Issue Date: February 16, 2004
VACCINES' LINK TO AUTISM UNCLEAR
There has been a long-running debate in the U.S. over whether vaccines routinely given to infants contribute to the development of autism. In particular, some people believe that thimerosal--sodium ethylmercurithiosalicylate--until recently used as a preservative in most pediatric vaccines, may cause autism.
On Feb. 9, researchers presented findings on the subject to the Immunization Safety Review Committee of the Institute of Medicine (IOM). The information did not resolve the dispute.
Epidemiologists showed data indicating that thimerosal in vaccines is harmless. However, the toxicologists and a chemist presented findings showing that it is biologically plausible for thimerosal in vaccines to cause autism.
The researchers seemed to agree that U.S. autism rates rose dramatically in the 20 years or so before 2000, a period in which mercury exposure to infants from thimerosal nearly tripled, primarily because many more childhood vaccines were developed.
In 2001, IOM issued a report saying that, although current evidence neither proves nor disproves a link between thimerosal and autism, exposure to the preservative should be reduced. As a result, drug firms removed thimerosal from most pediatric vaccines.
Anders P. Hviid, biostatistician at the State Serum Institute in Copenhagen, told IOM that he had found no connection between autism rates and thimerosal exposure among about 467,000 Danish children born between 1990 and 1996.
But Boyd E. Haley, professor of chemistry at the University of Kentucky, presented evidence that autistic children have more difficulty excreting mercury than normal children. This could explain why mercury may stay in their bodies long enough to interfere with normal brain development, he said.
Vaccine maker Aventis Pasteur says in a statement: "The hypothesis of any danger [from thimerosal] is not established or supported by either clinical or experimental evidence."
The IOM committee will issue a report in about three months.
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