Immunization Science

Thimerosal and Brain Development

The article

Safety of Thimerosal-Containing Vaccines: A Two-Phased Study of Computerized Health Maintenance Organization Databases. Verstraeten T, Davis RL, DeStefano F, Lieu TA, Rhodes PH, Black SB, Shinefield H, and Chen RT. Pediatrics 2003;112(5): 1039-48.

The question

Did children exposed to differing amounts of thimerosal develop at the right ages?

The study

Scientists looked for neurodevelopmental disorders (e.g. autism, speech problems) in children immunized with vaccines containing varying amounts of thimerosal at three health maintenance organizations (HMOs).

The findings

Researchers conducted the study in two phases. In the first phase, they analyzed 124,170 children who were born from 1991 through 1999 at 2 HMOs for many aspects of child development. Data from these HMOs suggested that thimerosal could be associated with tics and speech and language disorders. However, in the second phase of the study—which included 16,717 children at a third HMO—this association was not demonstrated. In none of the three HMOs was there any evidence of an association between thimerosal-containing vaccines and autism.

The researchers concluded that there was no significant association between thimerosal-containing vaccines and neurodevelopmental outcomes, but further studies are needed.

The relevance/bottom line

This study adds to the evidence that thimerosal-containing vaccines do not cause autism, significant speech or language disorders, or other neurodevelopmental disorders.

NNii’s comment

In research studies like the first phase of this study, when looking for a great many different things, there may be a statistical chance of finding the wrong answer about 5 percent of the time. Although the preliminary studies suggested there could possibly be a link between thimerosal-containing vaccines and some neurodevelopmental problems, an additional study failed to show an association.