Two large studies conducted in Great Britain looked at vaccines containing mercury-based preservative (thimerosal) and child development. The first study was a prospective study, meaning that it was designed to look forward and to control for other variables at the outset. It is a very large study of child development involving over 14,000 children. Not only did exposure to thimerosal show no negative effects, but children who had higher exposures actually had better outcomes, including less hyperactivity, and fewer problems with motor and speech development. The study showed eight potentially beneficial effects and only one negative effect — slightly higher risk of poor prosocial behavior at 47 months of age.
A much larger retrospective (looking back) study looked at over 109,000 children and found several positive associations; exposure to thimerosal was associated with decreased number of attention problems and also developmental delays. In one analysis, there was a slightly higher risk of tic disorders.
These large studies showed no evidence of a link between ethylmercury preservative (thimerosal) and neurological and psychological development in children in Britain. During the years these studies were conducted, the maximum exposure to thimerosal was less than that of infants in the United States; however, the dose was still significant, and these studies, like others, continue to show no link between ethylmercury preservative in vaccines and neurological or psychological problems.
