Thimerosal is a preservative that contains mercury. Before 2001, it had been commonly used in vaccines. Some believe it is responsible for neurologic and psychological problems in children. There have been many studies but no definitive proof that thimerosal causes these problems. The additive has been removed from current vaccines, but researchers continue to investigate its possible effects on children who had received thimerosal-containing vaccines.
Researchers from Italy conducted a survey of children that had been given a vaccine with thimerosal. The study, published in Pediatrics, reported on the neurological and psychological functioning of the children 10 years after the vaccines. Researchers found there was no clinically significant difference in neuropsychological testing between children exposed to higher and lower levels of mercury in their vaccines.
The cohort study contacted 1,403 children that had received a pertussis (whooping cough) vaccine containing thimerosal. The total amount of mercury received by each child was determined through medical records. Approximately 10 years after the vaccines were given, researchers gave all the children standard neuropsychological tests. The children were divided into two groups according to the total dose of mercury received:
Each child was then given 24 different neuropsychological tests covering such areas as language, memory, and motor skills. In most cases, there were no significant differences between the groups. The only exceptions were lower scores for girls in the high mercury intake group in the following test areas:
These were based on small differences in test scores that are unlikely to have any noticeable effect.
This is yet another study supporting the safety of vaccines. The slight differences seen in two of 24 neuropsychiatric tasks will have no discernable impact on the lives of these children. Even so, thimerosal is no longer routinely used as a vaccine preservative, and there is still no convincing evidence that low doses are harmful.
The indisputable benefits of vaccinating your child clearly outweigh the slight risk of harm. If you’re unsure about your child's vaccination schedule, be sure to speak to his or her doctor.
American Academy of Family Physicians
American Academy of Orthopaedic Surgeons
Tozzi AE, Bisiacchi P, Tarantino V, De Mei B, D'Elia L, Chiarotti F, Salmaso S. Neuropsychological performance 10 years after immunization in infancy with thimerosal-containing vaccines.Peditrics.2009 Feb;123(2):475-82.
Last reviewed April 2009 by Richard Glickman-Simon, MD