence of diseases considered vanquished long ago. In 2010, for example, California had an outbreak of whooping cough, which at its height put 455 babies in the hospital and killed 10 of them. Elsewhere there have been outbreaks of measles.
The case for vaccination is clear. First, it makes the vaccinated individual either immune or resistant to a disease. Second, and more important, it interferes with contagion and thus makes the entire community safer, including those members, such as newborn babies or the very sick, who cannot be vaccinated for medical reasons. The vaccination rate for herd immunity varies by disease, but usually falls between 85 percent and 95 percent.
The case against vaccination, by contrast, is not clear. One view seems to be that the diseases in question merely give you a rash and are a nuisance, whereas the vaccines will make your child autistic.
That particular myth, still peddled on the Internet, originated with Andrew Wakefield, a British doctor who in 1998 published a paper that suggested a link between autism and the common MMR shot, which guards against measles, mumps and rubella. The paper has since been entirely discredited, and Wakefield has been censured.
Other parents fret about thimerosal, a preservative that contains trace amounts of mercury. Not only were these amounts minute and safe, but thimerosal has been eliminated from all vaccines, with the exception of multidose flu shots. Often these parental conversations then turn into new-agey free-for-alls, where vaccines somehow join the list of conventionally grown avocados and unfiltered tap water as lethal menaces.
Nonetheless the issue runs straight into the classic American tension between individual choice, which is good, and public health and safety, which are also good. All states save Mississippi and West Virginia allow a religious exemption for vaccinations, and 20 states also allow a more vague philosophical exemption. So what can health officials do? Principally they can try to educate more parents with good science rather than Internet drivel. Pan is also pushing a law through the Legislature that would require parents to discuss vaccination with their pediatricians. If the parents still want to opt out, their doctors would have to sign a form attesting that the parents were told about all the risks and benefits.