An examination of scientific studies worldwide has found no convincing evidence that vaccines cause autism, according to a committee of experts appointed by the Institute of Medicine.
In particular, no link was found between autism and the measles-mumps-rubella vaccine or vaccines that contain a mercury preservative called thimerosal. The committee released its eighth and final report on Tuesday in Washington.
Some parents of autistic children immediately protested. Mark Blaxill, the father of an 8-year-old girl with autism, said the committee's conclusions were premature. Studies are under way that should not be dismissed, said Blaxill, who is a director of the Coalition for SafeMinds, an advocacy group that finances research on the possible connection between autism and vaccines.
Rep. Dave Weldon, R-Fla., a physician who is an advocate for the parents, said the report was "based on preliminary, incomplete information and may ultimately be repudiated."
The report will not "put to rest the concerns of parents who believe their children were harmed" by vaccines, Weldon said.
Autism is a disorder of brain development that has been the subject of much publicity in recent years as parents and researchers hunt for its underlying cause or causes. The issue has been fueled by a rise in the number of children found to have autistic traits in the past decade, though experts disagree on how large the increase is.
Dr. Marie McCormick, a professor of maternal and child health at Harvard who led the investigation, said that most "parents should be reassured" and should not worry about getting their children vaccinated. In the meantime, she said, research on autism should focus on "more productive" areas, like genetic and environmental factors. The debate over vaccines and autism began five years ago when a British researcher, Dr. Andrew Wakefield, reported that a small number of autistic children had signs of measles infection in their intestines after getting the measles-mumps-rubella vaccine.
A separate dispute soon broke out when researchers noted that many childhood vaccines contained a mercury preservative, though the measles formulation was not among them. Could an increase in the number of mercury-containing vaccines given to infants be an underlying cause of increased rates of autism?
The Immunization Safety Review Committee at the Institute of Medicine, which is affiliated with the National Academy of Sciences, was formed three years ago to examine those issues. The group, composed of expert physicians and scientists with no connections to the pharmaceutical industry, met nine times to gather evidence on the claims.
The committee emphasized that it carried out its mandate from a neutral position: The weight of evidence would indicate only whether it was possible to favor or reject a link between vaccines and autism. "You can never really prove a negative," McCormick said.
In 2001, the committee issued two reports. The first concluded that the measles vaccine was not likely to cause autism based on the epidemiological evidence. The second found that there was not enough evidence to reject or accept a causal link between vaccines with mercury and neurodevelopmental disorders like autism. To be on the safe side, it recommended that infants get vaccines without mercury preservatives. By 2002, mercury had been removed from most childhood vaccines.
The report released Tuesday was based on previous evidence and new studies since 2001, and goes further than ever in discrediting claims that vaccines cause autism. On the subject of vaccines with mercury, five epidemiological studies worldwide show there is no evidence of a link with autism. Three studies found evidence, but the committee said the research methods were flawed.
As for the measles-mumps-rubella vaccine, 14 epidemiological studies have shown no evidence of a link. The committee dismissed two studies that did show a link as flawed. The committee examined a number of possible biological mechanisms to explain how vaccines might cause autism, but said that all were theoretical and that there was not sufficient proof.
Fewer children today receive vaccines that contain mercury, Blaxill of SafeMinds said, so if the mercury hypothesis holds true, rates of autism should fall in the next couple of years. The number of cases in California, where autistic children are carefully tracked, declined slightly in the last six months, he said, but it is too soon to know if the drop is a trend.
Autism is notoriously complex, McCormick said. Many scientists believe that it may involve numerous genes that appear to interact with a variety of environmental factors and other nongenetic influences.