Thimerosal, an organomercury compound which has been used since the 1930's as a preservative in vaccines, is much faster cleared from the body than methylmercury, mainly taken up from contaminated fish. This is the result of a study recently published by a group of US researchers in Environmental Health Perspectives.
The group compared the systemic disposition and brain distribution
of total and inorganic mercury in infant monkeys following thimerosal
exposure with infants exposed to MeHg. The initial and terminal
half-life of Hg in blood following thimerosal exposure was 2.1 and 8.6
days, which are significantly shorter than the elimination half-life of
Hg following methylmercury exposure at 21.5 days. Brain
concentrations of total mercury were significantly lower by ~3-fold for
the thimerosal-exposed infants when compared to the
methylmercury infants and a higher percentage of the total
mercury in the brain was in the form of (less toxic) inorganic
mercury for the thimerosal-exposed infants (34% vs 7%).
This study indicates that methylmercury is not a suitable
reference for risk assessment from exposure to thimerosal derived
mercury. Toxicokinetics and developmental toxicity of thimerosal itself
have to be studied to afford a meaningful assessment of the effects of
thimerosal-containing vaccines.
Until such data is available, the current debate linking the use
of thimerosal in vaccines to autism and other developmental disorders
(IOM 2001, 2004) will probably continue (see news below).
Related Information:
Thomas M. Burbacher, Danny D. Shen, Noelle Liberato, Kimberly S. Grant, Elsa Cernichiari, and Thomas Clarkson, Comparison of Blood and Brain Mercury Levels in Infant Monkeys Exposed to Methylmercury or Vaccines Containing Thimerosal, Environ. Health Perspect., 113/8 (2005) 1015-1021. DOI: 10.1289/ehp.7712