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Thimerosal is a preservative used in some vaccines. It contains almost 50% mercury by weight and is at the centre of a long running controversy in the US about the harm it might do to children. In 1999, vaccine manufacturers were asked to phase out thimerosal from their products, but in the meantime researchers have been looking hard for evidence of neurological harm. So far, they have found little.
The latest study analysed multiple neuropsychological tests done by 7-10 year olds who were exposed to mercury from thimerosal in the womb, or in the first seven months of life. The authors looked at 42 different outcomes and found significant associations with thimerosal for only a handful. The associations were weak and inconsistent. About half implied harm, the others implied the opposite. For example, increasing mercury exposure during the first month of life was associated with poorer performance on one measure of speech articulation and better performance on one measure of fine motor coordination.
These authors and a linked comment (p 1278) say the weight of evidence is now firmly in thimerosal's favour. The only harm done by this agent was the general panic that followed the clumsy announcement that it was to be withdrawn, says the commentary.