Immunization Science

Thimerosal Exposure and Risk of Autism

This large case-control study looked at the relationships between thimerosal-containing injections—either before birth or during infancy—with the development of autism.

The article

Prenatal and infant exposure to thimerosal from vaccines and immunogolbulins and risk of autism. Price CS, Thompson WW, Goodson B, et al. 2010; 126: 656-64.

Explanatory note: The causes of autism spectrum disorders (ASD), a group neurodevelopmental disorders, are unknown. Most evidence, however, suggests that genetic factors are among the main causes of autism.

Thimerosal is a compound that is 49.6% ethyl mercury by weight which has been used as a preservative to prevent contamination of multi-dose vials of vaccines and other drugs since the 1930s. It was a preservative used with many (but not all) vaccines in use in the United States until 2001; it continues to be used in vaccines in many other parts of the world. It is also used as a preservative in some eye, ear, and other topical solutions, antivenins, immunoglobulins, and skin test antigens.

There is a large body of published studies which have shown no evidence that thimerosal exposure is associated with the development of autism. Nevertheless, some people have been concerned that there might be some link between exposure to thimerosal—either before birth or during infancy—particularly among children with ASD with regression.

The question

Are there relationships between thimerosal-containing injections—either before birth or during infancy—with the development of autism?

The study

This was a case-control study conducted in 3 managed care organizations (MCOs) that participate in the Center for Disease Control and Prevention’s Vaccine Safety Datalink. The study protocol was developed in consultation with an external panel of autism advocates, experts in autism, child development, toxicology, epidemiology, biostatistics, and vaccine safety. Before collecting the data, the researchers defined the subgroup analyses that were to be performed.

Study participants characteristics include:

  • Between 6 and 13 years of age at the time of data collection
  • Living with their biological mother since birth
  • Family fluent in English
  • Born between January 1, 1994 and December 31, 1999
  • Were continuously enrolled in the MCO until they were at least 2 years of age and until the sample had been selected to study
  • Lived within 60 miles of a study assessment facility

The researchers subdivided children with ASD into sub-groupings of ASD, autistic disorder (AD) and ASD with regression. Children were excluded if they had a medical condition with known links to ASD.

Approximately 3 control children were randomly selected and matched with case children for year of birth, gender and the MCO. Potential control children were screened by maternal interview to exclude children who might have unrecognized neurodevelopmental difficulties.

Thimerosal injection exposure was largely determined from computerized records and maternal interviews. The childrens’ thimerosal exposures were examined for the prenatal, birth-to-one-month, birth-to-7-month and birth-to-20-month periods.

The findings

After excluding ineligible cases and controls as well as those who could not be located and those who refused to participate, there were 256 ASD case-children and 752 control children who participated and met study criteria. Of the 256, 187 met the stricter criteria for AD and 49 met the criteria for ASD with regression.

Case-children and control children had similar levels of ethylmercury exposures during each of the study periods.

Exposure to ethylmercury during the prenatal and birth-to-one-month periods groups did not differ between the cases and controls. However, in the age ranges from birth to 7 months and birth to 20 months, increased ethylmercury exposure was associated with a decreased risk for all 3 autism diagnoses.

The relevance/bottom line

The findings from this large comprehensive study do not suggest any causal association between increasing exposure to ethylmercury in thimerosal—prenatally and early in life—and ASD, AD or ASD with regression at 7 to 10 years of age.

NNii’s comment

This study adds to the number of epidemiologic studies that have been unable to establish any association between ethylmercury exposure in thimerosal and the subsequent development of autism.