Vaccines containing the preservative thimerosal had no effect on the risk developing autism, a new Danish study concludes.
Thimerosal, which contains ethylmercury, has become a focal point in the debate over the cause of the worldwide rise in the number of diagnosed cases of autism. Mercury in high doses has been known to cause brain damage. Thimerosal was withdrawn from use in Denmark in 1992 and in the United States in 1999 as a precaution.
Last month, another study reported that autism cases in Denmark continued to rise even after thimerosal was withdrawn.
The new study, published last week in The Journal of the American Medical Association, involved a more detailed analysis: it drew on the individual medical records of nearly 500,000 children born from 1990 to 1996 to match individual exposures to thimerosal through the pertussis vaccine and cases of autism.
The study's lead researcher, Anders Hviid of the Statens Serum Institute, the government agency that distributes vaccines in Denmark, said the risk of developing autism was not significantly different among the children receiving either type of vaccine. That was true even of children who received three shots containing thimerosal instead of one or two, he said.
Mr. Hviid acknowledged that Danish children were given fewer shots containing thimerosal over the complete regimen of childhood vaccinations than American children received. But he said the individual doses used in Denmark were larger. Since the body cleanses itself of ethylmercury, the concern has always been about peak, not cumulative exposure, he said.