Children who weren't inoculated with the measles vaccine were 35 times more likely to contract the disease than those who were inoculated during a seven-year period, according to a study in last week's Journal of the American Medical Association.
The risk of measles today is relatively low--there were only 138 cases reported in the United States in 1997--but health officials fear that pockets of unvaccinated children could set the stage for an outbreak of the disease like the one in the late 1980s.
"When parents vaccinate children, they protect their child and the community," said Daniel Salmon, lead author of the study and a researcher with the National Immunization Program of the Centers for Disease Control and Prevention in Atlanta. "By not doing so, you are putting your child at risk and the community, too."
Using data collected from the Measles Surveillance System of CDC from 1985 to 1992, researchers found that about 0.5 percent of children who enter kindergarten each year are exempted from mandatory vaccination programs on grounds of religious or personal beliefs.
In the period studied, one major national outbreak of measles occurred, from 1989 to 1991. Thousands of children who had been inoculated, as well as many who had not, contracted the disease.
In the two years before the outbreak began, researchers found that the number of measles cases among vaccinated children declined by half, indicating that the vaccine had largely been working. During the same time, the number of measles cases among the unvaccinated was spreading fast--doubling over two years. The real outbreak began when the number of cases for both vaccinated and unvaccinated skyrocketed.
The data suggest that unvaccinated children played a role in spreading the disease during the outbreak but the evidence is not conclusive, Salmon said. Still, as the authors noted in their report: "The greater the increase in the number of exemptors, the more effect on the nonexempt population."
In the aftermath of the measles outbreak, public health officials recommended that children be given a measles booster shot. Since then, the number of measles cases has plummeted.
Because the overall measles vaccination rate is high, most unvaccinated children will not contract the disease. This reality led the authors to write that some parents may be claiming exemptions and not be vaccinating their children because they can avoid the small individual risk of an adverse reaction to the vaccine while "relying on the fact that others are vaccinated to provide protection."
However, those who support the right to decline vaccinations on religious and philosophical grounds were critical of the study.
"I think it is really irresponsible for an article like this to paint the exemptions as lessening public health in this country." said Barbara Loe Fisher of the National Vaccine Information Center. "We've had a very low number of measles cases for several years, and as far as we know the same number of exemptors."
The study concluded that being exempted from measles vaccinations should be considered a risk factor for contracting the disease, and that "it is important to discover the underlying reasons why individuals are claiming exemptions."
CAPTION: Not Quite Eradicated (This chart was not available)