The genesis of this report was a request from the Centers for Disease Control and Prevention and the National Institutes of Health, both of which recognized the need for an independent group of scientists to address growing concerns about vaccine safety in a timely and objective manner. In response to this request, the Committee on Immunization Safety Review was established in January. The committee comprises 15 members with expertise in a variety of relevant public health and medical disciplines.

During a three-year study period that began this year, the committee is charged with examining nine hypotheses selected by a working group of federal government vaccine researchers and policy-makers. For each hypothesized association between vaccines and adverse health effects, the committee will look at the science behind it, assess its significance in a broader societal context, and suggest appropriate actions to be taken by federal agencies, researchers, and policy-makers. In this study, our committee was asked to consider concerns about the potential relationship between thimerosal -- a mercury-containing preservative used in some vaccines -- and neurodevelopmental disorders such as autism, attention deficit-hyperactivity disorder (ADHD), and speech or language delay.

First, a bit of context. Childhood immunization is widely regarded as one of the world's most effective tools for preventing millions of cases of disease and death. In the United States alone, child-vaccination programs have eliminated smallpox and polio and made many debilitating and potentially life-threatening infectious diseases, such as diphtheria, pertussis, and measles, extremely uncommon. Along with these benefits, however, have come some concerns about whether children experience any adverse health effects from the vaccines themselves.

Because mercury at high doses is known to pose risks, some parents and researchers are concerned that thimerosal in vaccines puts children at increased risk for developmental disorders such as autism. Preliminary data from a few studies have suggested that thimerosal-containing vaccines could possibly -- very minimally -- affect some measures of normal child development. But the data are inconclusive.

Thimerosal has been used since the 1930s in several childhood and adult vaccines to prevent bacterial contamination in multi-dose vials. The active ingredient in thimerosal is ethyl mercury, a close chemical relative of methyl mercury. At high doses, methyl mercury and some other forms of the heavy metal are known to damage the nervous system. Ethyl mercury has not been well-studied, but the amount in vaccines is small.

Our committee has reviewed the limited body of toxicological, clinical, and epidemiological literature on ethyl mercury and the more extensive literature on methyl mercury. The committee also held an open scientific workshop in July in Boston. Based on information from these sources, our study has come to the following conclusion: The hypothesis that thimerosal exposure through the recommended childhood immunization schedule causes neurodevelopmental disorders is not supported by clinical or experimental evidence. Existing epidemiological evidence is inadequate to either accept or reject a causal relationship between exposure to thimerosal from vaccines and the neurodevelopmental disorders of autism, ADHD, and speech or language delay. However, there are some indirect associations concerning biological plausibility, which refers to a theoretical but unproven possibility. For example, high-dose thimerosal exposures are associated with neurological damage. There is also toxicological and epidemiological literature suggesting that methyl mercury is a toxicant to the developing nervous system. Some children who received the maximum number of thimerosal-containing vaccines on the recommended childhood immunization schedule had exposures to ethyl mercury that exceeded some safe exposure guidelines for methyl mercury. In addition, some children could be particularly vulnerable or susceptible to mercury exposures because of genetic or other differences.

So, where do we stand today with respect to thimerosal in vaccines? In 999 the U.S. Public Health Service, the American Academy of Pediatrics, and the American Academy of Family Physicians set a goal of removing thimerosal from vaccines in the United States as soon as possible. This decision was based on several factors. It was viewed as feasible as well as consistent with the public health goal of decreasing mercury exposures in general, as much as possible. Mathematical calculations also suggested that some infants received a total amount of mercury from vaccines that exceeded some federal agency guidelines for safe mercury exposure. It should be noted that exceeding the guidelines for safe exposure does not mean that harm is certain to occur. These guidelines take into account safety factors to protect sensitive populations, such as children. In addition, they were set for methyl mercury exposure, which occurs primarily from eating fish, and their relevance to ethyl mercury in vaccines is unclear. The policy action of 1999 was a means to decrease cumulative mercury exposures. Neither at that time nor today does any data exist to prove that thimerosal is dangerous at the level present in vaccines.

Our committee, in reaching its two major conclusions, considered the debate in a broad context. The government has a major responsibility to ensure that vaccines are as safe as possible and to understand the science base for safety concerns as well as it can. Although the vaccines on the recommended childhood immunization schedule are no longer made with thimerosal, the preservative remains in some other vaccines and some pharmaceutical products. If thimerosal can be removed from these products without risking bacterial contamination, it makes sense to try to do so as part of a broader, precautionary effort to decrease mercury exposures in general -- not because thimerosal in vaccines has been proven to be dangerous. It has not.

To these ends, the committee makes the following recommendations about public health actions regarding thimerosal. The committee supports previous policies of government and medical advisory bodies that efforts be considered to decrease use of thimerosal in vaccines and drugs as much as possible. While the health effects of thimerosal are uncertain, the serious consequences of not getting vaccinated are very certain. This means that if thimerosal-free vaccines are not available, thimerosal-containing vaccines should be used. And finally, the committee encourages increased basic, clinical, and epidemiological research to better understand whether thimerosal may, in the past, have increased children's risk of developmental disorders.