Kids' Vaccine Ingredient Not Likely Linked to Neurological Problems

WEDNESDAY, Sept. 26 (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.