The Committee considered a recently published pharmacokinetic study of thiomersal (contains ethyl mercury) in macaque monkeys.1 The systemic disposition and distribution in the brain of total and inorganic mercury were compared in infant monkeys following administration of thiomersal-containing vaccines and those exposed to methyl mercury. Differences between the kinetics of ethyl mercury and methyl mercury were examined. The findings confirmed the view that methyl mercury is not suitable for risk assessment of thiomersal. Brain concentrations of total mercury were threefold lower with thiomersal, compared with methyl mercury, and the average ratio of brain–blood concentrations of mercury was slightly higher for thiomersal-exposed infant monkeys. However, differences in primate and human brain development and species differences in the pharmacokinetics of mercury and mercury-containing compounds limit extrapolation of the results in these experimental conditions to what might be expected in human infants.
GACVS was informed of 2 neurodevelopmental studies currently being conducted in the United States and Italy of children who had received thiomersal-containing vaccines during infancy according to the national immunization schedules. Data analysis is presently in progress and the Committee will consider the results when they are available.
Currently, GACVS remains of the view that there is no evidence supporting a causal association between neurobehavioural disorders and thiomersal-containing vaccines.
1. Comparison of blood and brain mercury levels in infant monkeys exposed to methyl mercury or vaccines containing thiomersal. Environmental Health Perspectives, online 21 April 2005 (doi:10.1289/ehp.7712; http://ehp.niehs.nih.gov/docs/ 2005/7712/abstract.html).