Updated: Septiembre 20, 2004
Thimerosal does not cause autism; nor does the MMR vaccine. This is the conclusion reached by The Institute of Medicine’s Immunization Safety Review Committee in its report, Vaccines and Autism.1
The report states that “the body of epidemiological evidence favors rejection of a causal relationship between the MMR vaccine and autism” as well as a “rejection of a causal relationship between thimerosal-containing vaccines and autism.”
The hypothesis that the MMR vaccine was associated with autism was originally proposed in a highly publicized series of case reports published in The Lancet in 1998.2 The authors suggested that the onset of the symptoms of autism with gastrointestinal problems was temporally associated with the receipt of the MMR vaccine.
The IOM committee confirmed that this study by Wakefield and colleagues did not provide evidence that the MMR vaccine could cause autism. Indeed, in 2004, ten of the thirteen authors of that study formally retracted their suggestion of a possible link between MMR vaccine and autism.3
In a previous report in 2001,4 the IOM’s committee had rejected any causal relationship between the MMR vaccine and autism at the population level—that means the MMR vaccine did not cause autism in the general population. However, the available evidence at that time was not sufficient to exclude the possibility that MMR could contribute to autism in a small number of children with a genetic predisposition to that disorder.
More recent epidemiological studies, which are assessed in the new IOM report, have consistently shown no evidence that the MMR vaccine was associated with autism.5
The IOM report described two studies by Geier6 which had reported an association between MMR and autism as “characterized by serious methodological flaws and their analytic methods were nontransparent making their results uninterpretable, and therefore non-contributory with respect to causality.”
In other words, the studies by Geier could not establish a causal relation between MMR and autism because of their methods—such as using statistical measures incorrectly and omitting facts about their research approach. Similar problems were found in six other studies by Geier7 and one study by Blaxill8, which reported findings of an association between thimerosal-containing vaccines and autism. In addition, Geier’s expertise in neurological disorders has been questioned.9
Five large studies in Sweden, Denmark, the United States and the United Kingdom consistently found no evidence of an association between thimerosal and autism.10 For that reason, the IOM’s committee favored rejection of a causal relationship between thimerosal-containing vaccines and autism.
This rejection differs from the conclusion of a 2001 report11 by the same committee on thimerosal-containing vaccines and neurodevelopmental disorders.
The 2001 report stated that at that time the evidence was inadequate to accept or reject a causal relationship between thimerosal and the disorders of autism, attention deficit, and speech and language delay. The evidence now favors rejection of a relationship between thimerosal and autism. The current report did not evaluate the other disorders.
The IOM committee recommended that immunization schedules remain unchanged.
The IOM committee also recommended that research funding for autism be channeled towards more productive areas, such as the better understanding of the genetic causes of autism.12