Exposure to mercury can interrupt a child's development. That has led to worry about vaccines that contain the preservative thimerosal, which has a mercury compound in it. Because of those concerns, vaccines recommended for young children in the United States no longer contain thimerosal, with the exception of some flu vaccines. But many other countries still use thimerosal-containing vaccines, including the United Kingdom, where researchers looked for a connection between thimerosal and developmental problems.

What the researchers wanted to know: Does exposure to mercury in vaccines cause developmental problems?

What they did: The researchers used the Avon Longitudinal Study of Parents and Children, a study that followed more than 13,500 women and their babies in Avon, southwestern England starting in the early 1990s. They calculated mercury exposure based on the number of diptheria/tetanus/pertussis or diphtheria/tetanus shots the kids got, since those two vaccines contain thimerosal. Mothers completed questionnaires about their children's behavior, speech, motor skills, tics, and special education needs.

What they found: There was no evidence that getting lots of thimerosal-containing vaccines early in life had a negative effect on development. There was one just-barely-significant result—higher mercury exposure at 3 months was associated with being less helpful at 47 months—but sinceresearchers did 69 different statistical tests, it's hard to get excited about one turning up significant. In fact, several tests came up with the exact opposite of what people worry about—that getting more mercury early was associated with better development. The results were controlled for birth weight, the mother's educational level, and other factors that can affect development.

What the study means to you: Yet another failure to find a connection between vaccination and developmental problems.

Caveats: The study probably underestimated mercury exposure in some children at one time point.

Find out more: The FDA's discussion of thimerosal: http://www.fda.gov/

The Institute of Medicine examined thimerosal-containing vaccines and neurodevelopmental disorders: http://www.iom.edu/

Read the article: Heron, J., et al. "Thimerosal Exposure in Infants and Developmental Disorders: A Prospective Cohort Study in the United Kingdom Does Not Support a Causal Association." Pediatrics. September 2004, Vol. 114, No. 3, pp. 577-583.

Abstract online: http://pediatrics.aappublications.org/