The prospect that the MMR vaccination triggers autism first surfaced in 1998, when British researchers reported on a dozen children who developed the condition and other behavior problems shortly after getting immunized. However, studies since then have failed to support the connection.
Last year, a report from the U.S. Institute of Medicine, an arm of the National Academy of Sciences, said "a consistent body of epidemiological evidence shows no association at a population level between MMR" and autism. The World Health Organization, British health authorities, the American Academy of Pediatrics and other major medical groups have made similar statements.
Dr. Kreesten Madsen, the epidemiologist who led the latest research, says he hopes the new findings will finally quash concerns about the alleged connection.
"The risk was the same for vaccinated and unvaccinated children," says Madsen, of the Danish Epidemiology Science Center in Arhus. "Few studies can be said to be conclusive, but this one was state-of-the-art."
The researchers, who report their results in tomorrow's issue of The New England Journal of Medicine, reviewed the medical histories of 537,000 children born in Denmark between 1991 and 1998. Of those, 82 percent received the MMR vaccine, which was introduced to the country in 1987 and is identical to the one available in the United States.
Information from national disease registries showed that 316 of the children developed autism, and another 422 developed similar behavioral disorders. However, there was no difference in the risk of either diagnosis among those who received the combined vaccine and those who did not.
The scientists also looked for "timing clusters" of autism cases, to see if the condition appeared shortly after immunization.
"We would expect that a month or a year after vaccination kids would get [autism], but we didn't see any rise," Madsen says.
Still, critics of the vaccine say they aren't swayed by the new research.
Barbara Loe Fisher, co-founder and president of the National Vaccine Information Center, dismisses the Danish study as "numbers moved around on a piece of paper."
The issue "is not going away because Denmark has done an epidemiological study saying don't worry about it anymore," says Fisher, who claims her son developed neurological complications, though not autism, after receiving the MMR vaccine.
Fisher says the debate can only be settled by cell-level science that looks at the effects of the inoculation on the immune system and the brain -- research she is convinced will prove proponents of the MMR-autism link right.
Dr. Marie C. McCormick, a professor at Harvard University's School of Public Health and chairwoman of the Institute of Medicine committee that examined MMR's alleged tie to autism, says her group "couldn't rule it out in very, very rare cases with some sort of susceptibility" to the behavioral problem. In general, she says, parents have no reason to be concerned about the connection.
The MMR vaccine does use live, weakened measles virus. However, McCormick says there's no known biological mechanism for measles virus to cause autism.
Evidence is emerging, she adds, that genetics may be at the root of the condition.
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