Background

In 2000, the Centers for Disease Control and Prevention (CDC) and the National Institutes of Health (NIH) asked the Institute of Medicine (IOM) to establish an independent expert committee to review hypotheses about existing immunization safety concerns. The first report on MMR vaccine and autism was issued in April 2001. Today the IOM released its second report on thimerosal and neurodevelopmental outcomes.

CDC and NIH welcome the IOM report as a helpful contribution to the complex scientific and policy decision-making which is ongoing in regard to thimerosal. Given these challenges, CDC and NIH are most grateful to the individual scientists on the IOM Committee and IOM staff members for their dedicated and valuable service to the continued integrity of our national immunization system.

Conclusions

The IOM Immunization Safety Review Committee’s most important conclusions were 1) that the evidence is inadequate to accept or reject a causal relationship between exposure to thimerosal from vaccines and the neurodevelopmental disorders of autism, attention deficit hyperactivity disorder (ADHD), and speech or language delay, and 2) that although the hypothesis that exposure to thimerosal-containing vaccines could be associated with neurodevelopmental disorders is not established and rests on indirect and incomplete information, primarily from analogies with methyl mercury and levels of maximum mercury exposure from vaccines given in children, the hypothesis is biologically plausible.

Both conclusions are identical with much earlier Public Health Service (PHS) assessments of the evidence. The IOM conclusions are consistent with the goal, first articulated in July 1999 and reaffirmed in July 2000 by PHS agencies, the Advisory Committee on Immunization Practices (ACIP), the American Academy of Pediatrics, and the American Academy of Family Physicians. The goal was to continue using thimerosal-containing vaccines, while at the same time, taking the precautionary measure of removing or greatly reducing thimerosal from vaccines as soon as possible.

Recommendations

The Committee made the specific recommendation that thimerosal-free DTaP, hepatitis B and Hib vaccines be used in the United States for infants, children and pregnant women. This recommendation is also consonant with the 1999 and 2000 goal, particularly with the actions taken to implement the goal. Thus, since 1999, four replacement vaccines without thimerosal or with only greatly reduced trace amounts have been introduced in rapid succession for hepatitis B (September 1999 and March 2000), Hib (July 2000), and DTaP (March 2001). Since April 2001, all seven vaccines that are being made today and recommended for use among all children are now without thimerosal or with only trace amounts (hepatitis B, Hib, DTaP vaccines which formerly contained thimerosal as a preservative and MMR, Polio, Varicella, and Pneumococcal vaccines which have never contained thimerosal). Furthermore, as of October 2001, the vast majority of the supplies of DTaP, Hib, and hepatitis B vaccines are without thimerosal or with only trace amounts.

The committee also noted that in cases where only vaccines containing the preservative thimerosal are available, the vaccines should be administered rather than foregoing immunization.  The committee stated that "While the health effects of thimerosal are uncertain, we know for sure that these vaccines protect against real, proven threats to unvaccinated infants, children, and pregnant women."

Next Steps

The IOM Committee’s specific recommendation that only thimerosal free DTaP, Hib, and hepatitis B vaccines be used, even though there might be a very small number of remaining doses of thimerosal- containing vaccines in the supply chain, will be considered by the ACIP at its meeting on October 17-18 in Atlanta, Georgia. In the meantime, there are no changes in the ACIP’s current recommendations to use all available DTaP, Hib, and hepatitis B vaccines without regard to thimerosal content. All DTaP, Hib, and hepatitis B vaccines were judged to be safe and effective by experts in 1999 when the goal to remove thimerosal was first set. These vaccines are judged no less safe by experts today, even as they are being phased out.

The Committee has also made helpful recommendations about policy and research topics which are central to fully resolving the outstanding issues related to thimerosal. These also will be considered in depth by the PHS agencies and their advisory bodies over the next few weeks.

This page last reviewed and modified on May 18, 2004