This study shows that the mercury contained in some vaccines is handled very differently by the body than mercury found in foods such as fish or the mercury from industrial accidents.
Comparison of blood and brain mercury levels in infant monkeys exposed to methylmercury or vaccines containing thimerosal. Burbacher TM, Shen DD, Liberato N, Grant KS, Cernichiari E, Clarkson T. Environmental Health Perspective 2005; ehponline.org.
Explanatory note: Inorganic mercury enters the body readily, accumulating mostly in the kidney. It enters the central nervous system slowly but it is also slowly eliminated. Organic mercury compounds are more readily adsorbed and can cross into the brain much more readily than inorganic mercury.
Thimerosal is an organic mercury compound that is 49.6% mercury by weight, present in the form of ethylmercury. Little information about ethyl mercury distribution in the body or its elimination from the body has been available. The absence of good data about ethylmercury has necessitated the use of safety guidelines that were developed for methylmercury, another organic mercury compound that is a known toxin for the developing brain—often found in fish and pollutants. Guidelines for safe exposure to methylmercury, based on the analysis of cases where people were accidentally exposed to the compound, have been developed by three federal agencies.
The data available from small studies with children have suggested that ethylmercury is less toxic than methylmercury and that it is eliminated from the body more rapidly.
Is ethylmercury distributed in and eliminated from the body in the same way as methylmercury?
Forty-one newborn monkeys were treated in three ways. Seventeen were given thimerosal-containing vaccines by intramuscular injection, 17 were administered methylmercury oxide given by feeding tube in the stomach and 7 were untreated controls. The vaccines and the methylmercury were prepared by the investigators. These were given to the animals at 0, 7, 14 and 21 days of age. Blood was obtained 2, 4, and 7 days after the initial exposure and when the animals were sacrificed. Animals were sacrificed 2, 4, 7 or 28 days after their last dose for brain and other analyses.
The researchers then analyzed the total mercury in blood samples and both the total and inorganic mercury in brain. Utilizing the results, the researchers evaluated how they could best understand how the mercury was processed from the two types of mercury exposure.
The absorption and initial distribution of total mercury was similar for the thimerosal injection and the feeding of methylmercury.
However the elimination of ethylmercury was much faster and total mercury in the brain was significantly lower in the animals that received the vaccines. The pattern of elimination was also very different. Because the mercury was much more slowly eliminated from the methylmercury-fed animals, the levels of mercury in these animals increased with the additional doses given.
The researchers found higher levels of inorganic mercury in the brains and kidneys of the thimerosal treated animals than in the methylmercury-fed animals.
The mercury contained in some vaccines is handled very differently by the body than mercury found in foods such as fish or the mercury from industrial accidents. Mercury in vaccines is eliminated from the body much faster than methylmercury (8.6 versus 21.5 days). Methylmercury is not a suitable reference for extrapolating information about ethylmercury.
This study found similar blood levels and rates of mercury elimination to those reported in the small numbers of studies that have looked at children. Thimerosal is not used as a preservative in vaccines administered to children younger than 6 months in the United States. Because worldwide, thimerosal is used as a preservative for vaccines given to many children, more research about thimerosal continues to be needed.