Mercury levels in premature and low birth weight newborn infants after receipt of thimerosal-containing vaccines.

OBJECTIVE: We conducted a population-based pharmacokinetic study to assess blood levels and elimination of mercury after vaccination of premature infants born at > or =32 and <37 weeks of gestation and with birth weight > or =2000 but <3000 g. STUDY DESIGN: Blood, stool, and urine samples were obtained before vaccination and 12 hours to 30 days after vaccination from 72 premature newborn infants. Total mercury levels were measured by atomic absorption. RESULTS: The mean +/- standard deviation (SD) birth weight was 2.4 +/- 0.3 kg for the study population. Maximal mean +/- SD blood mercury level was 3.6 +/- 2.1 ng/mL, occurring at 1 day after vaccination; maximal mean +/- SD stool mercury level was 35.4 +/- 38.0 ng/g, occurring on day 5 after vaccination; and urine mercury levels were mostly nondetectable. The blood mercury half-life was calculated to be 6.3 (95% CI, 3.85 to 8.77) days, and mercury levels returned to prevaccination levels by day 30. CONCLUSIONS: The blood half-life of intramuscular ethyl mercury from thimerosal in vaccines given to premature infants is substantially shorter than that of oral methyl mercury in adults. Because of the differing pharmacokinetics, exposure guidelines based on oral methyl mercury in adults may not be accurate for children who receive thimerosal-containing vaccines.

The toxicology of mercury and its compounds

As an element present in the earth’s crust, in water, and in the atmosphere, living cells have been exposed to mercury throughout evolution of life on this p1anet. Elemental mercury vapor was probably present even in Archean times and may have been oxidized to the highly toxic divalent cation, Hg++, when oxygen first appeared in concentrations approaching present-day levels. As a result, cells have developed mechanisms to protect themselves from its toxic effects. Some of these mechanisms are diseussed. On the other hand, methylmercury compounds, although also present at the eariiest times, may not have presented a toxic hazard until the evolution of an advanced nervous system. This may be one reason why we lack lhe sarne degree of cellular protection as we enjoy for inorganic mercury. The production of methylmercury by biomethyiation in the environrnent, its bioaccumulation in aquatic food chains, and its potency to produce irreversibie brain damage makes it the species of mercury of greatest public health concern.