Background It has been hypothesized that early exposure to thimerosal, a mercury-containing preservative used in vaccines and immune globulin preparations, is associated with neuropsychological deficits in children. Methods We enrolled 1047 children between the ages of 7 and 10 years and administered standardized tests assessing 42 neuropsychological outcomes. (We did not assess autism-spectrum disorders.) Exposure to mercury from thimerosal was determined from computerized immunization records, medical records, personal immunization records, and parent interviews. Information on potential confounding factors was obtained from the interviews and medical charts. We assessed the association between current neuropsychological performance and exposure to mercury during the prenatal period, the neonatal period (birth to 28 days), and the first 7 months of life. Results Among the 42 neuropsychological patient outcomes, we detected only a few significant associations with exposure to mercury from thimerosal. The detected associations were small and almost equally divided between positive and negative effects. Higher prenatal mercury exposure was associated with better performance on one measure of language and poorer performance on one measure of attention and executive functioning. Increasing levels of mercury exposure from birth to 7 months were associated with better performance on one measure of fine motor coordination and on one measure of attention and executive functioning. Increasing mercury exposure from birth to 28 days was associated with poorer performance on one measure of speech articulation and better performance on one measure of fine motor coordination. Conclusions 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. ##### Comment: The authors, who expertly analyzed these data statistically, conclude that thimerosal does not produce meaningful clinical problems. However, the validity of this conclusion needs to be questioned because of possible confounding in the study sample. First, only 30% of cases were studied; how these cases may have differed from those who did not participate was not discussed. Second, the data were collected retrospectively. Third, the cases that were excluded — e.g., low-birth-weight babies — might be the ones most vulnerable to the effect of a neuroactive substance, but this issue was not addressed. Finally, the threshold dose for neurological impairment is unknown; therefore, to dismiss the vaccines given between ages 12 months and 7 years may be premature. Parents who ask about this issue still need to be informed of the complexities of studying this problem . ####### NYTimes: Vaccine Compound Is Harmless, Study Says, as Autism Debate Rages Yet another study has found that a controversial vaccine preservative appears to be harmless. But the study is unlikely to end the increasingly charged debate about vaccine safety. The study examined whether thimerosal — a mercury-containing vaccine preservative that was almost entirely eliminated from childhood vaccines by 2002 — is associated with neurological or certain psychological problems in children ages 7 to 10. Some parents' groups and prominent legislators contend that thimerosal has caused an epidemic of childhood autism. Several studies have examined this question and found no evidence that thimerosal is associated with autism. The most recent study did not assess thimerosal's association with autism directly. The Centers for Disease Control and Prevention is conducting a separate autism and thimerosal study that is expected to be published next year. In this study, published yesterday in The New England Journal of Medicine, researchers from the C.D.C. and several managed-care organizations subjected 1,047 children to 42 neurological and psychological exams, which included I.Q. tests, how well children recalled a list of names and whether they could repeat the names backward, their manual dexterity, and whether they stuttered or had tics. The researchers also took detailed medical histories to determine whether the subjects' mothers were exposed to thimerosal while pregnant, and how much thimerosal the children were exposed to in their first seven months of life. After subjecting the data to nearly 400 different statistical measures, researchers found 19 different possible associations between thimerosal and various mental outcomes — most of which suggested that thimerosal was actually beneficial. Researchers largely dismissed these associations as statistical flukes. "By chance alone, with that number of tests, we would estimate that 5 percent of the results would be significant," said Dr. Anne Schuchat, director of the National Center for Immunization and Respiratory Diseases at the C.D.C. "And that's what we found." The lone worrisome result was that, among boys, the study found an association between thimerosal and tics, which are involuntary movements or sounds. At least one earlier study had found a similar association. But Dr. Schuchat said that researchers had made no distinction between transitory tics — those that soon disappear and are not considered clinically important — and permanent, serious or disfiguring tics. "That particular finding is being evaluated further," Dr. Schuchat said. Dr. Jeffrey Baker, a pediatrician and vaccine expert who is director of the history of medicine program at Duke University, said that the study's findings should be reassuring for parents. "This study will further strengthen a growing consensus among researchers that there is no real evidence that thimerosal in vaccines led to any actual harm," Dr. Baker said. But Sallie Bernard, executive director of SafeMinds, a nonprofit parent organization whose members contend that thimerosal injured their children, said the study was inconclusive. Ms. Bernard served on a board of consultants that helped design and oversee the study, but she withdrew her support for the published version of the study, saying its conclusions were not supported by the underlying data. "There are some red flags here," Ms. Bernard said. Nearly 5,000 families have filed claims with the federal government contending that vaccines caused their children to become autistic. Even if the government dismisses their claims, many families have vowed to continue their fight in the courts. Since thimerosal's removal from vaccines, there has been no evidence that autism is on the decline.
ABSTRACT
Tuesday, November 25, 2008
Vaccines & Thimerosal (New England Journal of Medicine & NYTimes)
New England Journal of Medicine
Early Thimerosal Exposure and Neuropsychological Outcomes at 7 to 10 Years
Published: September 27, 2007
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