In February 1998, researchers in Britain suggested that receipt of the MMR vaccine may cause autism.17 This was based on a small series of 12 children, all of whom had inflammatory bowel disease and 8 of whom also had autism. This study was significantly flawed. First, case reports do not offer strong proof of causal association. Second, the exposure—MMR vaccination—relied on parental recall. Because these parents believed that MMR was responsible for their children's autism, it is not surprising that they reported a temporal association between MMR vaccination and the development of autistic symptoms. More importantly, the lead author was funded by a group of lawyers who were representing the families of the eight autistic children in a lawsuit against the manufacturer of MMR.18 Ten of the original 13 authors eventually retracted their statement of causality.19
Despite these limitations, this story was widely published in the popular media and on the Internet. In Britain, national MMR immunization rates fell from 92% to 75%, which resulted in several measles outbreaks and the first death caused by measles in a decade.20 In the United States, immunization rates have remained fairly high because of school-entry requirements, yet pediatricians do report that parents are concerned about the possible association between MMR and autism. These concerns led the Institute of Medicine (IOM) to investigate the potential relationship between MMR and autism.21,22 In 2001, and again in 2004, the IOM concluded that there was sufficient data to reject the hypothesis that MMR caused autism.
These decisions were based in part on several large epidemiologic studies. The most rigorous study was performed in Denmark in 2002.23 The investigators incorporated data from 537,303 children born in Denmark from 1991 to 1998, for a total of 2,129,864 person-years. Of these, there were 1,647,504 person-years of follow-up for children who received MMR and 482,360 person-years of follow-up for children who did not receive MMR. After adjusting for age, calendar year, sex, birth weight, gestational age, maternal education, and socioeconomic status, there was no significant difference in rates of autism (relative risk [RR] 0.92; 95% confidence interval [CI], 0.68–1.24) or other autistic-spectrum disorders (RR 0.83; 95% CI, 0.65–1.07) between children who had received MMR and those who had not.
Other research has relied upon ecologic analyses.24 Two studies compared rates of autism before and after the introduction of national MMR immunization programs, and found that the prevalence25 and incidence26 of autism decreased after the introduction of MMR. Other studies employing time-series approaches found that trends in the number of children diagnosed with autism did not parallel trends in MMR coverage.27–30 If MMR did cause autism, autistic children who received MMR might have developed symptoms at an earlier age than autistic children who were unvaccinated. This does not appear to be the case.23,26,29