Thimerosal, the mercury-containing preservative once widely used in vaccines, does not appear harmful to children's brains, a large U.S. study suggests.
The study of more than 1,000 children by the Centers for Disease Control and Prevention in the U.S. found that exposure to thimerosal in infancy or while in the womb is not associated with neuropsychological problems in kids aged seven to 10.
The report is published in the New England Journal of Medicine.
Thimerosal has long been fingered as a possible culprit behind the increasingly common disorder of autism. But studies have either not been able to prove a link or have been inconclusive.
In 1999, vaccine manufacturers agreed to reduce thimerosal as a precautionary measure. Today, all routinely recommended pediatric vaccines contain either no thimerosal or trace amounts.
While this study didn't look at autism - a further CDC study is currently examining that link -- it found that the ingredient didn't appear harmful to children's learning abilities, speech or motor skills, memory or attention levels.
"The bulk of the study really found very, very similar performance in children who were exposed to high amounts of thimerosal and children who were exposed to low or no thimerosal," Dr. Anne Schuchat, director of the National Center for Immunization and Respiratory Diseases at the CDC said Wednesday.
The researchers detected only a few significant associations with exposure to thimerosal, but the associations were small and almost equally divided between positive and negative effects. That is: some of those who reported high exposures to thimerosal-containing vaccines actually scored better on some of the tests, while others performed worse.
Some of those findings are likely due to statistical chance, said CDC epidemiologist William Thompson, lead author of the study.
However, the study did find that some children with high exposure to thimerosal from vaccinations had double the risk of motor tics -- mostly marked by finger-tapping --compared to those with a low exposure.
"The parental report of any kind of tics was not associated with high thimerosal exposure, but evaluators' observations of the children did suggest in boys - but not in girls - a higher risk of tics," said Schuchat, noting that two previous studies by different research groups had also found what appeared to be an increased risk of tics linked to vaccine use.
Because the children were with CDC evaluators for only three hours, it's unclear if the tics were transient or a more serious form.
"So this particular finding may or may not have importance and is being looked at a bit further," Schuchat said.
