What Is Thimerosal? Why Most Vaccines Don’t Contain Mercury Anymore
This week, the Consultative Committee for Centers for Disease Control and Prevention on the vaccination practices of Centers for Disease and Prevention meets to re -examine and vote on several vaccine recommendations. Just last week, however, the secretary of health and social services Robert F. Kennedy, Jr., added additional elements on the agenda, including a discussion on Thimérosal – a compound containing mercury used in certain vaccines. Thimérosal has already been withdrawn from all childhood vaccines and detailed research has shown that it does not cause neurodevelopmental disorders. Mercury in Thimérosal is quickly and easily eliminated by the body. Here’s how we know and why we always use the compound in certain adult vaccines.
What is Thimérosal and why was it used in vaccines?
Thimérosal is a preservative who was first added to the manufacture of vaccines in the 1930s. Because it is a very effective antiseptic, it can prevent the introduction of fungi or bacteria which could be harmful to inject. In weight, around 50% of thimerosal is ethylmercure, a compound that contains mercury. It seems scary for some people because it is well understood that mercury can be toxic to the brain. Many people are aware, for example, that eating too much tuna can be dangerous due to the amount of mercury that fish can contain.
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“I think everyone knows the concept that Mercury is toxic,” explains Ryan Marino, medical toxicologist in Cleveland university hospitals. “What is not transmitted is that there are several different forms of mercury with very different toxicities.”
Mercury is omnipresent in our environment, known as Marino, and it comes from natural and human sources. Volcanoes, forest fires and rocky alteration all release Mercury in the air, but the vast majority of the element comes from mining, coal combustion and other fossil fuels and industrial waste.
How does ethylmercure differ from elementary mercury and methylmercide?
Microorganisms converted inorganic mercury into the conspiracy methylmercury environment, which aquatic creatures consume inadvertently. Methylmercure accumulates the food chain, so that the predators of apex such as sharks, tuna and swordfish have the highest concentrations. This is why the Food and Drug Administration and the Environmental Protection Agency recommend weekly limits to the consumption of certain fish, in particular these fish consumed by children and the speakers.
Mercury in Thimérosal, however, is ethylmercure, and this missing letter makes a big difference.
The two molecules include the Mercury element, a metal which, in its elementary form, is silver in color, liquid at room temperature and well known for its use in ancient thermometers. And the two molecules are organic, which means that they include carbon atoms. More specifically, the chemical formula of ethylmercury is c2H5HG +, and methylmercury is ch3HG +. The different numbers of carbon and hydrogen atoms in these molecules mean that they have very different properties. To get an idea of the difference that one atom can do, consider that our bodies need – and are mainly composed of – water or h2O, but if you add another oxygen atom to this molecule, you get H2O2. The latter is hydrogen peroxide, something that we certainly should not drink.
Methylmercurus is more easily absorbed in neurological tissues and bioaccumulations, or accumulates in the body, says Marino. It can cross the blood-brain barrier, and too much can cause symptoms ranging from “forgetting, irritability and depression to dementia,” he said. The half-life of methylmercury is around 50 to 80 days, so it can stay in the body for almost four months. But ethylmercure is not absorbed as easily in tissues than methylmercury. The ethylmercury half-life is only three to seven days, so the body removes it in about a week and a half.
“Ethylmercure does not behave in the same way as methylmercury,” explains Marino. Although too much ethylmercury can also cause poisoning, “the body can erase the quantity that is very quickly.
When and why was the Thimérosal withdrawn from vaccines?
In the late 1990s, the US government took measures to reduce human exposure to Mercury, including the 1997 law on the modernization of Food and Drug Administration. This law obliged the FDA to make a list of all foods and drugs containing mercury compounds and the quantities of these compounds. At the time, three infant vaccines contained thimérosal: diphtheria, tetanus, acellular darling (DTAP), hepatitis B and Haemophilus influenzae Type B vaccines (HIB).
Although no evidence to this point suggested that the Thimérosal in the vaccines was harmful, only limited research existed at the time on the differences between ethylmercure and methylmercury. Total ethylmercure in infant vaccines to which infants could have been exposed to the fall below the safety limits recommended by the FDA for methylmercure: 0.4 microgram per kilogram of body weight per day (0.4 μg / kg / d). But they slightly exceeded the recommended limit of the EPA of 0.1 μg / kg / d. (Agencies have different limits depending on their data sources and their objective.) But again, they are limits for methylmercury.
“The medical and scientific establishment thought that ethylmercury behaved identically to methylmercury, and this has now been deeply refuted,” explains Marino.
During a meeting at the FDA in 1999, officials discussed the opportunity to withdraw the Thimérosal from vaccines, given the different security thresholds of federal agencies, explains Neal Halsey, a pediatrician of infectious diseases and professor emeritus at Johns Hopkins University, who was president of the American Academy of Pediatrics of Pediatrics (AAP) on infectious diseases at the time.
“I said that there was no way that I could approve the use of vaccines that contained more mercury than any federal agency recommended,” explains Halsey. No EPA or FDA directive existed on ethylmercure cumulative exposure, and Halsey was concerned with premature infants and mothers’ children who consumed many fish because those responsible also lacked data on a possible additive effect of ethylmercurity with the methylmercurus of the food regime. “There was no direct evidence of thimerosal damage in vaccines,” says Halsey, “but we had enough evidence to say that there was a theoretical risk.”
Thus, by abundance of prudence, in 1999, the AAP and the US Public Health Service published a joint declaration which recommended that the Thimérosal be removed from infantile vaccines. In 2001, the Thimérosal was removed from all vaccines with the exception of certain influenza vaccines in multidose bottles. These are the only vaccines that still use the curator today. “Since these bottles have several bites and could potentially be used for days for weeks, this prevents contamination,” explains Marino. Other preservatives include their own risks, and multidose bottle flu vaccines are mainly administered to adults in the United States during the 2024-25, 96% of all American flu vaccines and 98% of those in the Vaccines for Children program were without Thimérosish.
Other vaccines may contain traces of Thimérosal in the manufacturing process but not more than one microgram per dose, well below the limits considered as safe by the EPA and the FDA.
What have we learned from the Thimérosal since then?
After the elimination of the Thimerosal of Infant Vaccines, the researchers studied and learned more about the safety of ethylmercure. More than a dozen studies have concluded that Thimérosal does not increase any neurodevelopmental risk or the risk of any other health problem. In addition, in 2004, the Institute of Medicine, an independent non -profit organization which is now called the National Academy of Medicine, published a complete review of epidemiological research on vaccines containing Thimérosal which determine that they do not cause autism.
Halsey and Marino think that the elimination of Thimérosal from infantile vaccines has been the right decision, but opinions in the medical community differ on the way in which the public perception thus affected. “We want to reduce any exposure to Mercury,” says Marino, “but I think that, in retrospect, the appeasement of people who do no proof of people may have turned because now people think that it has been withdrawn due to problems when it has never been shown to cause any problems.”
Halsey, however, says he thinks that public confidence would have dropped if the Thimérosal was not withdrawn before research was clear. “Paying attention to Thimérosal has created controversy,” he said. “But if we had not taken the action, I think it would have been much worse.”
