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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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But three things happened. One was that kids started getting more and more vaccines containing thimerosal. Meanwhile, the U.S. Environmental Protection Agency got new information and lowered what it considered to be a toxic dose of methyl mercury.

"In the 1990s, those two lines merged," Burbacher says. "Someone at the FDA noted that when you add all the vaccines up they totaled more than the new EPA standard for methyl mercury. So then the other thing that occurred, during that same time period, was an increase in rates of autism diagnosis."

By 1999, thimerosal was in 30 U.S. vaccines -- some, like the DTaP, Hib, and hepatitis B vaccines, given to infants. In July 1999, the American Academy of Pediatrics and the U.S. Public Health Service recommended removing thimerosal from vaccines. By March 2001, all vaccines recommended for U.S. children were available in thimerosal-free versions. However, the preservative is still used in multiuse vials of flu vaccines and in childhood vaccines for use in developing nations.

Kids vaccinated between 1989 and 2003 are what Kennedy calls the Thimerosal Generation.

As it turns out, thimerosal is not as much like methyl mercury as previously thought. That's both good and bad, Burbacher notes. In recent monkey studies, Burbacher has found that the body eliminates thimerosal much more quickly than it eliminates methyl mercury. Thimerosal leaves two or three times less mercury in the body than methyl mercury.

But Burbacher also found that thimerosal deposits something called inorganic mercury in the brain -- twice as much as from the same dose of methyl mercury. Inorganic mercury isn't supposed to do anything. But there's troubling evidence that it might -- evidence Burbacher and others are only now beginning to investigate.

People who think thimerosal is safe usually point to the rapid-clearance finding. Those who think it unsafe, Burbacher says, point to the increased deposits of inorganic mercury in the brain.

The IOM as Jury

It's common, in matters of scientific dispute, to turn to the National Academy of Sciences for an answer. And when the question is medical, the dispute goes to the IOM, which then convenes a panel of nationally recognized experts to decide the matter.

For thimerosal, the IOM convened these juries not once, but twice.

In 2001, the first IOM committee concluded that there wasn't enough evidence to say whether thimerosal was safe or unsafe.

In 2004, the most recent committee rejected the idea that vaccines containing thimerosal cause autism.

Kennedy writes that the committee findings were preordained in "secret" meetings with drug companies playing the tune. He says the committee ignored "truckloads of studies" that show thimerosal accumulates in the brains of lab animals, and he says the studies of autism trends on which the IOM relied are "disastrously flawed."

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