Vaccine Link to Autism Unfounded, Experts Say

WASHINGTON, DC, May 19, 2004 (ENS) - Neither the mercury-based vaccine preservative thimerosal nor the measles-mumps-rubella (MMR) vaccine are associated with autism, according to a federal panel of experts.

Thimerosal is an organic mercury compound that is still used as a preservative in some adult vaccines - it has not been used to preserve most child vaccines since 1999.

The committee of the Institute of Medicine of the National Academies says its review of clinical and epidemiological studies is the most definitive to date on the suspected link between the thimerosal and MMR with autism.

Autism is not a single condition, but rather a complex set of severe developmental disorders - also referred to as autistic spectrum disorders - characterized by sustained impairments in social interaction and communication abilities, as well as restricted or repetitive patterns of behaviors and interests.

This wide range of characterizations makes it difficult to assess how many children have autism, but the panel said it afflict one in every 1,000 children.

The hypotheses regarding how the MMR vaccine and thimerosal could trigger autism lack supporting evidence and are only theoretical, according to the panel, which suggested scientists look elsewhere for the cause of autism.

"The overwhelming evidence from several well designed studies indicates that childhood vaccines are not associated with autism," said committee chair Marie McCormick of the Harvard School of Public Health.

The panel reports that 14 large epidemiological studies consistently showed no association between the MMR vaccine and autism.

In addition, five large epidemiological studies conducted in the United States, the United Kingdom, Denmark, and Sweden since 2001 consistently provided evidence that there is no association between thimerosal-containing vaccines and autism.

The committee also reviewed five studies that reported links between thimerosal and autism and two that indicated a connection between the MMR vaccine and the disorder. However, limitations in how these studies were conducted and how the data were analyzed led the committee to conclude that they did not provide evidence supporting an association between vaccines and autism.

The committee also reviewed evidence related to possible biological mechanisms by which immunizations might trigger autism.

It found none supporting the hypothesis that the immune system or its activation plays a direct role in causing autism.

Autism also has never been documented as a consequence of exposure to high doses of mercury, the panel reported.

While the committee agreed that the studies exploring these hypotheses raise interesting questions, they do not address the specifics of how autism could result. Therefore, evidence for any biological mechanism linking vaccines with autism can only be considered theoretical.

"We strongly support ongoing research to discover the cause or causes of this devastating disorder," 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."