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New Intensity to Debate Over Autism Cause

Parents and Researchers Grapple With Claims That Autism Is Linked to Thimerosal in Vaccines

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Gernsbacher says the findings are based on backward reasoning. Her main point: The study uses the current, looser definition of autism to look at children diagnosed under a more strict definition. Then it mistakenly concludes that since those kids meet the looser definition, the new definition made no difference.

A 2005 study by Craig J. Newschaffer, PhD, and colleagues at the Johns Hopkins Center for Autism and Developmental Disabilities Epidemiology shows "a drastic increase in the prevalence of the autism classification." The study concludes that it's important to find out how much of this is due to a change in diagnosis and how much is due to "real changes in risk."

Collision: Autism Rise, Vaccine Concern

Fact: Nobody knows what causes autism. Nobody is even sure whether autism in all its different forms has one cause or many. Since autism runs in families, nearly everyone agrees there's a genetic link. Might that link be something that makes some people especially sensitive to something in the environment?

One of the heartbreaking peculiarities of autism is that a child will seem to be developing normally. Then, suddenly, at age 2 to 4, everything seems to go wrong.

That's just when kids are getting their vaccinations. To many parents, a vaccine/autism link seems obvious. Until very recently, many parents believed the problem was with the measles/mumps/rubella vaccine. Although many people still cling to this idea, most have come to reject it.

But until recently, many other vaccines contained a mercury-based preservative called thimerosal. Vaccines used to get contaminated with germs. Those germs killed kids. So drug companies used thimerosal to save lives. It worked.

Oddly -- for a substance given to nearly every child in the U.S. -- nobody really knew much about thimerosal.

Even now, says toxicologist Thomas Burbacher, PhD, very little is known. Burbacher is associate professor of environmental and occupational health sciences and director of the infant primate research lab at the National Primate Research Center, University of Washington.

"It is incredible so many millions of kids have been vaccinated with this compound with so little data on it," Burbacher tells WebMD. "But line it up with everything else. A lot of information is missing on a lot of compounds in daily use. And a lot of people thought the amount the kids were getting was so small, it was not a priority."

The Thimerosal Debate and the EPA

Thimerosal is 49% ethyl mercury. A closely related form of mercury, methyl mercury, is a known toxin. Since nothing was known about thimerosal, safety measures were based on what was known about methyl mercury. Everybody thought that thimerosal would be safe if the doses given to kids were below the toxic dose for methyl mercury.

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