Immunizations, particularly measles-mumps-rubella (MMR), have been hypothesized to factor into the development of autism. The syndrome of autism has been associated with genetic factors and developmental neurologic defects. Manifestations of autism are usually seen as communication and behavior abnormalities later in childhood but, in the majority of cases, neuropathology is present at birth. This makes it unlikely that immunization could cause a majority of autism cases. A minority of children initially develop normally and then show symptoms of autism. These cases of “regressive autism” may more plausibly be associated with immunization.
DeStefano reviewed the hypothesis of an association between immunization and MMR vaccination. A 1998 report described 12 children with inflammatory bowel disorders who developed regressive conditions including autism. MMR vaccination was hypothesized to be associated with the bowel condition that resulted in malabsorption and subsequent neurodevelopmental problems. This report has been discredited because of the small number of subjects and the lack of an unaffected comparison group. The link between bowel disease and autism is also in doubt. Subsequent studies showed no evidence of measles virus in the mucosa of children with inflammatory bowel disease, and natural measles disease of immunization has not been shown to be associated with subsequent bowel inflammation.
Other researchers looked at the epidemiology of the increasing incidence of autism syndrome since 1979 in London and found no major increase following the initiation of universal MMR vaccination in 1988. Also, no time relationship has been found between MMR immunization and regression onset or exacerbation of autism symptoms.
Other observations supporting the link between autism and immunization include the recent increase in autism among children enrolled in special education or persons whose disability programs have increased their immunization coverage. These data are difficult to review because of changes in autism diagnosis criteria and an increased awareness of subtle symptoms. The increase in autism diagnosed in this group of children far exceeded the relatively small increase in MMR immunization coverage.
The author concludes that recent studies have found no association between MMR vaccination and autism. The frequent embryologic neuroanatomic abnormalities found in children with autism lessen the likelihood that MMR immunization is a major risk factor. The Immunization Safety Review Committee of the Institute of Medicine and a special American Academy of Pediatrics panel have concluded that evidence does not support MMR immunization as a risk factor for autism.