10 June 2013. Some vaccines contain a preservative, thimerosal, which is broken down into ethylmercury in the body. This additive is used to prevent microbial growth in the vaccine vials, especially those used for multiple dosages. It was recently decided to exempt thimerosal from the bans of mercury uses in the UN Minamata Treaty – but was that a wise decision?
Among major concerns, thimerosal has been linked to brain toxicity at the doses received from vaccinations. As some vaccines are given during pregnancy or at very early infancy, the exposure takes place at a very vulnerable age. But are safer substitutes available?
Thimerosal has a long history. A clinical trial was carried out as far back as 1931. From the  incomplete information that is publicly available, the substance was given intravenously to 22 meningitis patients. No assessment of brain toxicity was done, probably because it was impractical, given that the patients suffered a serious infection, and several of them died. Nonetheless, this very study is what the U.S. Food and Drug Adfministration (FDA) continues to rely on. Clinical trials have not been carried out more recently for the very reason that injection of this mercury compound would be considered unethical.
Thimerosal is used in seemingly tiny concentrations up to 0.01%. However, an 0.5 ml vaccine dose contains 25 micrograms of mercury. For comparison, the exposure limit for methylmercury set by the U.S. Environmental Protection Agency is 0.1 microgram of mercury per kilogram of body weight per day. For an infant weighing 5 kilogram, this limit would allow one-half of a microgram per day – the thimerosal dose is 50 times higher. While the vaccine additive seems less toxic than methylmercury, is the difference large enough to allow for 50-fold on a single day?
Experimental studies have only recently begun to focus on the possible effects of thimerosal on the developing brain. Of note is a just published report on lasting neurochemical effects in the hippocampus of the rat brain after prenatal thimerosal exposure at 10 microgram per kilogram body weight. Few other experimental studies focus on prenatal or neonatal exposures, but lacking information does not mean that thimerosal is safe.
When using neuropsychological tests known to be sensitive to methylmercury toxicity at school age, girls, but not boys, showed deficits at higher exposures to thimerosal from childhood vaccines. Similar tendencies have also been reported before age 3 years. Other studies reported no statistically significant effects. Although the evidence offers only limited support to a hypothesis that thimerosal causes chemical brain drain, is the safety record sufficient to support its continued use?
The history of thimerosal goes back a very long time, when mercury compounds were applied to inhibit the growth of microorganisms. Methylmercury was used to treat syphilis and other infections, but severe adverse effects were soon discovered (see Chapter 4 in ‘Only one chance’). Thimerosal was found to have much fewer side effects, and early applications included antiseptic ointments for treating cuts, nasal sprays, eye solutions, vaginal spermicides, and diaper rash treatments. These uses have now been phased out, as safer alternatives without mercury are available. For vaccines, 2-phenoxyethanol is a well established preservative, especially those aimed at immunization of infants or used during pregnancy. The FDA is currently considering the safety of methylmercury in fish. Thimerosal and its possible substitutes also need to be considered.