In a press release from Dec. 17, the American Academy of Pediatrics endorsed the World Health Organization’s recommendation that thimersol, an organomercurial compound used as a preservative, should not be removed from vaccines. This endorsement is a reversal from the AAP’s stance just over a decade ago.

In 1999, the AAP, the United States Public Health Service (USPHS) and various vaccine manufacturers actually agreed that thimerosal should be “removed as soon as possible” from vaccines due to its potential neurotoxicity. Since 2001, no new FDA-approved vaccines for children have contained thimerosal, and all vaccines for children under six years of age are thimerosal-free (or only contain trace amounts of the compound). Since the 1999 recommendation, researchers have reviewed data on the safety and efficacy of mercury compounds in vaccines, have monitored vaccine coverage and have reviewed the risks and benefits of thimerosal-containing vaccines and their alternatives. All of this because thimerosal is a mercury compound.


Mercury is a naturally occurring element, widespread in our environment. It can be found in the water, soil and air. Once in the air, mercury will settle into the land, and eventually wash into the water. Microorganisms in the water change mercury into methylmercury, an inorganic compound and neurotoxin. Methylmercury contaminates aquatic life and moves its way up the aquatic food chain. Humans are often exposed through ingestion of seafood. This form of mercury is easily absorbed but not easily eliminated from the body.

The neurotoxicity of methylmercury was first noticed in the 1950s and 60s. During that time there was significant discharge of industrial materials into Japan’s Minimata Bay, resulting in the consumption of mercury-contaminated fish. Similarly, in 1970s Iraq, a fungicide containing methlymercury was used on seed grain, which was then accidentally used in bread. Both episodes affected neurodevelopment. Maternal exposure to methylmercury was associated with disorders similar to cerebral palsy, sensory and motor neurologic dysfunction and development delays.


So, what does this have to do with thimerosal? Thimerosal has been used in vaccines since the early 1930s. A mercury-containing organic compound, thimerosal has been used to prevent microbial growth in vaccines. The compound is derived from ethyl mercury, which makes it a different chemical compound than methylmercury. According to the FDA, we are lacking in definitive comparative data between the two, and, erring on the side of caution, have considered both to have certain risks. Though, in 2004, the Institute of Medicine issued its safety review of thimerosal-containing vaccines. The report rejected that thimerosal-containing vaccines are causative of disorders such as autism, and further stated that the rejection of vaccines could lead to increases in infectious diseases such as whopping cough, measles, and bacterial meningitis. The CDC provided similar findings with a study of its own in 2010. The CDC also points out that there is no convincing evidence of harm caused by thimerosal in vaccines, and that since 2001, when thimerosal was no longer in vaccines, autism has actually gone up in the United States.

Weighing the Risks

In its Weekly Epidemiological Record, published on May 25, the Strategic Advisory Group of Experts (SAGE) on immunization recommended that thimerosal use continue. The group reaffirmed:

“thiomersal-containing vaccines were safe, essential and irreplaceable components of immunization programmes, especially in developing countries, and that removal of these products would disproportionately jeopardize the health and lives of the most disadvantaged children worldwide.”

Thimerosal plays an integral role in keeping multi-dose vaccine vials safe for use. As SAGE points out, removing thimerosal, and thus multi-dose vials, could have an enormous negative impact on routine immunizations. Multi-dose vials are cheaper per dose and occupy less cold chain capacity, making them popular in low-resource areas.

In making these recommendations, researchers and policy-makers had to weigh serious risks: the known negative impacts of more rampant infectious disease, or the uncertain negative impacts of a mercury compound. Though perhaps controversial, as Reuters suggests, the result is that the risk of infectious disease outweighs the yet to be fully understood risks of thimerosal. 

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