MMR Vaccine and Autism : Part 2

by CDC

(Page 3 of 5)

Madsen et al. (2002) conducted a study of all children born in Denmark from January 1991 through December 1998. There were a total of 537,303 children in the study; 440,655 of the children were vaccinated with MMR and 96,648 were not. The researchers did not find a higher risk of autism in the vaccinated than in the unvaccinated group of children. Furthermore, there was no association between the age at time of vaccination, the amount of time that had passed since vaccination, or the date of vaccination and the development of any autistic disorder. Though there were many more vaccinated than unvaccinated children in the study group, the sample was large enough to contain more statistical power than other MMR and autism studies. Therefore, this study provides strong evidence against the hypothesis that MMR vaccination causes autism.

DeStefano et al. (2004) conducted a study to see if there was a difference in the age at which children with autism and without autism received their first MMR vaccination. The study's findings showed that children with autism received their first MMR vaccination at similar ages as children without autism.

Are there studies that suggest there might be a connection between autism and MMR vaccine?

The existing studies that suggest a causal relationship between MMR vaccine and autism have generated media attention. However, these studies have significant weaknesses and are far outweighed by the epidemiologic studies described above that have consistently failed to show a causal relationship between MMR vaccine and autism.

The MMR-autism theory is based on the idea that intestinal problems, like Crohn's disease, are the result of viral infection and can contribute to the development of autism. The theory has its origins in research by Wakefield and colleagues (1989; 1990) which suggested that inflammatory bowel disease (IBD) is linked to persistent viral infection.

In 1993, Wakefield and colleagues reported isolating measles virus in the intestinal tissue of persons with IBD. However, the validity of this finding was later called into question when it could not be reproduced by other researchers (Afzal, 1998; Iizuka et al., 2000).

Thompson and colleagues (1995) suggested in a retrospective cohort study that MMR vaccine might be a risk factor for Crohn's disease. However, the selection and recall biases and the differences in data collection in this study were so substantial as to cast doubt on the validity of the findings.

Two studies out of Sweden linked measles infection in utero to the development of IBD (Ekbom et al., 1994; Ekbom et al., 1996). However, these studies involved a very small number of cases and when researchers identified the persons to be included in the 1996 study, they had prior knowledge that cases of Crohn's disease had occurred in the offspring of two women who were infected with measles during pregnancy. This is called "selection bias" and limits the strength of the study.

The MMR-autism theory came to the forefront when, in 1998, Wakefield and colleagues reviewed reports of children with bowel disease and regressive developmental disorders, mostly autism. The researchers suggested that MMR vaccination led to intestinal abnormalities, resulting in impaired intestinal function and developmental regression within 24 hours to a few weeks of vaccination. This hypothesis was based on 12 children. In 9 of the cases, the child's parents or pediatrician speculated that the MMR vaccine had contributed to the behavioral problems of the children in the study. There are a number of limitations in the Wakefield et al. (1998) study:

1. The study used too few cases to make any generalizations about the causes of autism; only 12 children were included in the study. Further, the cases were referred to the researchers and may not be a representative sample of cases of autism.

2. There were no healthy control children for comparison. As a result, it is difficult to determine whether the bowel changes seen in the 12 children included in the study were similar to changes in normal children, or to determine if the rate of vaccination in autistic children was higher than in the general population.

3. The study did not identify the time period during which the cases were identified.

4. In at least 4 of the 12 cases, behavioral problems appeared before the onset of symptoms of bowel disease; that is, the effect preceded the proposed cause. It is unlikely, therefore, that bowel disease or the MMR vaccine triggered the autism.

In 2004, 10 of the 13 authors of the study retracted the paper's interpretation, stating that the data were insufficient to establish a causal link between MMR vaccine and autism (Murch et al., 2004)

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The Centers for Disease Control and Prevention (CDC) is one of the 13 major operating components of the Department of Health and Human Services (HHS), which is the principal agency in the United States government for protecting the health and safety of all Americans and for providing essential human services, especially for those people who are least able to help themselves.