Infant Mercury Exposure

June 2000

Two articles and an editorial on thimerosal in vaccines appeared in the May 2000 issue of The Journal of Pediatrics. The new data reaffirm the IVS position that for infants, preference should be given to vaccines that do not contain thimerosal as a preservative.

Stajich and colleagues reported the results of testing premature and term newborns for a blood mercury levels before and after a single dose of hepatitis B vaccination using vaccines that contain thimerosal. Both groups showed significant rises in blood mercury concentrations; as expected, the highest levels were achieved in premature infants because of their smaller weights. (In this study, premature infants weighing less than 2,000 grams were immunized. The AAP and the Advisory Committee on Immunization Practices recommended waiting until infants weighed 2000 grams in 1994. In July 1999 the cut point was changed to 2,500 grams.) Stajich et al noted a mean blood mercury concentration of 7.36 mcg/L 48-72 hours after immunization (range of 1.3 to 23.6 mcg/L). These infants received an average dose of 16.7 mcg/kg of mercury. The smallest infants who receive only all thimer

osal-containing vaccines at 6-8 weeks could receive up to 20 mcg/kg on a single day.

Steuerwald, et al. found a statistically significant association between umbilical cord blood levels of mercury and a lower score on standardized neonatal neurodevelopment scale in infants born to women with mercury exposure primary from consumption of fish in Faeroe Islands. These data add to the cumulating evidence that low-to-moderate prenatal exposure to methylmercury pose a hazard to the neurologic development of infants. Further data will be forthcoming from long-term follow-up of cohorts of children in the Seyshelle Islands.

An accompanying editorial by representatives from the Centers for Disease Control and Prevention and the Agency for Toxic Substances and Disease Registry attempts to put some of these new findings into perspective including some of qualifications regarding the interpretation of these results. However, the authors do not point out that we now have an ample supply of vaccines that do not contain thimerosal as a preservative in the United States and that there is no reason for continuing to expose infants in this country to even a theoretical risk of neurodevelopmental delay. On June 21, 2000, the Advisory Committee on Immunization Practices of the Centers for Disease Control and Prevention will once again reassess the progress that manufacturers have made and policies on the use of these vaccines in this country.

The recently generated data only reaffirm the IVS opinion that we should be giving strong preference to vaccines that do not contain thimerosal for infants in this country. The situation is more complex in developing countries and efforts will need to be made to prepare vaccines without thimerosal as a preservative as rapidly as possible for use in these countries.