Preservative or poison? The science behind Thimerosal

By Elizabeth Ensink

In 2000, the Center for Disease Control announced that measles had been effectively eliminated from the U.S. In 2014, 644 cases of measles were reported in the U.S., including a multi-state outbreak that continues today. Epidemiological evidence points to a lack of vaccination as a major factor in the resurgence of this disease. The debate about vaccinations has a long, complex history; ever since Edward Jenner created the first vaccine in 1800, people have voiced religious, political and health concerns over the practice. One prevalent modern fear is that vaccination causes neurodevelopmental disorders such as autism – a fear started by a study in 1998 by Andrew Wakefield that was eventually retracted. Many studies in the past decade have confirmed the safety of vaccines, including a close examination of thimerosal, the mercury-containing preservative that has raised concerns in the past. Although it is just one piece of the issue, taking a closer look at the science behind thimerosal and its limited use in vaccines today can provide a clearer picture of vaccine safety.

Syringe and vialsThimerosal as a preservative
Thimerosal is a mercury-containing preservative formerly used in many vaccines. Some vaccines, such as the annual influenza vaccine, are produced and shipped in multi-dose form in order to provide adequate supplies across the country. Each time a new needle is inserted into a multi-dose vaccine, there is an opportunity for pathogens such as bacteria or fungus to contaminate the vaccine. Since the vaccine fluid is injected directly into the patient, preservatives are crucial to prevent contamination that could cause life threatening infections. Thimerosal is effective at killing bacteria and preventing fungal growth even at low concentrations and was the preservative of choice from about 1930 to 2001.

Methyl versus ethyl mercury
If you’ve been paying attention to recent health concerns about vaccination, you’ve likely heard thimerosal described as a dangerous neurotoxin instead of a necessary preservative. Thimerosal contains mercury, a substance known to cause severe neurological damage, especially in developing children. However, mercury only causes these toxic effects when it is in certain forms.

Most mercury on earth is found in water, soil and plants in the form of inorganic salts. When mercury is converted to an organic form by biological processes in the environment, it can accumulate in biological tissues, causing severe damage to the central nervous system. First recognized in the famous case of Minimata Bay, methylmercury poisoning can cause problems with muscle control, vision, hearing and, in severe cases, coma and death. In biological tissue, methylmercury readily forms a compound with the amino acid cysteine. By mimicking an amino acid, methylmercury can pass through the blood brain barrier and wreak havoc on nerve cells. Methyl mercury can also build up by bioaccumulation in fish which is why the FDA cautions against consuming large amounts of seafood while pregnant. Methylmercury is clearly dangerous, and it can take a year for the body to remove significant amounts of this toxic substance.

In contrast, when thimerosal enters the body, it breaks down into the organic molecules thiosalicylate and ethylmercury which are quickly excreted without harm. Though the names may sound similar, ethylmercury has one more carbon than methylmercury, giving it different properties. The exact behavior of ethylmercury in the body is still being studied. When researchers have examined infants receiving routine vaccinations, they found mercury levels in stool and blood to be well below levels considered safe. Because of its molecular structure, ethylmercury can be efficiently metabolized and excreted from the body, making it significantly less toxic than methylmercury.

Current use of thimerosal in vaccines
In addition to being different than methylmercury in its biological behavior, thimerosal has not been used in the majority of vaccinations since 2001. In response to growing concern over mercury toxicity, the Institute of Medicine formed a committee to perform an extensive review of the safety of thimerosal. After an initial report in 2001, the committee issued their final results in 2004, determining that there was not a causal link between thimerosal exposure from vaccines and neurodevelopmental disorders such as autism. They did, however, advise that thimerosal be removed from vaccines as a precaution “to reduce mercury exposure of infants and children as much as possible.”

Today, thimerosal is only used as a preservative in a few types of vaccines in very low concentrations. The most common of these is the influenza vaccine, but single-dose influenza vaccines that are thimerosal-free are becoming increasingly available. It is also important to note that thimerosal has never been used in the MMR vaccine, the vaccine that was first indicted in the alleged link to autism. The scientific facts clearly demonstrate that thimerosal is not a dangerous toxin lurking in our vaccines but is instead a useful preservative that has been removed to prevent even the slight possibility of harm. Although it doesn’t answer every concern over vaccination, understanding thimerosal certainly can help us embrace the benefits of vaccination in preventing the very real and present risk of diseases such as measles in the U.S.

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