Thimerosal in Vaccines

Some technology, like toilets, we're grateful for, while some, like genetically modified foods, only raise suspicion. And sometimes we're right to be suspicious -- mankind has made mistakes. After all, we used to think lead paint was fine.

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Activist Robert F. Kennedy Jr. speaks at a rally on Capitol Hill calling for "greener" vaccines on June 4, 2008 in
Washington, D.C.

For some people, thimerosal is the new lead paint. Why? Because thimerosal has been used as a preservative in vaccines since the 1930s, and some label the mercury-containing additive as the agent to blame for autism.

Mercury doesn't sound like the kind of substance you want coursing through your veins. The dangers of mercury in fish, for example, have been widely documented. But there's a difference between the kind of mercury in fish and the kind that's found in tiny amounts in vaccines.

Many fish contain methylmercury. People worry about the effects of methylmercury because it bioaccumulates, which means that over time, it becomes more concentrated in the body's tissues. This is why pregnant women are told to monitor their intake of fish. High methylmercury exposure in babies is tied to developmental delays and neurological impairment.

Ethylmercury, on the other hand, is what you find in thimerosal. Why have a mercury-containing chemical in your vaccine? There are two ways to dispense a vaccine: in single-dose or multi-dose vials. The danger of drawing out one dose from a multi-dose vial and then sticking a needle back in for another is contamination from bacteria and other organisms. Thimerosal is a preservative that keeps the vaccine from getting contaminated, so you're not getting a dose of bacteria to boot.

Ethylmercury and methylmercury aren't the same thing. From studies done on monkeys and on vaccinated babies, we know that the two compounds aren't metabolized and excreted the same way, and we know that ethylmercury doesn't seem to bioaccumulate [source: National Network for Immunization Information, NNIii]. But at the time when people began looking for possible autism causes, scientists didn't have this information. So people looked at the recommended limit for methylmercury and realized that the ethylmercury exposure from vaccines might exceed that limit.

­As it turns out, events unfolded messily. Among a flurry of other articles, Robert F. Kennedy Jr. wrote a piece for Salon and Rolling Stone that claimed to expose a government cover-up of the effects of thimerosal in vaccines because of ties to the pharmacy industry. Various autism advocacy groups got involved, as did other writers and doctors, some more reputable than others. The result? Vaccines in the United States today are almost entirely free of thimerosal or contain trace amounts (less than 1 microgram) by mandate of the Food and Drug Administration. The exception is the influenza vaccine, although you can find preservative-free versions. (For more background information on vaccine studies, see these article synopses from the NNii.)

Yet despite the removal of thimerosal from vaccines, autism rates haven't dropped. Denmark removed thimerosal from its vaccines in 1992, and its rates have continued to climb [source: NNii]. There is a possibility that all the money, time and brainpower being thrown into thimerosal-autism studies is wasted -- resources that could be spent trying to find to find the cause of and cure for autism.