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 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 national 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 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 virtually all childhood vaccines.
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.
“The totality of the results are quite reassuring,” she said.
The study, which took seven years to complete, examined the medical records of children between ages 7 and 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 age 7 months to determine each child’s total mercury exposure and also 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.
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