Kids' Vaccine Ingredient Not Likely Linked to Neurological Problems

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They then compared those findings to levels of thimerosal exposure, based on the child's vaccination exposure prenatally, in the first month of life, and in the first seven months of life.

"Our study does not support a causal association between early exposure to mercury from thimerosal-containing vaccines and immune globulins and deficits in neuropsychological functioning at the age of 7 to 10 years," concluded the study's authors.

That conclusion, however, seems to contradict some of the study's findings.

For example, the researchers found that boys with the highest levels of thimerosal exposure had about twice the risk of evaluator-observed tics compared to boys with the lowest exposure.

While these findings may seem disparate with this conclusion, Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases, said some of the findings are likely due to chance, because such a high number of statistical comparisons were done. She was not involved in the study, which was led by William Thompson of the CDC's influenza division.

According to Schuchat, who spoke at a special CDC teleconference held Wednesday, the researchers completed 42 tests of neuropsychological functioning and completed 378 statistical comparisons from those tests.

"By chance alone, 19 of the 378 statistical tests we ran would be abnormal," she said, noting that 19 is the exact number of tests that resulted in abnormal findings.

However, 12 of those 19 tests suggested a positive outcome from higher thimerosal exposure, and seven, including the tic finding, suggested a negative outcome.

"Each test doesn't tell us as much individually," she said. "Chance alone probably explained these findings. The totality of the results are quite reassuring."

Additionally, Schuchat said there was no increase in the incidence of parent-reported tics in children with the highest thimerosal exposure, so the tics observed by the evaluators may have been transient in nature.

But several other studies have found a possible link between thimerosal exposure and tics. Because of that, the CDC feels the finding should be further explored to ensure that it was due to chance finding and not an actual effect of the preservative.