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http://www.uic.edu/pharmacy/services/di/thimerosal.htm

The presence of thimerosal in marketed vaccines has been a cause of concern for many healthcare providers. Thimerosal is an organic mercurial compound that has been used since the early 1930s as a preservative in multi-dose vials of pharmaceutical and biological products. The preservative actions of thimerosal aid in preventing contamination in multi-dose vials after opening.

Thimerosal breaks down into 2 components, sodium thiosalicylate and ethylmercury, and is eventually metabolized into inorganic mercury. It is distributed primarily in the central nervous system, kidneys, liver, and skin. Mercury crosses the blood brain barrier and the placenta; infants and the fetus are the most at risk for toxicity to occur. Mercury exposure by expecting mothers has been shown to cause neurological abnormalities. Infants exposed in utero to mercury have shown developmental delays. In addition, the possibility of a link between mercury exposure and neurological disorders such as autism and attention deficit hyperactivity disorder has been evaluated.

Concentrations of thimerosal in vaccines and immunoglobulins range between 0.005 and 0.02%, a non-toxic concentration. However, a concern exists, not from exposure to a single vaccine, but that over a relatively short time span, children can be exposed to multiple vaccinations containing thimerosal, which is 49.6% mercury by weight. This repeated exposure might put children at risk for mercury toxicity.

There currently are no guidelines that set acceptable exposure levels to ethylmercury, the main mercurial component of thimerosal. The data used to determine toxicity potential have been correlated with levels of methylmercury, NOT ethylmercury. Since there is a lack of comparative data on metabolism, elimination, toxicity, and risk to fetus between ethylmercury and methylmercury, there are still questions as to whether or not exposure to ethylmercury poses the same threat as exposure to methylmercury. The Environmental Protection Agency (EPA), the World Health Organization (WHO), the Food and Drug Administration (FDA) and other organizations have developed recommended limits on methylmercury exposure levels in the diet. Levels range from 0.7 m g/kg bodyweight per week (EPA) to 3.3 m g/kg bodyweight per week (WHO). However, these are not absolute numbers, for example, the EPA reference range has an uncertainty factor of 10-fold as a protection for a developing fetus. The levels are meant to be a way to evaluate mercury exposure, NOT levels at which toxicity will definitively occur.

Researchers have estimated that an infant who received vaccinations based on the Expanded Programme on Immunization (EPI) during the first 14 weeks of life could have a mercury exposure ranging from 34 m g (EPA) to 159 m g (WHO). If an infant received the standard bacillus Calmette-Guerin (BCG), oral poliomyelitis, diphtheria-tetanus-whole cell pertussis (DTP), hepatitis B, and haemophilus influenzae type b (Hib) vaccines, the total potential exposure could have been as high as 187.5 m g. A review of thimerosal content of vaccinations, conducted by the FDA in 1997, concluded that the possibility existed for an infant to receive enough mercury from vaccinations within the first 6 months of life to exceed the recommendations regarding methylmercury intake stated by the EPA.

Although the FDA review found no evidence of harm caused by the amount of thimerosal in vaccinations beyond minor local reactions, the American Academy of Pediatrics (AAP), the American Academy of Family Physicians (AAFP), the Advisory Committee on Immunization Practices (ACIP), and the United States Public Health Service (PHS) issued a joint statement regarding thimerosal content of vaccines in 1999. This statement established a goal of removing thimerosal from all infant vaccines as soon as possible. Since publication of this statement, much effort from pharmaceutical manufacturers has been made to remove thimerosal as a preservative from pediatric vaccines. A summary of the thimerosal content of routine pediatric vaccines is presented in table 1.

*Contains less than 1 m g thimerosal/ 0.5 mL dose.

A more detailed listing of the thimerosal content of U.S. licensed vaccines is available at: http://www.vaccinesafety.edu/thi-table.htm.

In conclusion, the thimerosal content of vaccinations has been a concern for healthcare providers with regards to the potential for neurodevelopmental adverse events in children associated with the byproduct of thimerosal – mercury. Thimerosal is metabolized to ethylmercury for which there is no definitive toxicity level defined. Currently, available toxicity data is correlated with methylmercury levels. In the past, an infant who received all recommended vaccinations within the first 6 months of life could have exceeded the recommended intake of methylmercury as established by the EPA. A joint statement issued by the AAP, ACIP, AAFP, and PHS in 1999 called for the removal of thimerosal from vaccines as soon as possible. Since the issue of this statement, many companies have reformulated their vaccines to either remove thimerosal completely or significantly reduce the thimerosal content.

1. Clements CJ, Ball LK, Ball R, Pratt RD. Thiomersal in vaccines. Is removal warranted? Drug saf. 2001;24(8):567-74.
2. Ball LK, Ball R, Pratt RD. An assessment of thimerosal use in childhood vaccines. Pediatr. 2001;197(5):1147-54.
3. van’t Veen AJ. Vaccines without thiomersal. Why so necessary, why so long in coming. Drugs. 2001;61(5):565-72.
4. Hurie MB, Saari TN, Davis JP. Impact of the joint statement by the American academy of pediatrics/US public health service on thimerosal in vaccines on hospital infant hepatitis B vaccination practices. Pediatr. 2001;107(4):755-58.
5. Schwartz BA. New formulation of Tripedia. JAMA. 2001;285(13).
6. Brayden RM, Pearson KA, Jones S, Renfrew B, Berman S. Effect of thimerosal recommendations on hospitals’ neonatal hepatitis B vaccination policies. J Pediatr. 2000;138(5):752-55.
7. Centers for Disease Control and Prevention. Thimerosal and vaccines. Available at: http://www.cdc.gov/nip/vacsafe/concerns/thimerosal/faqs-thimerosal.htm. Accessed: 5/16/2002.
8. Food and Drug Administration. Thimerosal in vaccines. Available at: http://www.fda.gov/cber/vaccine/thimerosal.htm. Accessed: 5/16/2002.
9. American Academy of Pediatrics. Thimerosal in vaccines – Joint statement of the American Academy of Pediatrics (AAP), the American Academy of Family Physicians (AAFP), the Advisory Committee on Immunization Practices (ACIP), and the United States Public Health Service (PHS). Available at: http://www.aap.org/policy/jointthim.html. Accessed: 5/16/2002.

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