Thimerosal not associated with neuropsychological deficits

Physicians should continue to vaccinate children according to the current ACIP-recommended vaccine schedule.

Issue: December 2007

The controversial mercury-containing preservative thimerosal was not found to be associated with impaired neuropsychological functioning in children aged 7 to 10 years, according to research presented at the Advisory Committee on Immunization Practices meeting, held in Atlanta.

“Physicians should not change their behavior when vaccinating children,” William W. Thompson, PhD, epidemiologist of the National Center for Immunization and Respiratory Diseases, told Infectious Diseases in Children. “We recommend that physicians should continue to vaccinate children according to the schedule that is universally recommended by the ACIP, the AAP and the American Association of Family Practitioners.”

Following the 1999 joint statement issued by the AAP and the Public Health Service, which called for removal of thimerosal from vaccines, several studies were conducted to test the possible danger of this preservative in children.

For example, in a 2001 study by Ball et al, researchers assessed thimerosal use in childhood vaccines and found a well-recognized delayed-type hypersensitivity reaction from thimerosal exposure in children aged 6 months and in children aged 2 years.

“Our review revealed no evidence of harm caused by doses of thimerosal in vaccines, except for local hypersensitivity reactions,” the researchers wrote.

“However, some infants may be exposed to cumulative levels of mercury during the first SIX months of life that exceed Environmental Protection Agency recommendations. Exposure of infants to mercury in vaccines can be reduced or eliminated by using products formulated without thimerosal as a preservative.”

Study outcomes

In a cohort study with extensive assessments of relevant exposures and neuropsychological functioning, Thompson and colleagues examined whether early exposure to thimerosal may cause neuropsychological deficits.

The researchers tested 1,047 children aged 7 to 10 years from four Vaccine Safety Data link sites.

“The study participants had received vaccines during a time when thimerosal was used in some of the recommended childhood vaccines. With the exception of some childhood formulations of influenza vaccines, thimerosal is currently no longer used as a preservative in any of the recommended childhood vaccines,” Thompson said.

Thimerosal exposure from vaccines was confirmed by four sources, which included vaccine safety data, medical chart reviews, parent vaccine records and maternal interviews regarding potential prenatal exposures.

Exposure to thimerosal in the first seven months of life was estimated for the entire eligible population from computerized records. A sample of participants was then selected by using these data to include adequate numbers of children across a range of ages and estimated exposures, according to the researchers.

“We had an extensive battery of outcomes,” Thompson said. “Forty-two were assessed in the three-hour period when children were brought in for testing, but I want to make it clear that not all of these are related to autism.”

The measures included:

Speech and language (n=9).
Verbal memory (n=7).
Attention/executive functioning (n=7).
Behavior regulation (n=6).
Fine motor coordination (n=4).
Tics (n=4).
General intellectual functioning (n=3).
Achievement abilities (n=1).
Visual spatial abilities (n=1).

Results indicated that among these 42 outcomes, few significant associations with thimerosal exposure prenatally or during the first seven months of life were found; these few associations found were sex-specific, according to Thompson.

“In 12 outcome measures, increasing thimerosal exposure was associated with better outcomes vs. seven outcomes where increasing thimerosal was associated with poorer outcomes. This represents significant findings for 5% of the statistical tests conducted,” he said.

Thompson said that there is a need for future studies to look at the association found with tics.

This is particularly important because there were two other tic associations found in the 2003 study by Verstraeten et al and the 2004 study by Andrews et al.

“We are currently carrying out a separate autism study where we have evaluated 250 children with autism and will compare their vaccine histories with 750 matched controls. The results of this study should be available within the next year,” Thompson said. – by Jennifer Southall

For more information:

Thompson W, Price C, Goodson B, et al. Early thimerosal exposure and neuropsychological outcomes at 7 to 10 years. N Engl J Med. 2007;357:1281-1292.

Thompson W. Early thimerosal exposure and neuropsychological outcomes at 7 to 10 years. #A10. Presented at: The Advisory Committee on Immunization Practices; Oct. 24-25, 2007; Atlanta.

Ball LK, Ball R, Pratt RD. An assessment of thimerosal use in childhood vaccines. Pediatrics. 2001;107:1147-1154.

Verstraeten T, Davis RL, DeStefano F, et al. Safety of thimerosal-containing vaccines: a two-phased study of computerized health maintenance organization databases. Pediatrics. 2004;113:1039-1048.