WASHINGTON -- Former Texas Gov. Rick Perry (R) has stayed relatively quiet during the latest outbreak of potential 2016 presidential contenders stumbling on vaccination policy, even though he has perhaps the most interesting history on the issue. In 2007, Perry became the first governor in the U.S. to require young women to get vaccinated against a sexually transmitted disease that can lead to cervical cancer. The move ignited a firestorm among some conservatives -- who claimed the vaccine would promote promiscuity -- and Perry eventually backtracked and called what he did "a mistake."But in 2003, Perry severely undercut the public health community's push to increase the vaccination rate by signing into law a measure that made it easier for parents to opt out of vaccinating their children. The bill received less national attention than what he did in 2007, but Texas is still feeling the ramifications today.Every state requires parents to vaccinate their children against measles, mumps and rubella before they enter elementary school. They also all allow exemptions for medical reasons. Texas law used to say that a parent could exempt their child from immunization if it "conflicts with the tenets and practice of a recognized church or religious denomination of which the applicant is an adherent or a member." But the 2003 bill that Perry signed into law significantly loosened that opt-out clause, saying it was acceptable to skip vaccination "for reasons of conscience, including a religious belief." In other words, people can opt out for any reason. Texas is now one of 20 states that allow this "philosophical exemption," according to the National Conference of State Legislators. The result has been troubling to public health experts. Since that time, the rate of non-medical exemptions from vaccinations has climbed at an "alarming rate," according to The Immunization Partnership, a vaccination-advocacy group based in Austin. The problem isn't necessarily the number of people forgoing vaccinations. It's where they are located. "People who exempt out of vaccines tend to cluster in a geographic area," explained Anna Dragsbaek, president and CEO of The Immunization Partnership. "So you can have one or two schools that have extremely high exemption rates -- over 50 percent. If an outbreak were to strike, those schools could really have a problem. The clustering is what you have to be aware of. It's not just the pure number of kids that have exemptions."Dragsbaek said she would like to see non-medical exemptions repealed altogether, since they're not based in scientific fact. Short of that, she said, it should be harder to obtain an exemption. Texas has a process that's slightly more stringent than some other states. "Exemptions should be at least as hard, or harder to get, than a vaccine," said Dragsbaek. "So there are several states that have started to require a doctor's note stating that the parent has been counseled on the risks and benefits of immunization, and they're still declining that. In states where they've started that, we're starting to get some preliminary evidence that this does reduce the rates of people who are exempting out of vaccines."Perry spokesman Travis Considine noted that the vaccination provision was just one small part of H.B. 2922, which was a much larger health care bill. Texas doesn't allow a line-item veto, so Perry had to take the whole bill or send the entire thing back to the legislature. Considine added that the former governor would be pleased to see the state adopt stronger vaccination policies. "Gov. Perry thinks parents should vaccinate their children, and would support tightening Texas' vaccination guidelines," Considine told The Huffington Post Wednesday.Dragsbaek noted that Perry was "very supportive of immunization" and generally "pro-vaccine." But the law he signed, H.B. 2922, came during a wave of skepticism about vaccines and whether they caused autism, and a trend toward non-medical exemptions. That link between vaccines and autism was first raised in a 1998 medical study that has since been thoroughly discredited. More than 100 people in 14 states contracted measles in January, most of them stemming from an unvaccinated woman who visited Disneyland in December. According to Dr. Anne Schuchat, director of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention, 79 percent of unvaccinated people in the U.S. who caught measles last year weren't vaccinated because of personal belief exemptions.Vaccination policy shot into the news this week after President Barack Obama said in a pre-Super Bowl interview that parents should vaccinate their children. In response, a few potential GOP presidential candidates wavered on mandatory vaccination and called for more parental choice. New Jersey Gov. Chris Christie (R) and Sen. Rand Paul (R-Ky.) eventually walked back their comments. Click here to read full news..