This information is adapted from the Institute of Medicine (IOM) Report on Thimerosal-Containing Vaccines and Neurodevelopmental Disorders.

On October 1, 2001, the Institute of Medicine's Immunization Safety Review Committee released its report examining whether or not the use of vaccines containing the preservative thimerosal can cause neurodevelopmental disorders, specifically autism, attention deficit/hyperactivity disorder (ADHD), and speech or language delay.

The Committee concluded, "The hyphothesis that thimerosal exposure through the recommended childhood immunization schedule has caused neurodevelopmental disorders is not supported by clinical or experimental evidence." However, the Committee also acknowledged that the epidemiological evidence was inadequate to either accept or reject a causal relationship.

The full IOM Committee report, including the complete list of the Committee's recommendations, is available at www.nap.edu/catalog/10208.html. The IOM press release is at www4.nationalacademies.org/news.nsf/isbn/0309076366?OpenDocument.

A review by the FDA in 1999 found that some children could be exposed to levels of thimerosal higher than Environmental Protection Agency guidelines if they received thimerosal-containing vaccines for each recommended childhood vaccines. At that time, the American Academy of Pediatrics and the U.S. Public Health Service urged manufacturers to eliminate or reduce the amount of mercury in vaccines. Today the IOM Committee Chair, Marie C. McCormick, M.D., Sc.D., stated, "Neither at that time, nor today, does any data exist to prove that thimerosal is dangerous at the level present in vaccines."

Information reviewed by the Committee indicated that:

• low dose thimerosal exposure in humans has not been demonstrated to be associated with effects on the nervous system;
• thimerosal exposure from vaccines has not been proven to result in mercury levels associated with toxic response;
• signs and symptoms of mercury poisoning are not identical to autism, ADHD, or speech or language delay;
• autism is thought primarily to originate from prenatal injury;
• there is no evidence that ethylmercury causes any of the pathophysiological changes known to be associated with autism, such as genetic defects; and
• there are no well-developed pathological markers of ADHD or delay of speech or language that could be compared to effects of ethylmercury on the nervous system.

The Committee also concluded that the hypothesis that exposure to thimerosal-containing vaccines could be associated with neurodevelopmental disorders is biologically plausible, although it is not established, and rests on indirect and incomplete information. Because of this theoretical possibility and the need to avoid eroding public trust in the vaccine safety system, the Committee recommended the continuation of policy review and analysis, public health and biomedical research, and improved communications strategies on this issue in the U.S.

Since July 2000, all vaccines on the current recommended childhood immunization schedule have been available in formulations considered to be thimerosal-free (as defined by the IOM committee, this means that none of these vaccines use thimerosal as a preservative; some may contain trace amounts of thimerosal from the manufacturing process). However, some remaining supplies of DTaP and Hib vaccines that contain thimerosal may still be available. Committee Chair McCormick noted that, "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."

Although the focus of this review was on vaccines that are recommended for infants and children, the Committee considered vaccines that pregnant women might receive because of the potential that a developing fetus could be exposed indirectly from maternal exposure. The committee supports adherence to the recent recommendation of the Advisory Committee on Immunization Practices, which states that "because pregnant women are at increased risk for influenza-related complications and because a substantial safety margin has been incorporated into the health guidance values for organic mercury exposure, the benefit of influenza vaccine outweighs the potential risks for thimerosal."

The Committee urged that full consideration be given to removing thimerosal from any biological or pharmaceutical product to which infants, children and pregnant women are exposed.

The Committee's recommendations for additional epidemiological, clinical, and basic science research include:

• case-control studies examining the potential link between neurodevelopmental disorders and thimerosal-containing vaccines;
• analysis of neurodevelopmental disorders in cohorts of children who did not receive thimerosal-containing doses as part of a clinical trial of DTaP vaccine;
• comparison of the incidence and prevalence of neurodevelopmental disorders before and after removal of thimerosal from vaccines;
• an increased effort to identify the sources and levels of prenatal and postnatal background exposure to thimerosal and other forms of mercury in infants, children, and pregnant women;
• research on how children, including those diagnosed with neurodevelopmental disorders, metabolize and excrete metals, particularly mercury; and
• identification of a safe, effective, and inexpensive alternative to thimerosal for countries that decided to switch from using thimerosal as a preservative.

The IOM Immunization Safety Review Committee was convened as an independent committee to provide timely and objective assistance to the Department of Health and Human Services in reviewing emerging vaccine safety concerns. The 15 members of the IOM immunizations Safety Review Committee possess expertise in a number of fields, including pediatrics, neurology, immunology, internal medicine, infectious diseases, genetics, epidemiology, biostatistics, risk perception and communication, decision analysis, public health, nursing, and ethics.

For additional information, please visit:

• The Centers for Disease Control and Prevention
• The U.S. Food and Drug Administration
• The Johns Hopkins Institute for Vaccine Safety