Vaccine Safety > Issues of Interest > Autism
CDC Research on Vaccines & Autism

1. What was the goal of this study?

The main goal of this study was to see if there was a difference in the age at which children with autism and without autism received their first MMR vaccination. If an association existed between the MMR vaccine and autism, children vaccinated at younger ages (less than 24 months of age) would be expected to have a higher risk of developing autism. Autism is usually identified before 24 months of age.

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2. Why did CDC conduct this study?

Autism is a serious life-long developmental disability. CDC is dedicated to understanding better the causes of autism. Because the measles-mumps-rubella (MMR) vaccination has been suggested as a possible cause for autism, the CDC conducted a large-scale, population-based study in Metropolitan Atlanta to examine this relationship. This study is an example of our ongoing efforts to examine vaccine safety issues of concern to parents, health care providers, and others who are concerned about this condition. More information on CDC’s efforts related to understanding the causes of autism is available at http://www.cdc.gov/programs/bd02.htm.

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3. What were the major findings and conclusions of CDC's study on the age of first MMR vaccination for children with autism compared with children who do not have autism?

   The study findings showed:

   • Children with autism received their first MMR vaccination at similar ages as children without autism.
   • The majority (84%) of children with and without autism received their first MMR vaccination before they were 24 months old
   • Children who received their first MMR vaccination before they were 24 months old showed no increased risk for autism regardless of their age at the time of study, sex, race, maternal age or maternal education.
   • There was no increased risk for autism or any forms of autism including children whose condition included a loss of developmental skills (regression) associated with vaccination before 24 months of age.
   • Children with autism were slightly more likely to have been vaccinated before three years of age. It is likely that these results were influenced by factors related to the treatment of children with autism at this age. For example, children with autism might have been more likely than children without autism to have been vaccinated as a requirement for enrollment in preschool special education programs that start for children who are 3 years old. The majority of children (98%) with autism, between 3 to 5 years of age in this study were enrolled in these programs.

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4. How did CDC researchers determine which children had autism and at what age they received their first MMR vaccination?

This study was a large-scale, population-based, case-control study which also assessed the possible association between autism and the MMR vaccination for specific groups of children who may have been at increased risk for autism. CDC researchers, clinical psychologists and developmental pediatricians identified children with autism (624) and children without autism (1824) in Metropolitan Atlanta who were 3 to 10 years of age in 1996. They did this by reviewing data from the Metropolitan Atlanta Developmental Disabilities Surveillance Program (MADDSP) and by carefully reviewing information from special education records and treatment providers to ensure that children with autism were correctly identified. Vaccination records from children's school records were used to determine the age at which these children had received their first dose of MMR vaccination.

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5. What were the specific groups of children assessed to see if they were at increased risk for autism from their first MMR vaccination?

One of the main contributions of this study is that it assessed associations in specific groups of children who may have been at increased risk for autism from environmental exposures, such as vaccines. These groups included:

• Children who either lost developmental skills (regression) or failed to make progress (plateau);
• Children with developmental delays before one year of age or with a pre- existing condition that would likely contribute to a developmental delay;
• Children with or without mental retardation (which was defined as an IQ of less than or equal to 70); and
• Children grouped according to characteristics such as gender, race, and birth weight.

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6. What do these findings mean for parents?

These findings suggest that:

• The MMR vaccine did not cause autism in the general population or among specific groups of children who may have been at an increased risk for autism
• The current recommended age for the MMR vaccine (12 to 15 months) is not a contributing factor toward autism. The study findings showed that this was true for children who received their MMR vaccination before they were 24 months old; and applied to all children in the study regardless of their age, sex, race, or age and education level of their mothers.

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7. How many other studies have been done to investigate the possible association between autism and the MMR vaccination?

There have been several population-based published studies, including this one, to investigate the possible association between autism and MMR vaccination. They have been conducted in countries around the world including England, Sweden, Denmark, and the U.S. The weight of the evidence from these studies does not support an association between the MMR vaccine, or vaccines of any kind, and autism.

There have been two laboratory studies suggesting a possible connection between MMR vaccination and autism. These studies have raised the question of whether the MMR vaccine could have caused a persistent measles virus infection of the intestines resulting in a unique syndrome of regression in development and gastrointestinal disorders in some children with autism. The study, however, was not able to distinguish if the intestinal infection was from a vaccine virus or from naturally occurring measles virus. The mechanism of how an infection of the intestine could lead to brain damage resulting in autism has not been established. The population-based studies have not found evidence of a unique syndrome of developmental regression and gastrointestinal disorders associated with MMR vaccine.

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8. What are the known causes of autism?

No one knows exactly what causes autism, but scientists think that both genetic and environmental factors may play a role.

Family studies have shed the most light on the genetic contribution to autism. Studies of twins have shown that in identical twins there is about a 75% rate of both twins having autism, while in non-identical twins this occurs about 3% of the time. The risk of autism in siblings ranges from 2 to 6 percent. And in families with autism, there are often other developmental problems as well, such as language delays or learning disabilities. The inheritance pattern is complex and suggests that a number of genes are involved.

Autism also tends to occur more frequently than expected among individuals who have certain other medical conditions, including Fragile X syndrome, tuberous sclerosis, congenital rubella syndrome, and untreated phenylketonuria (PKU). Some harmful substances ingested during pregnancy also have been associated with an increased risk of autism, specifically, the prescription drug thalidomide.

We do know that parental child-rearing practices do not cause autism.

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9. Does the MMR vaccine contain thimerosal?

No MMR vaccine has ever contained thimerosal.

Today, with the exception of some flu vaccines, none of the vaccines used in the U.S. to protect preschool children against infectious diseases contain thimerosal as a preservative. The minute amounts of thimerosal found in a few of the recommended childhood vaccines are an unavoidable by product of the production process and no harmful effects have ever been reported or found with these doses of thimerosal in vaccine.

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10. Can vaccines that contain thimerosal cause autism?
     
    Although the evidence is strong that the MMR vaccine does not cause autism, new concerns have developed about possible neurodevelopmental effects, including autism, from exposure to the mercury-containing preservative, thimerosal. Thimerosal was used in certain vaccines given to infants prior to 1999. No association has been verified between any serious health outcomes and the use of thimerosal as a preservative in vaccines.

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This page last modified on June 15, 2006
Content on this page last reviewed on April 7, 2006