There is no evidence that the combined measles, mumps and rubella vaccine — given to nearly 4 million American toddlers each year — increases a healthy child's risk of developing autism, according to a report Monday by the Institute of Medicine in Washington.
The report said it was theoretically possible that in some toddlers the vaccine, or other vaccines, could act as a trigger for autism but that this problem, if it exists, would be extremely rare. Otherwise, "we would have seen some patterns of it in population-based studies," said Dr. Steven Goodman, an associate professor of oncology, pediatrics, epidemiology and biostatistics at the Johns Hopkins Schools of Medicine and Public Health in Baltimore, who is one of the experts on the institute panel that produced the report.
Every study that has looked at large numbers of vaccinated children has found "absolutely nothing" to show cause and effect between the so-called MMR vaccine and autism, Goodman said.
But the rate of autism seems to be on the rise. In California, which tracks autism incidence more closely than many other states, 700 cases of the severest form of autism were formally diagnosed and reported to the California Department of Developmental Services in the first three months of 2001. During the same period last year, the department identified 416 new cases. Five years earlier the number was 234 new cases in three months.
"The numbers are staggering," said Dr. Robert Byrd, an epidemiologist at the University of California at Davis. Byrd is carrying out a statewide study to find whether the apparent increase is real or the result of misdiagnosis or a greater and earlier awareness of autism among physicians.
If the rate of autism is increasing and the MMR vaccine is not to blame, what else might be causing it?
"We don't understand the pathophysiology and risk factors of autism very well," Goodman said.
Other states do not track autism as thoroughly as California does, so it is not clear if increases are happening elsewhere, Byrd said. Ten years ago it was estimated that autism affected one in every 10,000 children but recent trends suggest that one in 500 children have some form of the disorder.
Autism is a developmental disorder with a wide spectrum of symptoms that range from mild to severe.
In severe cases, autistic children exhibit ritualized body movements, repeated touching and sniffing of objects and an insistence on following precise routines.
Many parents of autistic children say that their child was developing normally until autism developed at around 15 to 18 months of age, when the MMR vaccine is normally administered. Their fears have been supported by researchers in England who found live measles virus in the intestines of recently vaccinated autistic children.