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Thimerosal: No Smoking Gun

CDC: No Evidence of Child Brain Damage Due to Vaccine Preservative
By
WebMD Health News
Reviewed by Louise Chang, MD

Sept. 26, 2007 - Thimerosal in vaccines has not affected children's brains or behaviors, the CDC says.

Thimerosal is a mercury-based preservative once common in vaccines. It's now found only in small amounts in some flu vaccines. However, thimerosal was once used in most vaccines given to children.

Although the amounts were tiny, they added up. Because symptoms of autism often appear suddenly around age 1 -- when kids have received a number of vaccinations -- many parents became convinced that thimerosal caused their children's autism. But the Institute of Medicine has twice rejected this idea.

The new CDC study carefully avoided the still-controversial issue of whether thimerosal is linked to autism. A separate CDC study of this issue is under way, with a report expected in a year's time.

"In this study there is nothing you can draw on regarding any relationship to autism," researcher William W. Thompson, PhD, of the CDC's National Immunization Program, said at a news conference.

Instead, the study looked for 42 different neurological and psychological problems in more than 1,000 7- to 10-year-old children exposed to various amounts of thimerosal while still in the womb, from birth to 28 days, or up to age 7 months.

"This was a large, very careful study to determine whether thimerosal exposures in early childhood was associated with bad neuropsychological outcomes," Anne Schuchat, MD, director of the CDC's National Center for Immunization and Respiratory Diseases, said at the news conference. "It came out with very reassuring results."

"Ninety-eight percent of the answers here are very reassuring," John Iskander, MD, MPH, acting director of the CDC's Immunization Safety Office, said at the news conference.

Thimerosal and Tics

What about the other 2% of the study findings? The study looked at 42 different outcomes, ranging from intelligence to fine motor coordination. The researchers performed 378 statistical tests on the data. Each statistical test had 5% odds of being a chance finding.

"By chance alone we would estimate about 5% of test results would be significant," Schuchat said. "And in fact that is exactly what we found: 19, or 5%, of the statistical tests did find significant results. They were pretty evenly split between [linking] better and worse outcomes [to thimerosal]."

One of those worse outcomes was a slightly increased risk of tics -- repetitive physical motions or vocalizations. What makes the finding plausible is that two previous studies identified tics as a possible thimerosal side effect.

However, there is good reason to think that the tic finding was simply chance:

  • The significant finding for tics came from observers who saw the child only once. Parents of children with the highest thimerosal exposure reported no more child tics than parents of children with lower thimerosal exposure.
  • The finding applied to boys, but not to girls.
  • The tic finding included transient tics, which experts do not consider a problem.

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