Mercury Levels in Premature and Low Birth Weight Newborn Infants after Receipt of Thimerosal-Containing Vaccines

The recommendation of the American Academy of Pediatrics (AAP)1 the Center for Disease Control (CDC) and the Food and Drug Administration (FDA)2 to reduce or remove thimerosal as a preservative in vaccine administered to children was made in 1999 and implemented by 2001. The recommendations were made because of expanding infant immunization schedule in the US, which would permit the injection of up to 62.5 μg of ethyl mercury (the active ingredient of thimerosal) at a single health care visit and up to 187.5 μg cumulatively across all health care visits during the first 6 months of life. Lacking specific data on ethyl mercury metabolism and half-life in infants receiving vaccines containing ethyl mercury, the AAP, CDC and FDA relied on metabolism and half-life data of another form of mercury, i.e., methyl mercury, as is present in fish and is ingested orally by adults. Calculations of possible persistence of ethyl mercury in the body of a low birth weight or prematurely born infant based on the ethyl mercury content of all the infant vaccines recommended in the first six months of life led to the conclusion that levels of ethyl mercury from injections could exceed those recommended by the Environmental Protection Agency for oral consumption of methyl mercury in fish.3 In two prior studies, we demonstrated that blood mercury levels were low after vaccines containing thimerosal were given to full term newborn, 2-month, and 6-month old infants, and that mercury was detected in increased concentration in the stools following vaccination, suggesting elimination by the gastrointestinal route 4,5. The calculated half life in blood was 3 to 7 days, significantly shorter than the 45-day half-life of oral methyl mercury in adults. The objective of this study was to evaluate the pharmacokinetics of mercury in blood in a cohort of prematurely born infants with low birth weight who received a birth dose of hepatitis B vaccine containing thimerosal in concentrations still in use in vaccines provided through the World Health Organization (WHO) and to evaluate the excretion of mercury in these prematurely born infants by examining mercury levels in their stools and urine.