As medical success stories go, few can match the overall results of America’s preschool vaccination program. Public health threats like polio, smallpox and diphtheria… which once killed or crippled many young people each year… have largely disappeared in the United States thanks to compulsory vaccination.

But that may be changing. Public health officials are now concerned that too many children are being allowed to opt out of the program, opening the door for old scourges to sneak back in and do harm.

Researchers at Johns Hopkins University and the University of Florida recently examined the vaccination exemption policies of all fifty states. Although states routinely permit exemption for medical or religious reasons, the authors found that some states make exemption widely available, either by offering a broader personal belief exemption or by making exemptions easy to obtain.

Their study correlated these exemption characteristics with the incidence of whooping cough, one of the few vaccine-preventable diseases experiencing a rebound in the United States, with nearly twenty-six-thousand cases reported in 2004. Indeed, the researchers found, states that readily grant exemptions or offer personal belief exemptions have about fifty percent higher rates of whooping cough, which is also known as pertussis.

In a separate study, the researchers found the number one reason why parents refuse vaccines and claim exemptions are concerns about vaccine safety, despite strong scientific evidence that vaccines are extremely safe.

But somehow that message doesn’t always get through, leaving a dangerous hole in the vaccine safety net.