A retrospective cohort study of autism in all children born in Denmark from January 1991 through December 1998 and those receiving measles, mumps, and rubella (MMR) vaccination is reported from the Danish Epidemiology Science Center, Aarhus, Denmark. Of 537,303 children in the cohort, 440,655 (82%) had received the MMR vaccine, 316 children were identified with a diagnosis of autistic disorder, and 422 with other autistic-spectrum disorders. The relative risk of autistic and other autistic-spectrum disorders in vaccinated compared to unvaccinated children was 0.92 and 0.83, respectively. No association was detected between age at time of vaccination, time since vaccination, and date of vaccination and development of autistic disorder. It is concluded that the evidence is against the hypothesis that MMR vaccination causes autism. 
COMMENT. It has been suggested that a reported increase in the incidence of autism in California may be linked to the widespread use of MMR vaccine. Symptoms of developmental regression and gastrointestinal disorders have coincided with MMR vaccination, and measles virus has been detected in the terminal ileum of these patients. Although these proposed associations may be suggestive of a causative role of MMR in autism, this retrospective cohort study fails to confirm the hypothesis and shows no temporal or other association between MMR and the onset of autism. The prevalence rates in 8-year-old children born between 1991 and 1998 in the Danish cohort were 7.7 per 10,000 for autistic disorder and 22.2 per 10,000 for other autistic-spectrum disorders. These rates are similar to those reported among French and US children. Recent observed increases in prevalence of autism in California and Denmark have occurred well after the introduction of the MMR vaccine and may possibly be related to heightened awareness of the diagnosis among physicians. The role of vaccines in certain neuropsychiatric illness is often controversial, but a link between MMR and autism appears to be unlikely.