Researchers looked for a link with learning delays, not autism, and a critic says the work was flawed.
A study of 1,047 children who received mercury-containing vaccines as infants has concluded the mercury does not cause learning difficulties or developmental delays.
The research released Wednesday said mercury exposure was associated with very small changes in 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 national Centers for Disease Control and Prevention (CDC), which paid for the $5.3 million study, said the agency was 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 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.
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“The finding may or may not have importance,” she said.
The new 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 virtually all childhood vaccines.
Thimerosal has not been used in childhood vaccines since 2001, although it is still in some flu shots. The new findings apply to children immunized before then, or exposed to the preservative through shots their mothers received while pregnant. Thimerosal was put in vaccines to prevent contamination from bacteria.
Although several large studies have found no causal link between thimerosal and autism, the issue is contentious, 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.
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 percent of the children selected for the study enrolled, she said, a low level of participation that could affect the result.
Lead author William Thompson, an epidemiologist with the CDC, said the level of participation in the study was better than expected, and any biases 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.
Thompson worked for vaccine maker Merck. Four other researchers have received fees from drug companies and one has served as a consultant to a CDC committee on immunization.
The study, which took seven years, examined the medical records of children between 7 and 10.
Material from The Associated Press is included in this report.