Immunization Science

Epidemiological Study on Autism and Thimerosal

Is the hypothesis that thimerosal-containing vaccines cause autism valid? This epidemiological study found no evidence of a cause-effect relationship between the two.

The article

Autism and thimerosal-containing vaccines: Lack of consistent evidence for an association. Stehr-Green P, Tull P, Stellfeld M, Mortenson PB, and Simpson D. American Journal of Preventive Medicine, 2003; 25(2): 101-6.

The question

Do thimerosal-containing vaccines cause autism?

The study

Stehr-Green and colleagues compared the occurrence of autism in California, Sweden and Denmark with average exposures to thimerosal-containing vaccines between the mid-1980s through the late-1990s. The researchers collected data on children with autism and vaccination coverage levels—along with data on the amounts of thimerosal in specific vaccines—from Sweden and Denmark. Data from California were already available from an ecologic analysis presented to the Institute of Medicine (IOM) Immunization Safety Review Committee.

The findings

The incidence of autism in the U.S., Sweden and Denmark began to rise in 1985, with a peak of reported cases in the early 1990s. In Denmark, for example, the annual number of autism cases rose from less than 10 among 2 to 10 year olds before 1990 to 181 in 1999. However, thimerosal in Sweden and Denmark was low in the late 1980s and after 1993 none of their vaccines contained thimerosal. The opposite happened in the United States, where the average amount of thimerosal received from vaccines increased throughout the 1990s. The researchers concluded that the available data, including the evidence from their study, “are not consistent with the hypothesis that increased exposure to thimerosal-containing vaccines are responsible for the apparent increases in the rates of autism in young children being observed worldwide.”

The relevance/bottom line

This study adds to the evidence that thimerosal-containing vaccines do not cause autism or similar disorders.

NNii’s comment

The definition of autism has changed substantially during the time period analyzed in the study. Indeed, many authorities believe that the increased incidence of autism is largely due to this fact. Nevertheless, the findings of all of this and other studies should provide additional reassurance to families and others that there continues to be no evidence of a relationship between autism and thimerosal-containing vaccines.