A study of 1,047 children who received mercury-containing vaccines as infants has concluded that the mercury does not cause learning difficulties or developmental delays.
The research released Wednesday said mercury exposure was associated with very small changes on some measures of attention, speech and motor control. But the changes varied by gender and were mostly beneficial, leading scientists to conclude they were the result of chance.
Dr. Anne Schuchat, an official with the Centers for Disease Control and Prevention, which paid for the $5.3-million study, said the agency was still trying to assess one finding: Boys with the greatest exposure to vaccines containing mercury had twice the risk of developing tics compared with boys with the lowest mercury exposures.
Schuchat, who heads the CDC's National Center for Immunization and Respiratory Diseases, said other studies have detected a correlation between mercury exposure and tics. She noted, however, that the tics were not reported by parents but by evaluators who assessed the children during the study, raising questions about whether the small muscular spasms posed a real problem.
"The finding may or may not have importance," she said.
The report in the New England Journal of Medicine did not examine whether mercury causes autism, as some scientists and advocacy groups have argued. Mercury is a component of thimerosal, which until recently was used as a preservative in childhood vaccines.
Although several large studies have found no causal link between thimerosal and autism, the issue is a contentious one, and several thousand parents are seeking legal compensation on behalf of children who developed autism after receiving vaccinations.
Schuchat reiterated during a conference call that there was no scientific support for the theory that thimerosal caused autism. She said the CDC is conducting two large epidemiological studies exploring the possible link. The latest study should reassure parents that vaccines are safe and do not cause other kinds of neuropsychological harm, she said.
"The totality of the results are quite reassuring," she said.
However, a consultant who worked on the study, Sallie Bernard, executive director of the advocacy group SafeMinds, disagreed.
Bernard said the research suffered from methodological problems that cast doubt on the findings, which she said were at best inconclusive. Only 30% of the children selected for the study enrolled, she said, a low level of participation that could bias the result.
"One interpretation is that this is due to chance. But another equally plausible and valid interpretation is possible. . . . We really don't know which is right," said Bernard, who was identified in the report as a "dissenting member" of the research team.
Lead author William W. Thompson, an epidemiologist with the CDC, said the level of participation in the study was better than expected, and any biases in the study would favor an association between thimerosal and harm because parents who believed their children were hurt by vaccines would be more likely to enroll them.
The study, which took seven years to complete, examined the medical records of children ages 7 to 10 who were enrolled in four health maintenance organizations and would have received shots containing thimerosal. (With the exception of the seasonal influenza vaccine, no childhood vaccines currently contain thimerosal).
Researchers assessed children's performance on tests measuring attention, memory, IQ and coordination, among other things. They used vaccination records from birth to 7 months to determine each child's total mercury exposure and considered prenatal mercury exposures.
Higher mercury exposures were related to better performance on 12 tests and worse performance on seven tests, an outcome no different from chance, researchers said.
Girls and boys performed differently on some measures, an inexplicable result that researchers attributed to statistical flukes. For example, greater levels of mercury during the first month of life were associated with lower verbal IQs in girls and higher performance IQs (a measure of spatial ability) in boys.
Dr. Paul A. Orfit, a vaccine researcher at Children's Hospital of Philadelphia, who wrote a commentary accompanying the study, said that the results were not a surprise and that he hoped that research efforts would shift from investigating thimerosal to developing new treatments.
Orfit said recent studies appear to point toward a genetic cause of autism. "A truth is emerging here," he said.