Immunization Issues

Thimerosal in Vaccines and Neurodevelopmental Outcomes

Updated: March 6, 2008

What is Thimerosal?

Thimerosal is a compound made up of ethylmercury attached to a chemical similar to aspirin. In the body it is split apart, releasing the ethylmercury which kills a number of micro-organisms, including many bacteria. Thimerosal is 49.6% mercury by weight. Although it was not used in all vaccines (for example, it has never been used in live virus vaccines such as MMR or chickenpox vaccines), it had been part of the manufacture of many vaccines in use in the United States and other countries until recently. It is still widely used in many parts of the world. It is also used as a preservative in some eye, ear and other topical solutions, antivenins, immune globulins and skin test antigens. Historically—in much higher concentration—thimerosal was used as an antiseptic for wound management. (More information from the FDA).

Thimerosal is currently used in vaccines for two purposes:

  • as a preservative to prevent bacterial and fungal contamination of multiple dose vials of vaccines during their clinical use. In this case, thimerosal is added at the end of the production process either to the liquid vaccine itself or—in the case of dry powder vaccines—to the liquid used to dilute the vaccine.
  • as a part of the manufacturing process to kill the bacteria and fungi that might enter the vaccine during the vaccine production process.

Except when used as a preservative, the final concentration of thimerosal in the vaccine is very small. In this case, the vaccine is labeled as containing “trace amounts” of thimerosal. The thimerosal content of vaccines either from the manufacturing process or as preservative can be found at the FDA Web site.


Methylmercury is NOT present in vaccines but it is in some of the food we eat. Of all the environmental contaminants that we are exposed to, this form of mercury has been the most extensively studied—because methylmercury accumulates in the food chain, in our bodies and affects the development of children, including those whose mother’s had been exposed to methylmercury before and/or during pregnancy.

Methylmercury is the most common form of mercury environmental contamination. Once methylmercury enters the food chain it becomes concentrated in certain foods, particularly predator fish. Methylmercury has also been used as a pesticide to protect seed grain from pests like rats and other rodents; it has caused poisoning by the accidental ingestion of seed grain.

Methylmercury is well established as a neurotoxin—that is, it can damage the brain and cause developmental disabilities in young infants. It can cause developmental problems in infants born to mothers have been eating contaminated foods and in the infant who eats contaminated milk and other foods.

After too much exposure over time, methylmercury can cause a number of neurologic symptoms including tremors, emotional lability, memory loss, weakness, tremors, sleeplessness, odd sensations, changes in the tendon reflexes, performance deficits (poor performance on cognitive and motor function testing), sensitivity to light, hearing and visual problems and other problems. However, it has never been associated with the constellation of signs and symptoms of autism.

Concerns that children with autism spectrum disorders had symptoms similar to methylmercury poisoning1 did not stand up to scientific scrutiny by the Institute of Medicine2 nor to side by side comparisons by neuroscientists.3 In their 2004 report, the Institute of Medicine said “… autism has never been documented as a consequence of high-dose mercury exposure…”2


Thimerosal contains ethylmercury, not methylmercury. When all the discussions about thimerosal began in 1999, there were very little data about ethylmercury toxicity. Because little information about ethylmercury toxicity was available in 1999, public health authorities worried that ethylmercury toxicity might be similar to that of methylmercury. If that were the case, exposure levels of young infants to mercury from thimerosal might theoretically be close to the safe intake level (the maximum amount to which an infant can safely be exposed on a daily basis, which is about ten times lower than where it might be neurotoxic).

A study in monkeys showed that the absorption and initial distribution of mercury in their bodies, was similar after injection of thimerosal-containing vaccines or after the feeding of methylmercury. However, the elimination of ethylmercury from the body was much faster, and the amount of mercury in the brain was significantly lower in the animals that received the thimerosal-containing vaccines than those that were fed methylmercury.4

Ethylmercury—given as thimerosal-containing vaccine—is also eliminated from the bodies of children much more rapidly5 than is methylmercury that has been eaten.

While methylmercury and ethylmercury sound similar, they are very different—just as are methyl alcohol (wood alcohol, a poison used in antifreeze) and ethyl alcohol (found in wine and beer).

Thimerosal in vaccines does not cause autism

This is the conclusion reached by The Institute of Medicine’s Immunization Safety Review Committee in its 2004 report, Vaccines and Autism.2 The report stated that the evidence favors “rejection of a causal relationship between thimerosal-containing vaccines and autism.” Additional studies since then have continued to find no association.

Many large studies looking at different populations, by many investigators, using different epidemiologic methods have consistently found no evidence of an association between thimerosal and autism.6 That is the reason that the IOM’s committee favored rejection of a causal relationship between thimerosal-containing vaccines and autism.

Reports by Mark and David Geier7, that supposedly showed links between thimerosal and autism, were faulty because of their methods—such as using statistical measures incorrectly and omitting facts about their research approach—and therefore were dismissed by the IOM as not contributing to their analysis. Similar problems exist with more recent reports by the Geiers.

If thimerosal in vaccines were a major cause of autism, the prevalence of autism was predicted to fall after the removal of thimerosal from the vaccines routinely given to young infants.8 However, a study in California showed no decrease in the prevalence of autism among children who are now 3 to 5 years of age—in fact they show a continuing increase of cases of autism in California since the reduction of thimerosal in pediatric vaccines.9

Thimerosal and other neurodevelopmental outcomes

The findings from a large comprehensive study did not suggest causal associations between increasing exposure to mercury in thimerosal—prenatally and early in life—and other neurodevelopmental outcomes at 7 to 10 years of age.10

More information about autism can be found at: