As many as 1.5 million Americans today are believed to have some form of autism and that number is estimated to grow to 4 million in the next decade. Autism is a complex developmental disability that typically appears during the first three years of life. It is characterized by poor or limited social relationships, underdeveloped communication skills, repetitive behaviors, unusual interests and activities, and altered and confused sensory input. Autism is four times more prevalent in boys than girls and knows no racial, ethnic, or social boundaries.

In the search for a cause of autism, researchers have looked at a multitude of childhood exposures including infections and environmental chemicals. The suggested link between autism and childhood vaccines is perhaps one of the most hotly debated. The measles, mumps, and rubella (MMR) vaccine has become suspect for two main reasons: (1) it’s widespread use has paralleled the increase in incidence of autism in some areas, and (2) it is routinely given at around the same time that autism symptoms first appear.

Past studies looking for a link between the MMR vaccine and autism have found no evidence to support it, but questions remained because these studies generally had small sample sizes, a lack of statistical power, and suboptimal data for risk assessment. A new study in the November 7, 2002 issue of The New England Journal of Medicine puts to rest doubts about the safety of the MMR vaccine. Using a large sample, researchers have convincingly shown that this common childhood vaccination does not cause autism.

About the Study

Researchers supported by the Centers for Disease Control and Prevention (CDC), the National Alliance for Autism Research, and the Danish National Research Foundation studied 537,303 children born in Denmark over a 7-year period to look for any relationship between autism and vaccination against measles, mumps, and rubella. All of the data was obtained from the Danish Civil Registration System and five other national registries.

MMR-vaccination status of Danish children at 15 months of age was determined from data reported to the National Board of Health by general practitioners, who administer all MMR vaccinations in Denmark. The MMR vaccine used during the study period is identical to that used in the United States. The Danish Psychiatric Central Register, which only records diagnoses made by specialists in child psychiatry, provided researches with information about the diagnoses of autism. Diagnoses were based on the International Classification of Diseases, 10th Revisions (ICD-10).

Children that had inherited medical conditions associated with autism (tuberous sclerosis and Angelman’s syndrome, fragile X syndrome, and congenital rubella) were excluded from this analysis. Information on other factors that might influence the outcome (mother’s education, birthweight, etc.) was obtained from the Danish Medical Birth Registry and the National Hospital Registry.

The MMR-vaccinated children were grouped according to their age at the time of vaccination, the time since vaccination, and the calendar period when vaccination was performed. Researchers compared the rates of autism in the 440,655 children (82%) who were given the vaccine with the 96,648 children (18%) who were not given the vaccine.

The Findings

Over the seven-year study period, the researchers found no increase in the risk of autistic disorder or other autistic-spectrum disorders among vaccinated children when compared with unvaccinated children. In addition, there was no association between the development of autistic disorder and the age at, interval since, or the calendar period of vaccination. These findings were not changed after adjusting for, sex, birth weight, gestational age, mother’s education, and socioeconomic status of the family.

As with any research, there are limitations to this study. First, because this study was done in Denmark, some may question whether these results can be applied to outside populations, though the percent vaccinated and the type of vaccine used were nearly identical to the United States. Second, because reporting of other autistic-spectrum disorders is less complete than that for autistic disorders, it is possibly that some diagnoses were missed. (However, since this misclassification is unlikely to be associated with vaccination-status, it would not be expected to, affect the results.) Finally, the analysis did not contain any information on when the first autistic symptoms were noted, so the researchers could not adjust for a delay in the diagnosis.

How Does This Affect You?

Despite these limitations, the findings of this study provide convincing evidence that there is no association between MMR vaccination and autism. Given the vaccine’s dramatic success in reducing the incidence of measles, mumps, and rubella – all potentially serious diseases – this study will make it harder to argue against the overall benefit of the MMR vaccine. The CDC recommends that children receive the MMR vaccine once at 12 to 15 months of age and again between ages 4 and 6 years.