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The report, released in May 2004, was prepared by a committee of independent experts established by the IOM in 2001 at the request of the Centers for Disease Control and Prevention (CDC) and the National Institutes of Health (NIH) to evaluate evidence on potential links between childhood vaccines and health problems. The agencies explored the issue because of growing controversy and questions from the public about vaccine safety.

Some parents have expressed concern because the symptoms of autism typically emerge in a child's second year of life, around the same time children first receive the MMR vaccine. Autism is a complex set of severe developmental disorders characterized by repetitive behavior and impaired social interaction and communication abilities. Other concerns the committee looked at include the use of thimerosal, a mercury-based compound used as a vaccine preservative, because many forms of mercury are known to damage the nervous system in high doses.

Review of the Research

This latest IOM report follows two reports on vaccines and autism published in 2001. The committee determined then that the evidence did not show an association between the MMR vaccine and autism, but that more evidence was needed regarding thimerosal. "The committee concluded that the evidence available at that time was inadequate to accept or reject a causal relationship between thimerosal and neurodevelopmental disorders," says Marie McCormick, M.D., Sc.D., chairwoman of the immunization safety committee and a professor at the Harvard School of Public Health.

The committee revisited these issues because several studies exploring possible links between vaccines and autism have been published since 2001. Committee members concluded that the hypothesis about how the MMR vaccine and thimerosal could trigger autism lacks supporting evidence. Their conclusions were based on a careful review of well-designed studies and other information from researchers and parents.

Five large studies in the United States, the United Kingdom, Denmark, and Sweden done since 2001 found no evidence of a link between autism and vaccines containing thimerosal. And 14 large studies consistently showed no link between the MMR vaccine and autism. The committee also reviewed several studies that did report associations between vaccines and autism and found that these studies had limitations and lacked supporting evidence.

The committee reviewed potential biological links between vaccines and autism and found them to be only theoretical. Examples of some of the hypothesized links include a suggestion that the measles virus in the MMR vaccine might lodge in the intestines and trigger the release of toxins that could lead to autism. Another hypothesis is that the MMR vaccine might stimulate the release of immune factors that damage the central nervous system. Yet another hypothesis is that thimerosal may interfere with biochemical systems in the brain, thereby causing autism. But according to the IOM report, no evidence has slaown that the immune system or its activation play a direct role in causing autism, and autism has not been documented as being a result of exposure to high doses of mercury.

"There is no convincing evidence of serious harm from the low doses of thimerosal in vaccines," says Karen Midthun, M.D., deputy director for medicine in the FDA's Center for Biologics Evaluation and Research (CBER). CBER regulates vaccines in the United States and works with the CDC and the NIH to study and monitor vaccine safety and effectiveness.

Limiting Thimerosal Use

Since the 1930s, small amounts of thimerosal have been used as a preservative in multi-dose vials of vaccines to prevent bacterial contamination. The active ingredient in thimerosal is ethylmercury.

Even though the risk of thimerosal is hypothetical, thimerosal began to be removed from childhood vaccines in 1999. The federal government, the American Academy of Pediatrics, and others agreed that thimerosal should be reduced and eliminated in vaccines as a precautionary measure. The FDA encouraged companies to comply with this recommendation. Currently, all routinely recommended vaccines manufactured for infants in the United States are either thimerosal-free or contain only trace amounts.

"We moved in this direction to address public concern and because it was feasible to eliminate mercury from vaccines," Midthun says. "We could eliminate thimerosal in vaccines as a way to reduce a child's total exposure to mercury, whereas other environmental sources of exposure are more difficult to eliminate."

In its latest report, the IOM's immunization committee reported that it does not dispute that mercury-containing compounds, including thimerosal, can be damaging to the nervous system. But the committee did not find that these damaging effects are related to the development of autism.

For the 2004-2005 flu season, the CDC is recommending that children ages 6 months to 23 months get vaccinated annually against the flu (influenza) with the inactivated flu shot. "The influenza vaccine is available both with thimerosal as a preservative and without it," Midthun says. "But the benefits of flu vaccination outweigh any theoretical risk from thimerosal." According to the CDC, the amount of flu vaccine without thimerosal as a preservative will increase as manufacturing capabilities expand. "To eliminate thimerosal as a preservative from flu vaccines, manufacturers will have to switch from multi-dose to single-dose preparations, which requires greater filling and storage capacity," Midthun says.

Based on federal guidelines on levels of mercury exposure, a child won't receive excessive mercury from vaccines, regardless of whether their inoculation against the flu contains thimerosal.

Recommendations

The IOM's immunization safety committee did not recommend any changes with the MMR vaccine or with the current schedule of routine childhood immunizations.

"While the committee strongly supports research that focuses on achieving a better understanding of autism, we recommend that future research be directed toward other lines of inquiry that are supported by current knowledge and evidence, and that offer more promise for finding an answer," McCormick said at a media briefing. "Given the current evidence, the vaccine hypothesis doesn't offer that promise."

The IOM is part of the National Academy of Sciences.

For More Information

Immunization Safety Review: Vaccines and Autism Immunization Safety Review Committee, Institute of Medicine http://books.nap.edu/catalog/10997.html

Thimerosal in Vaccines www.fda.gov/cber/vaccine/thimerosal.htm

COPYRIGHT 2004 U.S. Government Printing Office
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