MONDAY, Sept. 13 (HealthDay News) -- Infants exposed to the highest levels of thimerosal, a mercury-laden preservative that used to be found in many vaccines, were no more likely to develop autism than infants exposed to only a little thimerosal, new research finds.

The study offers more reassurance to parents who worry that vaccination raises their children's risk for autism, the researchers said.

"Prenatal and early life exposure to ethylmercury from thimerosal in vaccines or immunoglobulin products does not increase a child's risk of developing autism," concluded senior study author Dr. Frank DeStefano, director of the immunization safety office at the U.S. Centers for Disease Control and Prevention.

The study was released online Sept. 13 in advance of publication in the October print issue of Pediatrics.

Thimerosal has been used as a preservative in vaccines since the 1930s, according to background information in the article.

Concerns about the chemical began to crop up in 1999, when the U.S. Food and Drug Administration said that because of the increased number of thimerosal-containing vaccines added to the infant vaccination schedule, infants may be exposed to too much mercury. Thimerosal used to be found in hepatitis B, Hib (Haemophilus influenzae type B) and DTP (diphtheria, tetanus, pertussis) vaccines, among others.

During the ensuing years, the FDA worked with manufacturers to eliminate thimerosal from vaccines, according to the agency's Web site. Today, thimerosal has been removed or reduced to trace amounts in all vaccines routinely recommended for children 6 years of age and younger, with the exception of inactivated seasonal flu vaccine, according to the FDA. Parents who are concerned about thimerosal can ask for a preservative-free version, DeStefano said.

And thimerosal wasn't the only proposed autism-vaccine link. A 1998 paper in The Lancet suggested the MMR (measles-mumps-rubella) vaccine might trigger autism. The journal later retracted the paper, and numerous studies have refuted any link between the MMR vaccine and autism.

In February of 2009, a U.S. federal court ruled that there was no scientific evidence linking vaccines to autism.

In the new study, researchers examined medical records and conducted interviews with the mothers of 256 children with an autism spectrum disorder and 752 children matched by birth year who did not have autism. The children were all members of three health care management organizations in California and Massachusetts.

Researchers also gathered information about the manufacture and lot number of the vaccines that the children received, to determine how much thimerosal they were likely exposed to.

Children in the highest 10 percent of thimerosal exposure, either prenatally or between infancy and 20 months, were no more likely to have autism, an autism spectrum disorder or autism spectrum disorder with regression than children in the lowest 10 percent of exposure.

"This study adds to a large body of evidence indicating that early thimerosal exposure through vaccination does not cause autism," said Geraldine Dawson, chief science officer for a leading advocacy group, Autism Speaks. Dawson was not involved with the research.

She urged parents to have their children vaccinated.

"We encourage parents to have their children vaccinated and to establish a trusting relationship with their child's pediatrician so they can discuss any concerns they have," Dawson said.

More information

The U.S. Food and Drug Administration has more on thimerosal.