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Thiomersal (C9H9HgNaO2S) in Vaccines

James Odell, OMD, ND, L.Ac.


Thimerosal (or Thiomersal) is a trade name for an organic mercurial compound (sodium ethyl-mercury (Hg) thiosalicylate) that is 49.55% mercury (Hg) by weight, which rapidly decomposes in aqueous saline solutions into ethyl-Hg hydroxide and ethyl-Hg chloride. It has been widely used since the 1930s as a preservative in certain vaccines cosmetics, tattoo inks, eye drops and contact lens solutions as well as a disinfectant (e.g. Merthiolate). Because of its application as a vaccine preservative, almost every human and animal (domestic and farmed) that has received Thimerosal-containing vaccines have been exposed to ethyl-mercury. Concerns have been voiced about its use in vaccines because ethyl-mercury is highly toxic to human cells and has been shown to be nephrotoxic (kidney) and neurotoxic (Geier, D 2015; Clarkson, T 1997). There are over 165 studies that have focused on Thimerosal used as a preservative in many childhood vaccines, and found it to be harmful.

Since there is insufficient data available regarding the toxic profile of ethylmercury, the FDA guidelines for methylmercury (MeHg) have been used to predict the potential toxicity of Thimerosal. This assumes that the toxicokinetic profile of the two organic mercury salts is similar. However, new studies prove that this is not the case. The main route of human exposure to MeHg is oral ingestion of seafood; for ethylmercury, human exposure is mainly by injection of thimerosal-containing vaccines. Compared with inorganic mercury salts, organic mercury (Thimerosal) is absorbed more effectively and thus has a greater toxic potential.


Specifically, Thimerosal is initially metabolized into ethyl-Hg compounds and thiosalicylate and rapidly binds onto thiol groups found on many proteins in human blood. It is then actively transported throughout the body and even across the blood-brain barrier into neuronal cells, where it significantly accumulates and may persist for months to years following exposure. There it damages neurons (brain cells), particularly in the dentate gyrus of the hippocampus and thalamus. Although Thimerosal-containing vaccines have been banned in several countries, it continues to be added to some vaccines in the US and many vaccines in the developing world.


Thiomersal – Not an Effective Antimicrobial


Mercury compounds have been used as disinfectants or antimicrobials since bacteriology began. For a long period of time mercurial compounds, such as mercury chloride, were thought to be useful in the killing of bacteria and other microorganisms. However, as early as 1943, it was reported that plasma preserved with 1:10,000 Thimerosal was still contaminated with viable microorganisms, and it was concluded that Thimerosal cannot be considered the ideal preservative. Morton et al. found that Thimerosal is not highly germicidal and does not possess high antimicrobial value in the presence of serum and other protein mediums. They further stated that the loss of antibacterial activity of mercurials in the presence of serum proves their incompatibility with serum. Furthermore, these investigators described that Thimerosal was 35-times more toxic to embryonic tissue cells than it was to bacteria, as well as more toxic to leukocytes (white blood cells) than bacteria (Morton, H 1948).


More recently, the effectiveness of Thimerosal as a preservative in Diphtheria–Tetanus–Pertussis (DTP) vaccine was evaluated by the US CDC. The CDC researchers reported that the choice and level of the preservative for inclusion in the DTP vaccine were limited because of possible harmful effects on the vaccine's antigenicity, plus the need to ensure the safety of the preservative. These investigators reported that Thimerosal used in the production of the DPT vaccine as an organic-Hg bacteriostatic agent was only weakly bactericidal. These investigators concluded that at currently used concentrations, Thimerosal is not an ideal preservative (Stetler, H 1985). Higher concentrations were not recommended because it might reduce vaccine potency or pose a danger to individuals receiving the vaccine.


Other investigators have observed that Thimerosal failed to meet European Pharmacopoeia antimicrobial effectiveness acceptance criteria as a preservative due to lack of growth inhibition of Thimerosal on Staphylococcus aureus in both single and multi-challenge evaluations (Khandke, L 2011). These researchers have compared and described the toxicity levels of commonly used preservatives in vaccines and found that other non-mercurial, less toxic preservatives, such as 2-Phenoxyethanol, provided a superior antimicrobial effectiveness over Thimerosal for vaccine formulations.


Despite the evidence that Thimerosal is a potent biologically toxin (Geier, D 2010), is not an effective antimicrobial, and that there are other less toxic and more effective preservatives available, Thimerosal continues to be used as a preservative in several vaccines to date throughout the world. This compound is a considerable source of mercury exposure for children (Kern, J 2011).


Current use of Thimerosal in Vaccines


In the 1990s, an increasing number of different vaccines containing Thimerosal were introduced in vaccination schedules around the world, and thus, the average cumulative exposure to Thimerosal in infants has increased in recent years. In 1997, Frank Pallone, a U.S. congressman from New Jersey attached a simple, 133-word amendment to a Food and Drug Administration (FDA) reauthorization bill. This amendment gave the FDA 2 years to “compile a list of drugs and foods that contain intentionally introduced mercury compounds and [to] provide a quantitative and qualitative analysis of the mercury compounds in the list (21 USC 397 Section 413, 1997).” The bill — the FDA Modernization Act of 1997 — was signed into law on November 21, 1997. Neither the press nor the public took notice.


Eighteen months later, in May 1999, the FDA found that by 6 months of age, infants could receive as much as 75 μg of mercury from three doses of diphtheria–tetanus–pertussis vaccine, 75 μg from three doses of the Haemophilus influenzae type b vaccine, and 37.5 μg from three doses of the hepatitis B vaccine — a total of 187.5 μg of mercury. Thus, cumulative doses of Hg exposure from Thimerosal-containing vaccines can be as high as 18