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    Autism-Vaccine Link: Evidence Doesn't Dispel Doubts

    Many major medical groups say vaccines don't cause autism. Many parents say they do. So who's right?

    Vaccine-Autism Disconnect: Meet Ethyl and Methyl

    Recently, basic assumptions about thimerosal have come into question that may further confuse any links to autism.

    Thimerosal contains ethyl mercury. The government's assessments of health risks for thimerosal are based on what is known about another chemical form of mercury, called methyl mercury.

    The health assessments for methyl mercury are based on exposure by eating or drinking it. Warnings about eating certain kinds of fish, for example, are all about fish contaminated with methyl mercury.

    Thimerosal, on the other hand, isn't eaten; When experts called for reducing and removing thimerosal from vaccines in 1999, they assumed that methyl mercury and ethyl mercury were pretty much the same, and the health risks may be the same even though the way children are exposed is different. A study published in the journal Pediatrics in January 2008 shows that ethyl mercury leaves children's blood faster than methyl mercury does, so the health risks may not be the same.

    However, while it left the blood quickly, it doesn't necessarily mean it left the body. While the study did show that the mercury did not damage the kidneys, for instance, it didn't look at other organs.

    Vaccines and Autism: Other Studies Under Way

    Sue Swedo, MD, chief of the pediatric and developmental neuropsychiatry branch of the National Institute of Mental Health, says federal researchers have not closed the door to looking at whether vaccination might, in rare cases, be linked to autism.

    The strongest case for a link comes from children with regressive autism -- children who seem to be developing normally, but who then lose the social and language skills they had developed and slide into autism. To parents, such children seem to have been the victims of some environmental toxin. As this regression occurs at the same time children receive multiple vaccines, many wonder whether vaccines might carry such a toxin.

    "Our studies of regressive autism are taking a very shotgun approach to environmental factors in autism," Swedo tells WebMD. "We are saying we don't really know right now whether such factors might be involved."

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