The American Academy of Pediatrics announced that it supports the World Health Organization’s stance that the mercury-based preservative thimerosal should be left in vaccines, and not subject to a ban contained in a draft treaty from the United Nations Environment Program (UNEP).

Numerous studies have failed to find evidence of any harm from thimerosal in vaccines, and banning the preservative could adversely effect worldwide vaccine supply, WHO contends.

The AAP issued a brief statement in Pediatrics endorsing the recommendations drafted by the WHO’s Strategic Advisory Group of Experts (SAGE) on immunization and thimerosal. The Pediatric Infectious Diseases Society and the International Pediatric Association also support keeping the preservative in vaccines.

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In 2009, UNEP began drafting a binding treaty to reduce the hazards of environmental mercury. A provision banning thimerosal in vaccines was included in the draft document, which will be debated and potentially finalized next month.

But WHO has requested the provision be removed. Although the organization supports efforts to reduce environmental mercury, restricting thimerosal could threaten access to certain vaccines around the world, particularly in developing countries.

“The continued benefits of thimerosal use in vaccine manufacturing clearly outweigh any perceived risks,” Walter Orenstein, MD, of Emory University in Atlanta, and colleagues wrote in an accompanying editorial.

They explained that banning thimerosal would increase manufacturing costs, reduce manufacturing capacity because of the need to switch to single-dose vials, increase waste from single-dose packaging, and strain transportation and storage space.

“Thimerosal allows the use of multiuse vials, which reduce vaccine cost and the demand on already constrained cold-chain systems,” Orenstein and colleagues wrote.

In it’s recommendations, WHO’s SAGE expressed concern that replacing thimerosal with another preservative could “affect the quality, safety, and efficacy of vaccines,” noting that re-registration would be required by the National Regulatory Authority in each jurisdiction where a reformulated product was intended to be used.

Furthermore, currently available alternative preservatives could interact in “unpredictable ways with existing vaccines, and there are no consensus alternative preservatives for the near- or mid-term.”

“There would be a high risk of serious disruption to routine immunization programs and mass immunization campaigns if thimerosal-preserved multidose vials were not available for inactivated vaccines, with a predictable and sizable increase in mortality, for exceedingly limited environmental benefit,” SAGE members wrote.

Thimerosal was first included in vaccines in the 1930s to prevent the growth of bacteria and fungi in multidose vaccine vials. But because the preservative contains mercury — in the form of organic ethyl mercury — concern arose about the potential for neurotoxic effects and an association with autism.

With growing public pressure and mounting media coverage, the AAP and the U.S. Public Health Service (USPH) called for thimerosal to be removed from all vaccines in 1999. Although thimerosal was removed from most vaccines in developed countries by 2001, it is still used in some seasonal influenza and other adult vaccines, and widely used in many developing countries.

At the time AAP and USPH recommended removing thimerosal, there were no studies that evaluated the potential risks associated with ethyl mercury exposure — as opposed to its inorganic counterpart, methyl mercury — from vaccines. Since then, numerous studies looking for harms associated with vaccines containing the preservative have failed to find such associations. However, serious neurotoxic effects have been observed with methyl mercury.

Current opposition to removing the ban proposed in UNEP’s draft treaty is based on the idea that it is unjust to allow thimerosal to be used in low- and middle-income countries when it’s use has been nearly phased out in wealthier nations.

But in an accompanying editorial, Louis Cooper, MD, of Columbia University in New York City, member of the AAP board of directors in 1999, and Samuel Katz, MD, of Duke University in Durham, N.C., former chair of the AAP’s infectious diseases committee, argue that the AAP and USPHS would have never issued a decree to remove thimerosal in the first place if data from recent studies had been available then.

In a separate commentary, Katherine King, PhD, of St. Michael’s Hospital in Toronto, and colleagues, agree that there is no potential for injustice if there are no health risks associated with thimerosal.

“[T]he real threat of injustice comes from considering the removal of this currently necessary and irreplaceable compound from the global vaccine supply, and the avoidable increases in morbidity and mortality that would inevitably result from disruptions to vaccination programs targeting already marginalized populations in low- and middle-income countries,” King and colleagues wrote.


  1. AAP. Pediatrics. 2012; doi: 10.1542/peds.2012-2262.
  2. Orenstein W et al. Pediatrics. 2012; doi:10.1542/peds.2012-1760.
  3. King K et al. Pediatrics. 2012; doi:10.1542/peds.2012-2976.