Thimerosal has been used widely as a preservative in certain vaccines and has been thought to increase the risk of certain neurodevelop-mental disorders, such as autism, language and speech delay, and attention-deficit/hyper-activity disorder. Despite the lack of evidence for a causal relationship, the biologic plausibility of a link remains. Hviid and colleagues performed a study to compare children receiving vaccines with and without thimerosal and followed them to note the incidence of autism and other autism-spectrum disorders in both groups.

Using information from Denmark's registry system, the authors were able to link data on vaccinations, diagnoses of autism, diagnoses of other autistic-spectrum disorders, other diagnoses, and confounders to the children in a cohort receiving thimerosal-based pertussis vaccine (before June 1, 1992) and thimerosal-free pertussis vaccine (after June 1, 1992).

During 2,986,654 person-years of follow-up, the authors identified 440 cases of autism and 787 cases of other autism-spectrum disorders. In the original cohort, 20,755 (4.4 percent) did not receive any whole-cell pertussis vaccine, 446,695 (95.6 percent) were vaccinated at least once, 416,081 (89.0 percent) were vaccinated twice, and 293,186 (62.7 percent) received all three doses. In those receiving at least one dose of vaccine, 407 cases of autism were identified, of whom 303 received thimerosal-free vaccine and 104 received thimerosal-containing vaccine; of the 751 cases of other autistic-spectrum disorders identified, 430 received thimerosal-free vaccine and 321 received thimerosal-containing vaccine.

Comparing children who had received at least one dose of thimerosal-containing vaccine as opposed to thimerosal-free vaccine, a fully adjusted relative risk of 0.85 was found for autism and a relative risk of 1.12 for autistic-spectrum disorders. There was no evidence of a dose-response association between the dose of ethyl mercury received and the incidence of autism and autistic-spectrum disorders. The authors found a statistically significant increase in the incidence of autism and autistic-spectrum disorders over the study period.

The authors conclude that there is no evidence of an association between thimerosal-containing vaccine and autism in children. They also found no dose-response association between autism and the amount of ethyl mercury received via thimerosal.

editor's note: This study adds to the mounting evidence that childhood vaccinations do not cause autism. Other studies have shown no association between the measles, mumps, and rubella vaccine and autism.¹ The use of the mercury-containing vehicle thimerosal was discontinued in the United States because of possible implications in the development of autism. Given the high frequency of vaccination, which coincides temporally with the diagnosis of autism in the first years of life, it is understandable that vaccinations have been implicated by association. However, based on current evidence, this association is merely coincidental.—c.w.