Autism is a complex developmental disability affecting as many as 1.5 million Americans. It appears during the first three years of life and impacts the functioning of the brain. Children and adults with autism find it difficult to communicate and form normal social relationships.

No one knows exactly what causes autism. Many scientists believe that a child’s environment plays some role, but they have yet to determine what those environmental factors are. Others contend the problem lies in childhood vaccines (like DTP or diphtheria, tetanus, pertussis and MMR or measles, mumps, rubella) containing mercury, a substance known to be toxic to the nervous system at high doses.

To investigate this claim, researchers in Denmark reviewed national data on autism. Their results, which are published in the October 1, 2003 issue of the Journal of the American Medical Association , indicate there is no relationship between vaccines containing thimerosal, a mercury- based preservative used in vaccines, and autism.

About the Study

The researchers studied records of children born in Denmark from January 1, 1990 until December 31, 1996 who were vaccinated with either a thimerosal-containing pertussis (whooping cough) vaccine—at doses similar to those given in the U.S.—or with the same pertussis vaccine formulated without thimerosal. They compared these two groups with respect to autism and other autistic-spectrum disorders (such as Asperger disorder and Rett’s disorder ).

The researchers used codes similar to the Diagnostic and Statistical Manual of Mental Disorders , Fourth Edition (DSM-IV) to define autism and other disorders resembling autism.

The Findings

The researchers identified 440 cases of autism and 787 cases of other autism-like disorders. The risk of autism and similar disorders did not differ significantly between children vaccinated with thimerosal-containing vaccine and children vaccinated with thimerosal-free vaccine.

The researchers also found no indication of a dose-response association between autism and the amount of mercury received through thimerosal. That is, children who were given a vaccine with a higher dose of mercury had no greater chance of developing autism than those who received a lower dose.

A limitation of this study is the fact that the researchers only recorded the diagnosis of autism and its related disorders at one point in time. Because diagnosis is typically a lengthy process, the researchers might have counted too many or too few cases.

How Does This Affect You?

The results of this study do not support the theory that thimerosal-containing vaccines cause autism or similar disorders. Although it may not relieve concerns of parents of autistic children trying to make sense of the disorder, this study may help steer them in a more productive direction. In any case, all of the routinely administered pediatric vaccines in the U.S. today are either thimerosal-reduced or thimerosal-free. Thus, in the future, this will be a non-issue.

The cause of autism remains a mystery. While research is underway to find a reason for this complex developmental disorder, studies do suggest that early and intensive educational and psychological interventions can help autistic children grow, learn and adapt to their environments.