Thimerosal, ethylmercurithiosalicylic acid, is one of the most widely used preservatives in vaccines. Patented in 1928, thimerosal finds use as an effective antiseptic compound. 
Although it seems that controversy about thimerosal use in vaccines is a recent phenomena, an insider memo at Merck & Co shows concern among top scientists in the field as early as 1991.  The main point of contention is whether doses of thimerosal, a known neurotoxin, given in routine vaccinations are sufficient enough to cause toxicity. From 1999 onwards, the CDC and FDA contend that no convincing evidence exists that links thimerosal to autism or other major toxicities in the amounts given during routine vaccination; still it was recommended that thimerosal use in childhood vaccinations be phased out.  Today, thimerosal is still found in some vaccines, such as fluzone® (Sanofi™ flu shot). The 1991 confidential memo mentioned earlier was leaked by the LA times in 2005. Interestingly, in the memo, Dr. Hilleman, a renowned scientist, addressed the concern of dosage guidelines and safety. An except below reads:
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This highlights an interesting component of the debate that requires a background in science to fully elucidate. Its important for us to establish what thimerosal is chemically, how it relates to other mercury compounds, and finally what its safety and efficacy profile is.
Thimerosal is metabolized to ethylmercury and thiosalicylate. Ethylmercury has a long half-life of 18 days. It may be further metabolized to inorganic mercury and other metabolites.  Early concerns were based on the idea that methylmercury and ethylmercury shared a similar pharmacodynamic/pharmacokinetic and toxicology profile. Recent studies show this to be unfounded. “The data showing significant kinetic differences in tissue distribution and metabolism of mercury species challenge the assumption that ethyl mercury is toxicologically identical to methyl mercury.” Furthermore, “Ethylmercury seems to be eliminated from blood rapidly via the stools after parenteral administration of thiomersal in vaccines.”  Thus, it seems evident that thimerosal and its metabolites differ significantly from other mercury compounds and do not share identical safety profiles.
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Make no mistake; thimerosal and ethylmercury have their own set of problems. These molecules work as antimicrobial agents by increasing reactive oxygen species (ROS) and by inhibiting various enzymes. This causes cell death. It’s important to understand that this mechanism extends to our cells as well. A 2014 study by Carneiro, et al. reads, “Interestingly, after 0.5 h of TM exposure, the highest levels of both etHg and inoHg were found in kidneys (accounting for more than 70% of the total Hg in the animal body), whereas the brain contributed least to the Hg body burden (accounts for <1.0% of total body Hg).”  Surprisingly, it seems that the burden of toxicity lies not in the brain, as originally feared by some, but in the kidneys! A more recent study agrees, “Exposure to Hg0 vapor and MeHg produce symptoms in CNS, whereas, the kidney is the target organ when exposures to the mono- and di-valent salts of mercury (Hg+ and Hg++, respectively) occur.”  It states further that chronic exposure to inorganic mercury produces symptoms of stomatitis, erethism, and tremors. Moreover, recent articles linking thimerosal and autism have surfaced, this time with clout. “Evidence suggests that thimerosal induces apoptosis through a mitochondrial pathway and that it is toxic to the mitochondria, reducing mitochondrial respiration and inducing mitochondrial DNA damage and superoxide production… Our data suggest that the abnormal mitochondrial function and increased susceptibility to ethylmercury in the autistic disorder group [sic] may be related to impaired glutathione-mediated antioxidant capacity and chronic oxidative stress, since NAC pretreatment, which could improve glutathione status, appears to partially correct the atypical mitochondrial function in the autistic disorder group [sic] and protect the cells against the toxic effects of ethylmercury.” 
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In medicine, it is said that dosage is everything. This fact is vitally important for thimerosal exposure. For anyone with an impaired immunity or metabolism, suffering from any serious/chronic disease – such as Autistic Disorder with mitochondrial dysfunction, it seems thimerosal can exacerbate your condition. For the average adult person, it appears that “autophagy might play a dual role in EtHg-induced renal toxicity, being both protective following treatment with low doses of EtHg and detrimental following treatment with high doses.” Notwithstanding, our bodies are able to cope with thimerosal quite well. However, the fact that it puts any strain on our body, young or old, healthy or disabled, is rather concerning.
Despite the very real concerns about thimerosal, it’s important to recognize the need, the potential, and the success of vaccines. From Jenner to Salk, pioneering is what’s needed for future success. It’s as simple as this; the champions that are vaccines have an outdated and potentially fatal flaw, thimerosal and compounds alike. More clinical data with better efficacy and safety, transparency, accountability and accessibility on the part of vaccine manufactures/developers needs to come to fruition. I call for an outright ban on thimerosal, as seen in other countries, to be put into effect immediately. Only when we are firm in our decision, albeit backed with proper evidence and a broad prospective, can we make actual change.
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[unnumbered] Burbacher, Thomas M., Danny D. Shen, Noelle Liberato, Kimberly S. Grant, Elsa Cernichiari, and Thomas Clarkson. “Comparison of Blood and Brain Mercury Levels in Infant Monkeys Exposed to Methylmercury or Vaccines Containing Thimerosal.” Environmental Health Perspectives. National Institute of Environmental Health Sciences, 1 Aug. 2005. Web. 14 Oct. 2016.