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BMJ. Sep 11, 2004; 329(7466): 588.
PMCID: PMC516680

Thiomersal doesn't cause developmental disorders

Two studies report that vaccines that contain thiomersal do not cause behavioural problems or developmental delay in young children. The studies, published in Pediatrics, are the University of Bristol's Avon longitudinal study of parents and children (Pediatrics 2004;114: 577-83 [PubMed]) and a review article by US authors (2004;114:793-804).

The Bristol study, also known as the “children of the 90s” study, is the largest of its kind in the world. It followed nearly 13 000 children from their births in 1991 and 1992 in southwest England. The children were born when the recommended vaccines contained thiomersal and when an accelerated vaccination schedule was introduced.

Thiomersal, known as thimerosal in the United States, is a preservative that was used until recently in many children's vaccines. In the United States it is either not used at all in vaccines or used in very small amounts. Its use in vaccines in the United Kingdom will be discontinued from the end of this month.

Thiomersal contains about 50% ethylmercury, an organic compound that is metabolised into mercury. Concern about thiomersal arose because a related compound, methylmercury, was toxic in several environmental disasters. Low doses of methylmercury seem to have adverse developmental effects on children who are exposed before birth or in the first few months of life.

It was suggested that low doses of ethylmercury used as a preservative in vaccines might have similar detrimental effects, although ethylmercury is more quickly metabolised and excreted than methylmercury.

In the Bristol study, immunisation records showed when the children received the vaccines. Researchers calculated the mercury exposure the children received at 3, 4, and 6 months. The children's cognitive and behavioural development between 6 and 91 months was assessed in standard international tests of social, emotional, hyperactivity, and motor skills.

The researchers found no convincing evidence that early exposure to the additive in vaccines had any deleterious effect on neurological or psychological outcomes.

In the review article researchers from the University of Colorado and the Centers for Disease Control and Prevention reviewed 10 epidemiological and two pharmacokinetic studies of ethylmercury, seeking evidence that the additive might cause autism or related disorders. They reported that the epidemiological studies did not show a link and that the pharmacokinetic studies made the link unlikely.


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