Vaccinating from multi-dose vials is cheaper than from single-dose ones. Multi-dose vials also take up less space, reducing the amount of refrigerated storage required to get them to where they are needed. They are thus particularly important for poorer countries, which do not have the money or facilities to use single-dose vials for large-scale immunization programs.
Currently 120 countries, accounting for 64 percent of global births, depend on thimerosal-containing vaccines. These prevent an estimated 1.4 million child deaths a year, according to the World Health Organization. At present there is no substitute.
Thimerosal is also added to influenza vaccines, which can be important in developed countries. The consequences of banning the compound are therefore wide-reaching and dramatic.
A number of developing countries have expressed concern over thimerosal's proposed ban. Public health experts around the world, including the WHO, have no doubt about the importance of allowing it to remain in vaccines.
So why has thimerosal been dragged into the negotiations? The debate is partly fuelled by a historic confusion between risks ascribed to methyl mercury and the ethyl mercury in thimerosal. In 1999, the American Academy of Pediatrics and the U.S. Public Health Service issued a joint statement recommending the removal of thimerosal from vaccines as a precautionary measure, following a U.S. Food and Drug Administration review.
At the time there was abundant evidence that methyl mercury was toxic, but little evidence on ethyl mercury. Additional pressure came from rumors of a link between thimerosal and autism. Since then, however, numerous studies have shown that thimerosal is harmless.
In 2006, an expert panel convened by the WHO issued a statement on thimerosal in vaccines, concluding that there was “no evidence of toxicity.” It highlighted the fact that while methyl mercury builds up in the body, ethyl mercury is excreted rapidly. The American Academy of Pediatrics has since endorsed the WHO's position.
Nonetheless, a handful of well-meaning campaigners still believe that thimerosal is harmful. Led by two groups—the Coalition for Mercury-free Drugs and SafeMinds—they have brought the thimerosal “debate” into negotiations designed to address environmental problems.
What happens next depends on the negotiators. The latest draft treaty does not specifically name thimerosal, but there is a clause that leaves the door open for additional items to be added.
There is no question that mercury is dangerous. But thimerosal is not a threat, and banning it would create far more human misery than failing to negotiate a treaty at all.