Thimerosal and Autism
Some parents are concerned that thimerosal, a mercury-containing preservative contained in the influenza vaccine, causes autism. However, during the past few years a series of biological and epidemiological studies have shown this concern to be unfounded. Here is a summary of the evidence showing that, while some things do cause autism, mercury in vaccines isn’t one of them.
All mercury isn't the same: methymercury vs. ethylmercury
Mercury is a naturally occurring element found in the earth’s crust, air, soil and water. Since the earth’s formation, volcanic eruptions, weathering of rocks and burning of coal have caused mercury to be released into the environment. Once released, certain types of bacteria in the environment can change mercury to methylmercury. Methylmercury makes its way through the food chain in fish, animals, and humans. At high levels, it can be toxic to people.
Thimerosal — a preservative still used in the influenza vaccine — contains a different form of mercury called ethylmercury. Studies comparing ethylmercury and methylmercury suggest that they are processed differently in the human body. Ethylmercury is broken down and excreted much more rapidly than methylmercury. Therefore, ethylmercury (the type of mercury in the influenza vaccine) is much less likely than methylmercury (the type of mercury in the environment) to accumulate in the body and cause harm.
Evidence that mercury doesn't cause autism
- In 1971 Iraq imported grain that had been fumigated with methylmercury. Farmers ate bread made from this grain. The result was one of the worst, single-source, mercury poisonings in history. Methylmercury in the grain caused the hospitalization of 6,500 Iraqis and killed 450. Pregnant women also ate the bread and delivered babies with epilepsy and mental retardation. But they didn’t deliver babies with an increased risk of autism.
- Four large studies have now compared the risk of autism in children who received vaccines containing thimerosal to those who received vaccines without thimerosal. The studies were consistent, clear and reproducible — the incidence of autism was the same in both groups. Denmark, a country that abandoned thimerosal as a preservative in 1991, actually saw an increase in the disease beginning several years later.
- Studies of the head size, speech patterns, vision, coordination and sensation of children poisoned by mercury show that the symptoms of mercury poisoning are clearly different from the symptoms of autism.
- Methylmercury is found in low levels in water, infant formula, and breast milk. Although it is clear that large quantities of mercury can damage the nervous system, there is no evidence that the small quantities contained in water, infant formula, and breast milk do. An infant who is exclusively breast-fed will ingest more than twice the quantity of mercury that was ever contained in vaccines and fifteen times the quantity of mercury contained in the influenza vaccine.
Adapted from www.immunize.org on May 22, 2009. We thank the Immunization Action Coalition.
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