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Children's Vaccines Health Center

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Did CDC Conspire to Hide Vaccine Risk?

'Simpsonwood Conspiracy' Claims Debunked; Concerns Remain

Simpsonwood and Vaccine Safety continued...

The focus on a Simpsonwood "conspiracy" also rankles Sallie Bernard, co-founder and president of SafeMinds, a group that advocates for research into environmental mercury as a possible cause of autism. The SafeMinds web site indicts thimerosal as a dangerous source of mercury.

But what worries Bernard about the focus on Simpsonwood is that it's a distraction from the issue of vaccine safety.

"Why the focus on Simpsonwood or on conspiracies? It's as if the concerns over vaccine safety were some narrow partisan group thinking these things up," Bernard tells WebMD.

Thimerosal and Autism

In the decade since the Simpsonwood meeting, scientists have come to a better -- but still incomplete -- understanding of what thimerosal does in the human body.

As it turned out, thimerosal is based on a form of mercury called ethyl mercury, while the EPA limits were based on methyl mercury. Why is that important? The body processes ethyl mercury far differently than it does methyl mercury.

The body takes ethyl mercury out of the blood much faster than methyl mercury, turning it into less-toxic inorganic mercury. Before the Simpsonwood conference took place, it was assumed that ethyl mercury was just as toxic as methyl mercury. Fortunately, it is not.

That's not to say that ethyl mercury has been proven 100% harmless. Ongoing studies offer reassurance, and studies of millions of children identify no risk to children with the highest levels of exposure to vaccines containing thimerosal. Nor do these studies find evidence of a subgroup of children particularly sensitive to thimerosal toxicity. But the fact that inorganic mercury accumulates in the brains of monkeys given high doses of thimerosal suggests there's more to learn about the preservative's safety.

In 2006, Clements reviewed studies of thimerosal toxicology and vaccine epidemiology. He found 12 studies published since the Simpsonwood conference. Six found no link.

Six did find a link -- all by the father-and-son research team of Mark Geier, MD, PhD, and David Geier. The Geiers’ research has been roundly criticized by medical reviewers. And it was found not credible in a 2010 decision by George L. Hastings Jr., special master of the U.S. District Court, also known as the vaccine court, which hears claims against vaccines. Hastings was charged with evaluating whether there is any evidence that thimerosal might cause autism.

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