Autism and MMR vaccination or thimerosal exposure: an urban legend?

form of a cautionary tale that is related or transmitted as if true.1 The tale is plausible enough to be believable and indeed is a type of modern folklore that represents the beliefs of ordinary people. Frequently, the legend addresses a vexing aspect of modern life and carries the moral message that new technologies may ultimately prove to be a hazard. Mass media, including the internet, may serve to propagate and establish an urban legend as “truth”, however careful examination of the legend’s origin and its contents ultimately reveals it to be false and without basis. The belief that either MMR vaccination or excessive thimerosal exposure may be causally linked to the occurrence of an autistic spectrum disorder has many of the features of an urban legend. For MMR vaccination it originates in a single small study of 12 children published in the Lancet in 1998,2 whose essential conclusion was later retracted by ten of its coauthors.3 This article serves as the core of the ‘vaccination and autism’ legend that is now believed to be true by a majority of parents surveyed.4 For thimerosal, the legend’s origin lies not in a scientific article per se, but in a theoretical proposal published in Medical Hypothesis in 2001.5 However, as systematically surveyed by Doja and Roberts6 in this issue of the Canadian Journal of Neurological Sciences, the vast preponderance of epidemiological data subsequently collected in a variety of geographic settings, utilizing a number of investigative approaches, refutes any causal link between autism and either prior MMR vaccination or excessive thimerosal exposure. Particularly impressive are the results obtained from large datasets in Denmark,7 the United Kingdom,8-10 Finland,11 Sweden,12 United States,13 Japan14 and most recently Canada itself15 that each independently have refuted any MMR vaccination and autism causal association. Other systematic reviews, to which that of Doja and Roberts can be appended, have supported these conclusions.16-18 Furthermore, recent rigorous and independent molecular biologic investigations of one corollary hypothesis, that of the persistence of the measles virus in autistic children, have yielded negative results and unmasked more methodological flaws in the original work.19,20 Similarly, epidemiologic studies have refuted, utilizing varying approaches and analyses (cohort and ecological), convincingly any link between excessive thimerosal exposure and later autism.15,21-26 Biological studies have also not validated any thimerosal and autism link.27,28 Yet despite the overwhelming scientific epidemiologic evidence and the lack of any plausible scientific hypothesis as to why the MMR vaccine or thimerosal should cause autism, the Can. J. Neurol. Sci. 2006; 33: 339-340