Joe Raedle/Getty Images(NEW YORK) -- An increasing number of parents are getting state approval to allow their children to opt out of school-mandated vaccinations for non-medical reasons, according to a new analysis published Wednesday.
Dr. Saad Omer, author of the correspondence published in the New England Journal of Medicine, warned that this trend is leaving large populations of children at risk for developing potentially deadly illnesses that haven't been seen in the United States in many years.
"Rates of exemption are substantially higher today than several years ago," said Omer, assistant professor of global health, epidemiology and pediatrics at Emory University in Atlanta. "Previously, rates were only rising in states with easy exemption policies, but now they are even rising in states that make it more difficult."
Exemption policies vary from state to state and can range from not allowing any non-medical exemptions to allowing opt-outs for religious or philosophical reasons. Some states make it very difficult to get approval for exemptions by requiring notarized letters from clergymen, letters written by parents with specific wording, or completion of standardized forms that can only be obtained from special locations such as health departments. Others make it very simple to skip vaccinations: Parents need only check a box on a short, standardized form.
Omer and his colleagues analyzed data compiled by the U.S. Centers for Disease Control and Prevention on vaccination exemption rates for school years 2005 to 2011. They compared vaccine opt-out rates in each state to the ease with which exemptions could be obtained.
They found that parents were 2.5 ***** more likely to opt out in states that permit philosophical reasons compared with states that require religious objections. They were also more than two ***** more likely to opt out in states with easy exemption processes.
In most cases, fears among parents over vaccines -- many of them unfounded -- may be at play.
"The CDC and health departments are doing a good job of increasing vaccine coverage," Omer said. "Therefore, rates of vaccine-preventable disease are going down substantially. Parents aren't seeing the actual diseases, so when they hear about real or perceived adverse effects of vaccines, their perception of the risks versus benefits is shifted."
Dr. William Schaffner, chair of preventive medicine at Vanderbilt University, agreed.
"Parents are just more skeptical about benefits of vaccines," he said. "Most young parents today have never seen a case of polio or measles, and they didn't learn about the seriousness of these diseases and importance of the vaccinations in school."
Schaffner added that he finds it interesting that some states do not allow exemptions while others "really oblige parents wishing to opt out."
Past research has shown, though, that in states with a substantial proportion of unimmunized or incompletely immunized children, many kids are susceptible to these classic diseases. A 2006 study showed a 50 percent higher incidence of pertussis -- commonly known as whooping cough -- in states where it is easier to get exemptions. There have been similar findings with respect to measles.
Schaffner said he is very concerned that unvaccinated children who go abroad will bring back diseases, such as measles, that are still a major problem in other parts of the world. Not only will they suffer, but they will spread the illnesses to children who are unable to be vaccinated for medical reasons.
Schaffner said that part of the solution to the problem is that "states with easier process need to tighten up." However, he said, this is not a fool-proof approach. He said some research has shown an increase in medical exemptions in states that have tightened up their policies -- suggesting that parents are pressuring doctors to give medical exemptions. He encourages doctor to not let themselves be "brow-beaten into providing dubious excuses."
Copyright 2012 ABC News Radio