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Autism Spectrum Disorders Health Center

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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


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.

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."

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