Vaccines and Autism

At a glance: The weight of currently available scientific evidence does not support the hypothesis that the MMR vaccine causes autism. CDC recognizes there is considerable public interest in this issue, and therefore supports additional research regarding this hypothesis. CDC is committed to maintaining the safest, most effective vaccine supply in history.

Frequently Asked Questions

1. What is autism?

Autism is a term that refers to a collection of neurologically-based developmental disorders in which individuals have impairments in social interaction and communication skills, along with a tendency to have repetitive behaviors or interests. The severity of autism varies greatly, from individuals with little speech and poor daily living skills, to others who function well in most settings. Autism is typically diagnosed during the toddler or preschool years, although some children are diagnosed at older ages. It has been reported that approximately 20 percent of children with autism experience a "regression;" that is, they have apparently normal development followed by a loss of communication and social skills. Boys are three-to-four times more likely to have autism than girls. Autism occurs in all racial, ethnic, and social groups. A variety of factors could be associated with some forms of autism, including infectious, metabolic, genetic, neurological, and environmental factors. Genetic factors and brain abnormalities at birth are considered to be some of the most recognized causes of autism. For more information, see CDC's autism site.

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2. Does the measles-mumps-rubella (MMR) vaccine cause autism?

Current scientific evidence does not support the hypothesis that measles-mumps-rubella (MMR) vaccine, or any combination of vaccines, causes the development of autism, including regressive forms of autism. The question about a possible link between MMR vaccine and autism has been extensively reviewed by independent groups of experts in the U.S. including the National Academy of Sciences, Institute of Medicine. These reviews have concluded that the available epidemiologic evidence does not support a causal link between MMR vaccine and autism.

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3. What have studies found regarding MMR vaccine and autism?

Epidemiologic studies have shown no relationship between MMR vaccination in children and development of autism:

• In 1997, the National Childhood Encephalopathy Study (NCES) was examined to see if there was any link between measles vaccine and neurological events. The researchers found no indication that measles vaccine contributes to the development of long-term neurological damage, including educational and behavioral deficits (Miller et al., 1997).
  
  
• A study by Gillberg and Heijbel (1998) examined the prevalence of autism in children born in Sweden from 1975-1984. There was no difference in the prevalence of autism among children born before the introduction of the MMR vaccine in Sweden and those born after the vaccine was introduced.
  
  
• In 1999, the British Committee on Safety of Medicines convened a "Working Party on MMR Vaccine" to conduct a systematic review of reports of autism, gastrointestinal disease, and similar disorders after receipt of MMR or measles/rubella vaccine. It was concluded that the available information did not support the posited associations between MMR and autism and other disorders.
  
  
• Taylor and colleagues (1999) studied 498 children with autism in the UK and found the age at which they were diagnosed was the same regardless of whether they received the MMR vaccine before or after 18 months of age or whether they were never vaccinated. Importantly, the first signs or diagnoses of autism were not more likely to occur within time periods following MMR vaccination than during other time periods. Also, there was no sudden increase in cases of autism after the introduction of MMR vaccine in the UK. Such a jump would have been expected if MMR vaccine was causing a substantial increase in autism.
  
  
• Kaye and colleagues (2001) assessed the relationship between the risk of autism among children in the UK and MMR vaccine. Among a subgroup of boys aged 2-5 years, the risk of autism increased almost 4 fold from 1988 to 1993, while MMR vaccination coverage remained constant at approximately 95% over these same years.
  
  
• Researchers in the U.S. found that among children born between 1980 and 1994 and enrolled in California kindergartens, there was a 373% relative increase in autism cases, though the relative increase in MMR vaccine coverage by the age of 24 months was only 14% (Dales et al., 2001). For more on this study, see California Data on Theory of Autism and MMR Immunization.
  
  
• Researchers in the UK (Frombonne & Chakrabarti, 2001) conducted a study to test the idea that a new form, or "new variant," of Inflammatory Bowel Disease (IBD) exists. This new variant IBD has been described as a combination of developmental regression and gastrointestinal symptoms occurring shortly after MMR immunization. Information on 96 children (95 immunized with MMR) who were born between 1992 and 1995 and were diagnosed with pervasive developmental disorder were compared with data from 2 groups of autistic patients (one group of 98 born before MMR was ever used and one group of 68 who were likely to have received MMR vaccine). No evidence was found to support a new syndrome of MMR-induced IBD/autism. For instance, the researchers found that there were no differences between vaccinated and unvaccinated groups with regard to when their parents first became concerned about their child's development. Similarly, the rate of developmental regression reported in the vaccinated and unvaccinated groups was not different; therefore, there was no suggestion that developmental regression had increased in frequency since MMR was introduced. Of the 96 children in the first group, no inflammatory bowel disorder was reported. Furthermore, there was no association found between developmental regression and gastrointestinal symptoms.
  
  
• Another group of researchers in the UK (Taylor et al., 2002) also examined whether MMR vaccination is associated with bowel problems and developmental regression in children with autism, looking for evidence of a "new variant" form of IBD/autism. The study included 278 cases of children with autism and 195 with atypical autism (cases with many of the features of childhood autism but not quite meeting the required criteria for that diagnosis, or with atypical features such as onset of symptoms after the age of 3 years). The cases included in this study were born between 1979 and 1998. The proportion of children with developmental regression or bowel symptoms did not change significantly from 1979 to 1988, a period which included the introduction of MMR vaccination in the UK in 1988. No significant difference was found in rates of bowel problems or regression in children who received the MMR vaccine before their parents became concerned about their development, compared with those who received it only after such concern and those who had not received the MMR vaccine. The findings provide no support for an MMR associated "new variant" form of autism and further evidence against involvement of MMR vaccine in autism.
  
  
• 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. More information about this study can be found on the CDC's research on vaccines and autism web page.