14 April 2014. A chemical preservative is usually added to multi-dose vials of vaccines in order to prevent microbial growth, should the vaccine be accidentally contaminated. This may well happen with repeated puncture of multi-dose vials. More than 100 years ago, various mercury compounds were found to possess antimicrobial properties. The history of therapeutic usage of mercury is not pretty, as has been recapitulated at this site and in Chapter 4 of “Only one chance”. Nowadays, childhood vaccines in most industrialized countries are generally formulated in single-dose vials that do not require a preservative. Among childhood vaccines in the US, only certain types of influenza vaccines contain the thimerosal preservative. However, the single-dose vials are more expensive, and in most of the world the use of multi-dose vials is the only option.
Is there still reason to worry about the safety of vaccines? According to several authoritative sources, the evidence available does not support a causal link of thimerosal exposure to brain toxicity, including autism. This conclusion was most recently confirmed by the World Health Organization’s Global Advisory Committee on Vaccine Safety. Still, a small risk may potentially exist, even though not documented so far.
Where multi-vial vaccines remain in use, the cumulated dose of thimerosal from a complete schedule of childhood vaccinations may exceed the safe limit for methylmercury exposure. But thimerosal differs from methylmercury by being broken down and excreted more rapidly. To explore the possible magnitude of this risk, researchers from the US Food and Drug Administration calculated the fate of thimerosal in the body. As limited data are available from human studies, they relied on an experimental study in infant macaque monkeys. The FDA researchers considered the variability in thimerosal distribution and break-down observed among the 17 macaque monkeys, and they also included variability in infant body weight at the time of scheduled vaccinations.
The results just published appear reassuring. US infants who receive annual influenza vaccines with thimerosal will receive less than 1% of the acceptable exposure level for methylmercury. Even short-term peak mercury concentrations remain below the US Environmental Protection Agency’s limit for methylmercury. But is that the whole story?
Not quite. Thimerosal is still being used much more widely in other parts of the world. The cumulated doses are much greater than those originating from flu vaccine only. And then there is one more concern. The data from the 17 monkeys showed that blood-mercury concentrations could vary by at least two-fold. Are we sure that this variability, in addition to differences in body weight, covers the full range of susceptibility also in human infants?
One concern that the FDA researchers did not consider is genetic predisposition. Evidence is building that mutations in the catechol-O-methyltransferase (COMT) gene can make children – especially boys – much more vulnerable to brain toxicity from inorganic mercury. And other common gene variants may substantially increase the risk of brain drain from methylmercury. These issues may take years to elucidate. In the meantime, safe alternatives to thimerosal need to be pursued.