Laws that allow parents to opt out of school immunization requirements due to philosophical beliefs are linked to increases in the number of children exempted from vaccinations, a new study finds.

The study examined changes in childhood immunization rates that occurred among Arkansas schoolchildren before and after the 2003 passage of legislation allowing philosophical exemptions.

"The change in the law in Arkansas resulted in a modest increase in the rates of parents taking exemptions," said Daniel Salmon, Ph.D., a study co-author.

More nonmedical exemptions could potentially increase the risk of vaccine-preventable diseases, the authors say in the March issue of the American Journal of Preventive Medicine.

Before 2003, Arkansas children could be exempted from school immunization requirements for medical or religious reasons. However, in 2003, the Federal court ruled that Arkansas' ability to determine if a religion is "recognized" as a basis for exemption was unconstitutional.

New legislation implemented in Arkansas in fall 2003 retained medical exemptions but also enabled parents to apply for exemptions for their child based on philosophical objections. Religious exemptions were no longer recognized.

From 2001 to 2002, the total number of immunization exemptions granted increased by 23 percent, from 529 students in 2001 to 651 students in 2002.

From 2002 to 2003 the first year that philosophical exemptions were allowed the total number of exemptions granted rose to 764 students, an increase of 17 percent. By 2005, the total number of exemptions increased another 50 percent, with 1,145 students claiming exemptions

The percentage of exemptions that were nonmedical increased from 79 percent in 2001 to 95 percent in 2005.

Although the percentage increases in exemptions were substantial, the increases in the number of children exempted were fairly small, said Salmon, an associate professor of epidemiology at the University of Florida College of Medicine in Gainesville.

"If [the exemption rate] continues to increase 30 percent to 50 percent per year, it's concerning. If it levels out, it's less of a concern. What happens in the next few years will be quite telling," Salmon said.

"I believe we do need to have the availability of exemptions for people who hold deep convictions that this is absolutely wrong for their child," said Neal Halsey, M.D., a professor at the Johns Hopkins Bloomberg School of Public Health.

However, "exemptions are associated with increased risk of disease in the community, so it's a reason to be a bit concerned," said Halsey, who did not participate in the study but was familiar with the Arkansas legislation.

"We can't afford to have very many children not getting immunized or we will be getting outbreaks of disease," such as measles and pertussis, Halsey said.

American Journal of Preventive Medicine: Contact the editorial office at (858) 457-7292.

Thompson JW, et al. Impact of addition of philosophical exemptions on childhood immunization rates. Am J Prev Med 32(3), 2007.

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