May 19, 2004 (CIDRAP News) – The Institute of Medicine (IOM) says it can find no evidence that thimerosal-containing vaccines or the measles-mumps-rubella (MMR) vaccine causes autism and recommended that the search for the cause of the disorder focus on other possibilities.
Three years ago, the IOM's Immunization Safety Review Committee said the idea of a link between thimerosal and autism was "biologically plausible" despite a lack of clear evidence. But in a new report, the committee says several large epidemiologic studies since 2001 have turned up no evidence of a link between thimerosal in vaccines or MMR vaccine and autism. In addition, researchers have found no evidence of any biologic mechanism by which vaccines could cause autism, the committee says.
"The overwhelming evidence from several well-designed studies indicates that childhood vaccines are not associated with autism," committee Chair Marie McCormick stated in an IOM news release. McCormick is the Sumner and Esther Feldberg professor of child and maternal health at the Harvard School of Public Health in Boston.
Adding that the IOM strongly supports continued research on the causes of autism, McCormick said, "Resources would be used most effectively if they were directed toward those avenues of inquiry that offer the greatest promise for answers. Without supporting evidence, the vaccine hypothesis does not hold such promise."
Thimerosal, which contains ethyl mercury, has been used as a preservative in vaccines since the 1930s. Since 1999 it has been removed from most of the vaccines routinely recommended for infants and children. It is still used in injectable influenza vaccine, though some thimerosal-free flu vaccine is expected to be available this year.
This year the Centers for Disease Control and Prevention (CDC) recommended for the first time that all 6- to 23-month-old children receive annual flu shots. In doing so, the agency did not call for the use of thimerosal-free vaccine, saying the risk of flu complications far outweighs any possible risk from thimerosal in the vaccine. (Removing thimerosal from the flu vaccine requires extra steps that waste some vaccine and increase its cost, according to vaccine manufacturers.) The CDC position has stirred protests from groups that have asserted for years that vaccines, and thimerosal in particular, may cause neurodevelopmental disorders.
The IOM revisited the MMR and thimerosal issues because of evidence that has accumulated since 2001, according to the IOM statement.
"Five large epidemiological studies conducted in the United States, the United Kingdom, Denmark, and Sweden since 2001 consistently provided evidence that there is no assocation between thimerosal-containing vaccines and autism," the IOM said. "Similarly, 14 large epidemiological studies consistently showed no association between the MMR vaccine and autism."
The panel also reviewed five studies that linked autism with thimerosal and two that linked the disorder with MMR vaccine. Those studies were found to have flaws that invalidated their stated findings, the IOM said.
The committee also reviewed studies of suggested mechanisms by which immunizations could cause autism. For example, it has been proposed that thimerosal may interfere with biochemical systems in the brain and that MMR vaccine may trigger the release of immune factors that damage the central nervous system, according to the IOM.
The panel found no evidence for these suggested mechanisms and also determined that autism "has never been documented as a consequence of exposure to high doses of mercury," the IOM statement said.
In view of the current evidence, the committee recommended that cost-benefit assessments concerning the use of thimerosal-containing vaccines versus thimerosal-free vaccines not include autism as a potential risk. The committee also said hypotheses about thimerosal and autism do not justify a review of current childhood immunization recommendations.
However, the committee said surveillance for vaccine-related adverse events should be strengthened in several ways, including the development of "Clinical Immunization Safety Assessment Centers" to study adverse events at the individual level. In addition, the group called for continued monitoring of the incidence of autism as exposure to thimerosal declines.
Watchdog groups concerned about mercury exposure criticized the IOM report as biased or otherwise flawed, according to news service reports today. For example, Reuters reported that Barbara Loe Fisher, founder of the National Vaccine Information Center, labeled the report "political" and called for research by scientists with no connections to the "public health community."
McCormick said the IOM committee was independent of vaccine manufacturers and the government, according to the Reuters report. The IOM, part of the National Academies, is described as a private, nonprofit organization that works under a congressional charter to conduct studies for federal agencies.
May 18 IOM news release about the report on vaccines and autism
Table of contents page for the IOM report
May 6 CIDRAP News story, "CDC neutral on thimerosal in flu vaccine for toddlers"