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No association between thimerosal-containing pertussis vaccine and autism

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COMMENTARY

An article in the October 1, 2003 issue of JAMA found no association between the use of thimerosal-containing pertussis vaccine and the incidence of autism. Between 1970 and 1992, whole-cell pertussis vaccine was the only thimerosal-containing vaccine used in Denmark for routine immunization of infants. In 1992 the last batch of this vaccine was phased out. Whole-cell pertussis vaccine without thimerosal was used from June 1992 through 1996. Thereafter, only acellular pertussis vaccine without thimerosal was used. The Danish Civil Registration System assigns a unique identifier to each person and can track the health and immunization status of all children. A cohort of children born between January 1, 1990 and December 31, 1996 was identified and the number later diagnosed with autistic disorder or other autism spectrum disorder (ASD) was determined. The incidence of autism or other ASD in children receiving thimerosal-containing vaccine was compared to the number of new cases identified who did not receive thimerosal-containing pertussis vaccine and a rate ratio (RR), based on the number of person-years at risk of exposure to thimerosal, was computed. The RR was also adjusted for increasing cumulative dose of thimerosal in increments of 25 mcg of ethylmercury (which is about one-half by weight of thimerosal). The results are summarized in the abstract below. The incidene of autism or other ASD was not different between the two groups. Also, there was no dose-dependent trend of increasing incidence as the cumulative dose of ethyl mercury increased. These data also show a general trend of increasing incidence during the years that thimerosal-containing pertussis vaccine was not used.

These data confirm the findings reported by a separate group of Danish researchers, Madsen et al, PEDIATRICS 2003;112:604-606, [see Abstract], who reported a time trend analysis of the incidence of autism during the period, 1971-2000. The number of children diagnosed with autism (using the criteria of the ICD-8, 1971-1993, and, since 1994, the ICD-10, which included atypical autism, at the time of initial inpatient or, since 1995, outpatient contact) was extracted from a central data registry. There was no significant trend of increasing incidence during the two-decade period, 1971-1990. Immunization with thimerosal-containing vaccines was discontinued in 1992. Between 1991 and 2000 there was an approximately 10-fold increase in the incidence of autism in children 2-4 years of age at the time of diagnosis. These children were born after the discontinuation of thimerosal-containing vaccine.

Although millions of children have received thimerosal-containing vaccines over the years, the association with autism seemed unlikely but biologically plausible. This paper convincingly shows that there is no association.


ABSTRACT

Title: Association Between Thimerosal-Containing Vaccine and Autism

Authors: Anders Hviid, MSc; Michael Stellfeld, MD; Jan Wohlfahrt, MSc; Mads Melbye, MD, PhD

Publication: J Amer Med Assoc 2003;290:1763-1766.

Context: Mercuric compounds are nephrotoxic and neurotoxic at high doses. Thimerosal, a preservative used widely in vaccine formulations, contains ethylmercury. Thus it has been suggested that childhood vaccination with thimerosal-containing vaccine could be causally related to neurodevelopmental disorders such as autism. Objective To determine whether vaccination with a thimerosal-containing vaccine is associated with development of autism.

Design, Setting, and Participants: Population-based cohort study of all children born in Denmark from January 1, 1990, until December 31, 1996 (N = 467 450) comparing children vaccinated with a thimerosal-containing vaccine with children vaccinated with a thimerosal-free formulation of the same vaccine. Main Outcome Measures Rate ratio (RR) for autism and other autistic-spectrum disorders, including trend with dose of ethylmercury.

Results: During 2 986 654 person-years, we identified 440 autism cases and 787 cases of other autistic-spectrum disorders. The risk of autism and other autistic-spectrum disorders did not differ significantly between children vaccinated with thimerosal-containing vaccine and children vaccinated with thimerosal-free vaccine (RR, 0.85 [95% confidence interval {CI}, 0.60-1.20] for autism; RR, 1.12 [95% CI, 0.88-1.43] for other autistic-spectrum disorders). Furthermore, we found no evidence of a dose-response association (increase in RR per 25 µg of ethylmercury, 0.98 [95% CI, 0.90-1.06] for autism and 1.03 [95% CI, 0.98-1.09] for other autistic-spectrum disorders).

Conclusion: The results do not support a causal relationship between childhood vaccination with thimerosal-containing vaccines and development of autistic-spectrum disorders.

Author Affiliations: Danish Epidemiology Science Centre, Department of Epidemiology Research (Messrs Hviid, Wohlfahrt, and Dr Melbye) and Medical Department (Dr Stellfeld), Statens Serum Institut, Copenhagen, Denmark.

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Commentary by Ronald J. Kallen, M.D.
This page last updated on October 5, 2003