Scientists disagree on whether there could be a link between the rising number of children with the developmental disorder autism and certain childhood vaccines.
At a daylong meeting here last week before a federal panel examining the issue, scientists presented an array of studies on the sharply debated topic. Some research showed no connection between thimerosal, a mercury-based preservative once widely used in childhood vaccines, and the prevalence of autism, while other studies indicate the potential for mercury from vaccines to have played a role.
Some scientists also disputed a separate theory about vaccines and autism. That theory suggests that the vaccine for measles, mumps, and rubella causes the complex neurological disorder, which leaves children with difficulties communicating and forming relationships. Other scientists said the potential for a link between the MMR shot and autism still existed.
Those theories have developed, some experts say, because children experience the onset of symptoms of autism around age 2, which coincides with their receiving the vaccines and so has led some parents to suspect a connection.
Public-health officials are anxious over the stakes of the debate. They fear that parents worried about autism might not get their children vaccinated against potentially deadly diseases.
The Immunization Safety Review Committee of the Institute of Medicine, an arm of the National Academy of Sciences that provides scientific information to Congress, will review the information submitted at the Feb. 9 meeting in Washington and issue a report on the subject in three to six months, staff members said.
In 2001, the panel concluded that no clear evidence existed to prove or disprove a link between vaccines and autism, and it called for additional research. In the meantime, the committee recommended that thimerosal be removed from childhood vaccines.
Since 1999, as a precautionary measure, vaccine manufacturers have removed the preservative from most vaccines given to young children in the United States. But it remains in some, such as the flu shot, which is recommended for pregnant women in their second or third trimesters during the flu season.
Since the committee’s last report, several high-profile studies have cast doubt on a connection between vaccines and autism.
One large-scale study in Denmark, published in November 2002, looked at the health records of 537,303 children—every child born in Denmark between 1991 and 1998.
The rate of autism was virtually the same for children who had received the MMR vaccine and those who had not. (“New Study Discounts Autism-Vaccine Linkage,” Nov. 20, 2002.)
Other population studies have come to similar conclusions disputing a vaccine and autism link.
A new study presented last week looked at 103,000 British children born between 1988 and 1997, who as babies were given vaccines with thimerosal. The study showed no evidence that exposure to thimerosal at a young age increased the risk of autism, said Elizabeth Miller of the Public Health Laboratory Service at the Communicable Disease Surveillance Center in London.
But other scientists argued that despite such studies, other research has shown children with autism have higher levels of mercury in their systems.
One toxicologist, H. Vasken Aposhian, a professor of molecular and cellular biology at the University of Arizona, said thimerosal can enter the developing brain of young children, adding to their overall mercury-exposure level.
He hypothesized that autism will one day prove to be related to a child’s inability to effectively rid his or her systems of mercury.
“We may find thimerosal doesn’t cause it [autism], but it adds to the mercury load,” he said.
Another scientist, Dr. David Baskin, a professor of neurosurgery and anesthesiology at Baylor College of Medicine, urged the committee to reserve judgment about studies discounting a link between autism and vaccines because the science is still evolving.
“With such an increase [in autism diagnoses] in these last years, it can’t just be a genetic disorder,” Dr. Baskin said. “There has to be a mix of environmental and genetic factors.
Twenty years ago, between two and five cases of autistic disorders were reported per 10,000 people nationwide. Now the disorders have been identified as affecting as many as one in 500 people, making it more common than childhood cancer or Down syndrome, according to the National Academy of Sciences.