In a Commentary in the Dec. 17, 2012 issue of the journal Pediatrics, vaccine and public health experts recommend retaining thimerosal (ethyl mercury) as a preservative in the global vaccine supply. Since the 1930s, thimerosal has been used as a preservative in multi-dose vials of vaccines to prevent contamination from bacteria and fungi, which could be introduced into those vials on repeated entry to withdraw additional doses.

As part of an international treaty to eliminate controllable mercury pollution and exposure, the United Nations Environmental Program (UNEP) is considering a recommendation to remove thimerosal from all vaccines worldwide. However, the World Health Organization (WHO) Strategic Advisory Group of Experts (SAGE) recently recommended that this part of the ban be removed from the UNEP treaty.

The Pediatrics authors strongly support the WHO recommendations, contending that many recent studies support the safety of thimerosal in vaccines and that removal of thimerosal globally has the potential to adversely affect the worldwide vaccine supply and lead to more disease and death.

The authors are Walter A. Orenstein, MD, associate director of the Emory Vaccine Center; Katherine Seib, MSPH, Rollins School of Public Health, Emory University; Jerome A. Paulson, MD, George Washington University and Children’s National Medical Center; Michael T. Brady, MD, The Ohio State University and Nationwide Children’s Hospital; and Louis Z. Cooper, MD, Columbia University.

In 1999 the American Academy of Pediatrics and the U.S. Public Health Service recommended moving toward thimerosal-free vaccines as a precautionary measure, and thimerosal has been removed from most vaccines in the U.S., with the exception of some seasonal influenza vaccines. This means most U.S. vaccines are now distributed in single-dose rather than multi-dose vials.

Thimerosal is still used as a preservative in resource-poor countries, however, which allows distribution in multi-dose vials. Removing thimerosal as a preservative would result in the use of single-dose vials, which would reduce manufacturing capacity, increase transportation and storage requirements, and result in the need for increased cold storage capacity that would be unavailable in many areas of the world, a consequence of which may be that non-thimerosal vaccines would compete for space in the cold chain.

Although there are clear neurotoxic effects from methyl mercury, dozens of research studies over the past 15 years from countries around the world have failed to yield any evidence of significant harm from ethyl mercury (thimerosal), in the quantities used to preserve vaccines, and have consistently supported the safety of thimerosal-containing vaccines, the authors note. Moreover, in 2002 the American Academy of Pediatrics retired its 1999 statement after evaluating new research studies showing the lack of adverse effects from thimerosal.

"Had the evidence that is available now been available in 1999, the policy reducing thimerosal use would likely have not been implemented," the Commentary authors write.

"Thimerosal remains an important vaccine preservative in resource-poor countries. Thimerosal allows the use of multiuse vials, which reduce vaccine cost and the demand on already constrained cold-chain systems. Even in the United States, thimerosal could be critical for dealing with emergencies and the need to rapidly increase vaccine supply and delivery, such as during a serious pandemic of influenza."

"The continued benefits of thimerosal use in vaccine manufacturing clearly outweigh any perceived risks," the authors state.