Foul shot? Autism and the MMR vaccine.

THE PERCENTAGE of American children who receive childhood vaccinations is dropping, and educated, well-off parents are leading the retreat. What has spooked them? Parents fear that the measles-mumps-rubella (MMR) vaccine may trigger autism, a neurological disorder that typically appears before a child reaches the age of three.

The MMR/autism hypothesis took off in 1998 with the publication of a study of 12 autistic children by Canadian gastroenterologist AndrewWakefield. Wakefield's study found traces of the measles virus in the guts of children he tested. He concluded that the virus derived from the MMR vaccination, and suggested that it caused inflammation possibly related to the children's neuropsychiatric dysfunction. The connection was boosted by a report published in 2001 by autism activist Sallie Bernard, who argued that the mercury in the vaccine preservative thimerosal was "a novel form of mercury poisoning" responsible for autism.

But the preponderance of subsequent research has failed to find a connection between autism and the vaccine. A new study published by McGill University researchers in the July 2006 issue of the journal Pediatrics found that autism rates rose from 52 per 10,000 to 70 per 10,000 in Quebec a decade after the preservative thimerosal was removed from the vaccines. In 2005, a comprehensive review of 31 studies by the nonprofit authoritative medical collaboration, the Cochrane Library, found "no credible evidence behind claims of harm from the MMR vaccination."A 2004 National Academy of Sciences report produced similar results.

This and other research suggests that parents are confusing correlation with causation. The symptoms of autism just happen to emerge at the about the same time as recommended vaccinations are given. It's a coincidence, not a cause.

The real danger may not be the MMR vaccine, but fear of using it. Public health experts worry that many parents, having never seen a child afflicted with whooping cough or polio, are putting their kids at risk. The threat posed by the vaccine is likely illusory; that of infectious disease is surely not.