Q&A

Vaccinations containing thimerosal do not increase rates of autism

J Fam Pract. 2004 February;53(2):93-105
By
Jennifer J. Buescher, MD
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  • BACKGROUND: Thimerosal has historically been used as a preservative in many vaccines. Thimerosal contains ethyl mercury that has been postulated as a cause of neurodevelopmental disorders such as autism. Between 1970 and 1992 the only thimerosal-containing vaccine used in Denmark was the whole-cell pertussis vaccine; after 1992 only a thimerosal-free pertussis vaccine was available.
  • POPULATION STUDIED: All children born in Denmark between January 1, 1990, and December 31, 1996, were included in this retrospective cohort study. The researchers examined vaccination records for all children and followed the subjects until diagnosis of autism or an autistic-spectrum disorder, age 11 years, or December 31, 2000, whichever came first. A total of 467,450 children were followed for an average of 6.4 years, with 440 cases of autism and 787 cases of autistic-spectrum disorders identified.
  • STUDY DESIGN AND VALIDITY: Denmark stopped administering vaccines containing thimerosal in June 1992. Through a national registry, the researchers were able to identify how many thimerosal-containing vaccines each child received. All persons in Denmark are given an identification number in the Danish Civil Registration System, thus follow-up information and comparative analyses through links to other databases are readily obtainable and highly reliable.
  • OUTCOMES MEASURED: The primary outcomes were the rate of autism and autistic-spectrum disorders in children vaccinated with thimerosal-containing and thimerosal-free vaccines. A secondary outcome was the change in overall incidence of autism and autistic-spectrum disorder over time.
  • RESULTS: In this cohort, 95.6% of the children received at least 1 whole-cell pertussis vaccine, with or without thimerosal, during the study period. Autism rates between children vaccinated with thimerosal-containing and thimerosal-free vaccines were similar, with a relative risk of 0.85 (95% confidence interval [CI], 0.60–1.20). Rates of autistic-spectrum disorders were similar in both groups as well, with a relative risk of 1.12 (95% CI, 0.88–1.43). When comparing children receiving 0, 1, 2, or 3 thimerosal-containing vaccines, there was no evidence of a dose-response correlation between thimerosal and autism or autistic-spectrum disorder. Among all children, the age-adjusted relative risk of diagnosis of autism increased each year from 1990 to 1996
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PRACTICE RECOMMENDATIONS

This retrospective cohort study found that autism rates in children receiving vaccines containing thimerosal were not statistically different than for children receiving thimerosal-free vaccines.

This study refutes any connection between the thimerosal preservative and autistic disorders, but does not evaluate the risk of autistic disorders in vaccinated children compared with those who are not vaccinated. No routine pediatric vaccinations currently used in the United States contain thimerosal.

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