As the rate of kids going to school unvaccinated rises and outbreaks of once-eradicated diseases are tied to low vaccination rates, some states are trying to make it tougher for parents to opt their children out of mandatory immunizations. The latest battleground is Colorado.

Colorado’s House of Representatives passed a bill that would limit vaccine exemptions earlier this year by a wide margin. But before sending it to the governor, the Senate stripped the bill of its toughest measure, which would have required parents to learn about the risks and benefits of vaccines before exempting their child, arguing that it impedes parental choice.

It’s a heated argument that’s played out in many states since vaccine exemptions took off decades ago, but a recent report and legislation elsewhere show that the trend of states relaxing vaccine laws may be reversing.

Between 2009 and 2012, 31 bills expanding vaccine exemptions were filed in statehouses, but not one passed, according to a study in The Journal of the American Medical Association. But in that same period, three of the five bills filed to restrict exemptions passed and became law. Oregon joined California, Vermont and Washington last year in adding limits to its exemptions.

“I think the tide is turning because people are seeing outbreaks of vaccine-preventable diseases like measles, chicken pox, Pertussis, and there’s evidence that a contributing factor was parents who chose not [to] vaccinate their kids,” said Diane Peterson, associate director of the Immunization Action Coalition and an author of the study.

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Every state requires kids to get vaccinated against diseases like measles, mumps, polio and whooping cough to enroll in public school. Every state also allows parents to exempt their kids for doctor-approved medical reasons, and most states (except Mississippi and West Virginia) let parents opt out for religious reasons. But Colorado is one of 20 states that also allow parents to refuse to vaccinate their children because it conflicts with other “personal beliefs” that are often grounded in fears dismissed by major medical organizations that vaccines come with risky side effects or cause autism.

Peterson and others argue that the majority of stories about bad reactions falsely link the vaccine to an unrelated health event. “It’s not based in science; they’re beliefs,” she said. “They witnessed their child, who was developing normally, until coincidentally they took a vaccine and then something changed.”

Both Oregon and Washington state's laws now require parents seeking an exemption from a school-required vaccination to show that they were consulted about the risks and benefits of vaccination by a doctor or that they watched an online education module. That same mandatory education provision is what the Colorado Senate removed. The Oregon law just took effect earlier this year, but since the Washington law took effect in 2011, the exemption rate for students entering kindergarten has dropped 27 percent, according to the Associated Press.

Colorado is one of 11 states where at least 4 percent of its public school students are exempt from mandatory vaccinations. According to a 2012 study in The New England Journal of Medicine, exemption rates are higher in states make it easier to opt out and 2.54 times higher in states that allow personal belief exemptions than states that only allow religious exemptions. But exemptions and outbreaks of disease are on the rise -- even in states that allow only a religious out.

Public health experts have long held that exemptions pose a threat because diseases spread more easily among populations with lower rates of vaccination. Colorado, for example, recently experienced a higher number of cases of whooping cough, which has surged nationally in recent years and puts infants, people with compromised immune systems and unvaccinated kids most at risk -- though the effectiveness of the available vaccine has also been blamed. A study of California’s 2010 whooping cough outbreak published last year in the journal Pediatrics found the disease was significantly more prevalent in areas with lower vaccination rates.

Outbreaks fueled Colorado's efforts to toughen exemption laws, but it's an uphill battle. The most recent bill passed the House with a 42-19 bipartisan vote, but the Senate removed the education requirements.

“In the Senate, there are only 35 members. If there are a few members who don’t have a pro-public health viewpoint, their position may carry more weight,” said Sundari Kraft, a spokeswoman for a coalition of groups that supports a more stringent law. “It was just a different political climate.”

Kraft argued that what the Senate passed is still an improvement over current law because the bill includes reporting requirements from schools and requirements for parents to reapply for exemptions.

Opponents in the Senate argued the mandatory education requirements take too much control from parents, some of whom crowded into hearings to say their children experienced serious reactions from vaccinations.

State Sen. Kevin Lundberg, a Republican who voted against the amended bill, said he believes the threat of adverse reaction is downplayed through institutional bias within the medical establishment. “I believe there’s a place for immunizations, but there’s a place for parent’s choosing what’s best for their children,” he said.