Paul Offit, infectious disease expert
There's clear evidence that the MMR vaccine does not cause autism. Autism, a serious developmental disorder that causes problems in communication, social interaction, and behavior, has been on the rise since the 1970s and, by some estimates, now affects one in 160 children in the United States. No one knows what causes the condition or why it's becoming more prevalent, so parents are understandably alarmed.
Concern about a link between the MMR vaccine and autism began in 1998, after the British medical journal The Lancet published a study connecting the measles-mumps-rubella vaccine with autism. The researchers were investigating the theory that intestinal problems, like Crohn's disease, can result from viral infection and can contribute to the development of autism. The study was very small, however (only 12 children participated), and has since been called into question by several of the original researchers.
Another British study, published in 2002, seemed to suggest an association between measles (not necessarily the vaccine), intestinal bowel disease, and autism or related developmental disorders.
In that study, researchers found measles virus fragments in 75 of 91 children with intestinal bowel disease. Traces of measles were found in only five of the 70 controls. But the way the study was designed made it impossible to know whether the MMR vaccine caused the bowel disease and developmental delays or if the association was a coincidence.
In 2004, a much larger study in The Lancet compared 1,294 children with autistic spectrum disorders with 4,469 unaffected children and concluded that the MMR vaccination doesn't raise the risk of autism or other autism spectrum disorders.
A number of other studies have compared the incidence of autism among children who received the MMR vaccine and those who didn't, and concluded that autism isn't more common in vaccinated children.. Ten studies performed on three continents involving tens of thousands of children have now clearly shown that MMR does not cause autism.
Most experts think that autism may be at least partly genetic, and point out that there's no plausible way for a vaccine to trigger it. After all, there's no known connection between measles, mumps, or rubella and autism. It doesn't make sense that a vaccine would cause a condition that the disease itself doesn't cause, since a vaccine is essentially a symptomless infection.
It's also important to point out that the MMR vaccine never contained thimerosal, the mercury-based preservative that some people believe may be linked with autism. (Thimerosal has now been removed from all childhood vaccines except the flu vaccine, so it's no longer a concern.) Nevertheless, the Centers for Disease Control continues research in this area to try and resolve the issue.