Immunization Science

Mercury Levels After Vaccines with Thimerosal

The article

Mercury levels in newborns and infants after receipt of thimerosal-containing vaccines. Pichichero ME, Gentile A, Giglio N, et al. 2008. Pediatrics 121(2) e208-14.

Explanatory note: Thimerosal is a compound made up of ethylmercury attached to a chemical similar to aspirin. In the body it is split apart, releasing the ethylmercury which kills a number of micro-organisms, including many bacteria. It had been widely used as a preservative in a number of vaccines routinely used in newborns and infants in the United States until 2001. It is still widely used in many parts of the world.

When all the discussions about thimerosal preservatives began in 1999, there was very little data about ethylmercury toxicity, its distribution in the body after injection, or its elimination from the body. The only known toxicities to thimerosal were allergic reactions and deaths associated with massive exposures.

However, in 1999 much was known about methylmercury, an environmental contaminant in some foods. It distributes everywhere in the body, including to the brain; it is eliminated very slowly from the body (the half-life for elimination from blood is about 44 days and from the entire body is about 70 days); it is excreted by the liver into the stool where it is then converted into inorganic mercury (which is poorly reabsorbed into the body). Because it remains in the body for such a long time, it accumulates with repeated exposure. It is known to cause neurologic and developmental problems.

In 1999, because so little information about ethylmercury was available, public health authorities assumed “a worst case scenario;” that is, they assumed that until they had more information, they should act as if ethyl- and methylmercury were metabolized in the same manner and that the two compounds were equally toxic.

Animal studies showed that both both ethyl- and methylmercury are widely distributed in the body but there were lower levels of ethylmercury in the brain and ethylmercury was eliminated much faster than methylmercury. Studies in a small group of infants also showed that infants very rapidly excrete ethylmercury.

The question

How rapidly is mercury from vaccines eliminated? Is it eliminated in the stool or the urine?

The study

This study, included healthy newborns, infants 2 months of age, and infants 6 months of age (72/group) in Argentina. The researchers collected serum, urine and stool samples before and at timed intervals after routine immunizations with vaccines containing differing—but known—amounts of thimerosal. Each child had two sets of samples collected. Pre and post vaccine blood samples were available from 40 newborns, 50 2-month old infants, and 38 6-month old infants.

Samples were assayed for total and inorganic forms of mercury; organic forms were calculated by subtracting the one from the other. Vaccines and a subgroup of the blood samples were tested to determine how much of the organic mercury was ethylmercury (from the vaccine) and how much was methylmercury (from the mother or from food). The researchers who tested the samples were “blinded” (that is, they did not know which samples they were testing).

The findings

Regardless of the age group, blood mercury levels were relatively low, the highest samples being at 12-24 hours after receipt of vaccine. Mercury was detected in almost all stool samples and increased substantially after immunization; all of the mercury in stool was inorganic mercury. There was almost no mercury in urine.

The half-life (rate of elimination) of mercury from the blood was estimated to be 3.7 days. Pre-vaccine blood levels in 2-month and 6-month old infants did not differ.

All the mercury in the vaccines was ethylmercury. Of the blood samples they tested, between 1 and 50% of the organic mercury was methylmercury.

The relevance/bottom line

Ethylmercury from thimerosal-containing vaccines is eliminated much more rapidly than methylmercury and appears to be eliminated in the stool as inorganic mercury.

NNii’s comment

Ethylmercury is much more rapidly excreted than methylmercury. Based upon the prevaccination blood levels in 6-months old infants, ethylmercury does not appear to accumulate after routine immunizations. Because samples also contained methylmercury, the measured blood levels may have overestimated the amount of ethylmercury present—but the levels were low in any case. Because 24-hour collections of stool were not tested, how much and how fast all of the administered ethylmercury is eliminated can not yet be estimated; it appears likely that most of the dose is rapidly excreted, however.

Because injected ethylmercury is much more rapidly excreted than methylmercury, exposure guidelines developed for the oral consumption of methylmercury were and are not accurate for the assessment of risk for infants who receive thimerosal-containing vaccines.