02/22/2009 05:12 am ET | Updated Nov 17, 2011

Don't Risk Going Unvaccinated

This past year the United States witnessed a measles epidemic that was the largest in more than a decade. About 135 people, mostly children, were infected with measles; some of those children were hospitalized with severe dehydration and others with pneumonia caused by the virus.

Why did this happen? The answer can be found in a study published in December 2008 in the American Journal of Epidemiology that received little attention from the media. The authors, epidemiologists from Johns Hopkins Bloomberg School of Public Health, examined school children in Michigan whose parents had chosen not to vaccinate them. They compared clusters of unvaccinated children with clusters of documented whooping cough (pertussis) outbreaks. Not surprisingly, the clusters overlapped. The authors concluded: "Geographic pockets of vaccine exemptors pose a risk to the whole community."

This is exactly what had happened during the measles epidemic in 2008. Almost all of the children who caught and transmitted measles were unvaccinated. The authors of this study had provided an insight into the obvious. If parents choose not to vaccinate their children, not only do they put their own children at risk, they put others at risk. Because no vaccine is 100 percent effective, some vaccinated children can still get pertussis. Others at even greater risk include children who haven't completed the entire series of pertussis vaccines or those who can't get vaccines because they are receiving steroids for asthma or chemotherapy for cancer.

The findings of the Hopkins researchers would have been somewhat more tolerable if the choice not to vaccinate was because of legitimate problems with vaccines. But the reason that some parents are choosing not to vaccinate their children is based on the mistaken notion that vaccines cause autism; or that vaccines cause diabetes or multiple sclerosis or asthma or allergies; or that vaccines weaken or overwhelm the immune system; or that vaccines have not been adequately tested. Many studies have addressed these concerns and should have reassured parents. But there appears to be a rift between studies that exonerate vaccines and the public's knowledge of those studies.

This latest study is not the first piece of evidence that a choice not to vaccinate is not a risk-free choice. In 2005, a 17-year old unvaccinated girl visited Romania, caught measles, returned to her home in Indiana, and proceeded to infect at least 34 more people, most of whom were also unvaccinated.

These outbreaks have not, apparently, been sobering. If anything, the number of parents choosing to delay or withhold or separate vaccines is increasing. So what will it take? Certainly, as more and more children contract measles and pertussis, deaths from these diseases will follow. And it's not a leap to believe that we could see other deadly diseases, like polio and diphtheria; both of which still occur commonly in some areas of the world; and both of which are only a plane ride away from causing outbreaks in relatively unvaccinated communities in the United States.

We can only hope that parents have not been lulled into a false sense of security by the success of vaccines -- or that our inattention to history will not cause us to relive it.

Paul A. Offit, MD is Chief of the Division of Infectious Diseases at the Children's Hospital of Philadelphia and the author of Autism's False Prophets: Bad Science, Risky Medicine, and the Search for a Cure.