A mercury-based vaccine preservative did not appear to affect language or other similar brain functions in children, US researchers said on Wednesday in the first of a series of studies meant to lay to rest the controversy over thimerosal.
Their study of more than 1 000 children aged 7 to 10 showed that having been exposed to thimerosal in vaccines before and after birth did not affect neuropsychological functions such as verbal ability, fine motor control, memory and attention.
The study did not look at autism - another study, to be released within the next year, is doing that.
"The weight of the evidence suggests there is no association," said William Thompson of the US Centres for Disease Control and Prevention, who led the study.
"Among the 42 neuropsychological 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," the research team wrote in their report, published in the New England Journal of Medicine.
In other words, Thompson said in a telephone interview, some language, attention and motor skills seemed to be slightly enhanced when children or their pregnant mothers had received more thimerosal-containing shots, and others seemed worse.
This probably means all the associations are chance, he said. One exception was a type of tic called a motor tic, which seemed to be more common among boys who got more thimerosal.
The CDC researchers said it was a subtle tic that none of the children's parents had noticed, mostly marked by finger-tapping. They said they would investigate further.
"There is no massive surprising result here. The findings are very reassuring," Dr Anne Schuchat, director of the CDC's National Centre for Immunization and Respiratory Diseases, told reporters.
Thimerosal has been used as a preservative in vaccines for decades. While it contains mercury, the mercury is in a chemical form called ethyl mercury that scientists say cannot affect the body in the same way as methyl mercury - found in contaminated fish and in industry.
Concerns first arose when activists began to link cases of autism to vaccines. Eventually, some settled on thimerosal as a potential cause of the condition, which can cause symptoms from poor social skills to profound disability and retardation.
The CDC has always maintained that thimerosal is safe, but eventually persuaded manufacturers to take it out of all childhood vaccines except for flu shots by 2001. – (Reuters Health)
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