WEDNESDAY, Sept. 26, 2007 (HealthDay News) -- The latest study on thimerosal, a preservative used in childhood immunizations, should provide parents with reassurance that exposure to the agent will not cause neuropsychological problems later on, experts say.
"We found no consistent pattern between increasing mercury exposure from birth to seven months and performance on neuropsychological tests," concluded the study's authors in the Sept. 27 New England Journal of Medicine.
Widely used before 2000, thimerosal has been the center of controversy for some years now. Some parents of autistic children believe that the mercury contained in the preservative is responsible for their children's autism.
However, the U.S. Centers for Disease Control and Prevention (CDC) maintains there is no scientific evidence of such an association. And this latest CDC study did not specifically look at links between thimerosal exposure and autism. According to the study authors, a separate CDC case-control study focused on autism and mercury exposure, is currently under way.
In 1999, the U.S Food and Drug Administration (FDA) concluded that children who were vaccinated according to the recommended childhood immunization schedule could be exposed to levels of mercury from the thimerosal contained in those vaccines that were higher than the maximum levels considered safe by the FDA.
In response, the CDC and the American Academy of Pediatrics (AAP) asked vaccine manufacturers to remove thimerosal from vaccines. That move, not surprisingly, concerned many parents who wondered if the preservative was being removed because it was harmful.
In an effort to allay some of those fears, the current CDC study looked at more than 1,000 children between the ages of 7 and 10. They compared the youngsters' neuropsychological functioning with their level of thimerosal exposure.
Using information from a three-hour neuropsychological assessment or information provided by parents and teachers, the researchers measured 42 neuropsychological outcomes, including speech, language, verbal memory, fine motor coordination, achievement, behavior regulation, tics and general intellect.
They then compared those findings to levels of thimerosal exposure, based on the child's vaccination exposure prenatally, in the first month of life, and in the first seven months of life.
"Our study does not support a causal association between early exposure to mercury from thimerosal-containing vaccines and immune globulins and deficits in neuropsychological functioning at the age of 7 to 10 years," concluded the study's authors.
That conclusion, however, seems to contradict some of the study's findings.
For example, the researchers found that boys with the highest levels of thimerosal exposure had about twice the risk of evaluator-observed tics compared to boys with the lowest exposure.
While these findings may seem disparate with this conclusion, Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases, said some of the findings are likely due to chance, because such a high number of statistical comparisons were done. She was not involved in the study, which was led by William Thompson of the CDC's influenza division.
According to Schuchat, who spoke at a special CDC teleconference held Wednesday, the researchers completed 42 tests of neuropsychological functioning and completed 378 statistical comparisons from those tests.
"By chance alone, 19 of the 378 statistical tests we ran would be abnormal," she said, noting that 19 is the exact number of tests that resulted in abnormal findings.
However, 12 of those 19 tests suggested a positive outcome from higher thimerosal exposure, and seven, including the tic finding, suggested a negative outcome.
"Each test doesn't tell us as much individually," she said. "Chance alone probably explained these findings. The totality of the results are quite reassuring."
Additionally, Schuchat said there was no increase in the incidence of parent-reported tics in children with the highest thimerosal exposure, so the tics observed by the evaluators may have been transient in nature.
But several other studies have found a possible link between thimerosal exposure and tics. Because of that, the CDC feels the finding should be further explored to ensure that it was due to chance finding and not an actual effect of the preservative.
"This is what happens when you try to sub-stratify data," said Dr. Paul Offit, chief of infectious diseases at Children's Hospital of Philadelphia. "What's important is if you add up all the studies, do you find greater problems from thimerosal?" And, he said that study after study has shown that thimerosal does not cause harm.
"Parents should feel enormously reassured that another study has found that the levels of thimerosal contained in vaccines before 2001 are not harmful," said Offit, who also wrote one of two accompanying perspective pieces in the journal. Both pieces offered details on the legal history and the public health implications of the autism-vaccine controversy, even though that was not addressed in the actual study itself.
Thimerosal has already been removed from all but one vaccine, according to Schuchat. Some versions of the influenza vaccine still contain thimerosal, though thimerosal-free versions are also available, she said.
"Parents shouldn't take a theoretical risk and elevate it above the real risk of influenza. There's nothing theoretical about the dangers of influenza," Offit explained, adding that even healthy children can be at risk of serious flu complications.
"Vaccines are safe and effective. They prevent 33,000 deaths and 14 million infections annually," Schuchat said.
To learn more about vaccines, visit the U.S. Centers for Disease Control and Prevention.