A mercury-containing vaccine preservative is not associated with problems in speech, intelligence, memory, coordination, attention, or other measures of childhood development, a large new study finds.
Child-health experts say that the results should allay concerns that thimerosal, a preservative first added to vaccines in the 1930s, affects children’s brains.
“The study was enough to convince me that this small amount of mercury … was not harmful to the children,” says Michael Goldstein, vice president of the St. Paul, Minn.–based American Academy of Neurology.
“I think it’s one more piece of evidence that thimerosal doesn’t have any negative association with health outcomes,” says Penelope Dennehy, professor of pediatrics at the Brown University School of Medicine in Providence, R.I.
The study, funded by the Centers for Disease Control and Prevention (CDC) in Atlanta, enrolled 1,047 children, from 7 to 10 years old, whose health needs had been covered from birth by four health maintenance organizations. The researchers combed the health plans’ records to assess how much thimerosal each child received through the first 7 months of life.
Each child took a battery of 42 tests of language, memory, motor coordination, attention, and intelligence. The researchers then attempted to correlate the amount of mercury each child received—from 0 through 187.5 micrograms—with performance on the tests.
“We found no consistent pattern between increasing mercury exposure … and performance on neuropsychological tests,” CDC epidemiologist William W. Thompson and his team say in the Sept. 27 New England Journal of Medicine.
However, of 378 different statistical measures derived from the test results, 19 did show some small associations with thimerosal exposure. Twelve such measures were positive—that is, higher thimerosal exposure led to better outcomes—while seven were negative. “Five percent of the [measures] showed significant associations, and that’s what you would expect by chance,” says Thompson.
If thimerosal did hurt the children, “you would have found results in one direction only,” says Paul Offit, chief of the division of infectious diseases at Children’s Hospital of Philadelphia. “If you want to believe that thimerosal’s positive association with tics [one of the results of the analysis] is real, then you also have to believe that it makes [the children] perform better in school.”
Concerns about thimerosal flared in 1999 when the U.S. Public Health Service and the American Academy of Pediatrics recommended that the preservative no longer be used in childhood vaccines. With more and more vaccines being recommended, the groups were concerned that children could be exposed to amounts of mercury that exceeded Environmental Protection Agency guidelines. By 2003, no childhood vaccines contained thimerosal.
Because mercury can cause brain damage, many parents of children with autism latched on to thimerosal as a possible cause for soaring rates of the disease. In 2004, the Washington, D.C.–based Institute of Medicine concluded that available evidence refutes the thimerosal-autism connection. The current study did not assess autism directly, but another ongoing CDC study is expected to weigh in on the issue next year.
One of the 14 outside experts that the CDC consulted while designing and analyzing the current study disagrees with the published conclusions. Sallie Bernard, executive director of the parent-advocacy group SafeMinds in Tyrone, Ga., says that the study’s limitations, including a low participation rate, mean that it “was unable to prove either the presence or absence of a causal relationship” between thimerosal and developmental problems.