Friday, 28 September 2007

Journal Watch on limitations of Thimerosal Study in New England Journal

Thimerosal and Neurotoxicity
The effect of mercury exposure in children once again proves difficult to determine.
Considerable controversy surrounds the question of whether mercury preservatives in vaccines, such as thimerosal, are associated with impaired behaviors or cognition in children (see Journal Watch Psychiatry Aug 4 2004). These researchers examined 42 measures of neuropsychological and behavioral functioning in children aged 7 to 10 years from four HMOs. Using histories of vaccination, immunoglobulin experience, and other records, the researchers retrospectively calculated mercury exposure at three time points: neonatal (i.e., maternal), the first 28 days of life, and the first 7 months.
Children with low birth weight, meningitis, encephalitis, or other serious neurological diagnoses were excluded. The 1047 tested children represented only 30% of those eligible. Children of parents with the highest socioeconomic indicators had the highest exposure. Small yet significant negative and positive associations, some sex-specific, were identified between some individual neuropsychological tests and thimerosal exposure.
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.
Barbara Geller, MD
Published in Journal Watch Psychiatry September 26, 2007
Thompson WW et al. Early thimerosal exposure and neuropsychological outcomes at 7 to 10 years. N Engl J Med 2007 Sep 27; 357:1281.
Original article (Subscription may be required)
Medline abstract (Free)