This week the Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices is meeting to review and vote on several vaccine recommendations. Just in the past week, however, Secretary of Health and Human Services Robert F. Kennedy, Jr., added additional items to the agenda, including a discussion of thimerosal—a mercury-containing compound that is used in some vaccines. Thimerosal has already been removed from all childhood vaccines and detailed research has shown it does not cause neurodevelopmental disorders. The mercury in thimerosal is quickly and easily cleared by the body. Here’s how we know and why we still use the compound in some adult vaccines.
What is thimerosal, and why has it been used in vaccines?
Thimerosal is a preservative that was first added to the manufacturing of vaccines in the 1930s. Because it is a highly effective antiseptic, it can prevent the introduction of fungi or bacteria that could be harmful to inject. By weight, about 50 percent of thimerosal is ethylmercury, a compound that contains mercury. That sounds scary to some people because it’s well understood that mercury can be toxic to the brain. Many people are aware, for example, that eating too much tuna can be unsafe because of how much mercury the fish can contain.
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“I think everyone is pretty familiar with the concept that mercury is toxic,” says Ryan Marino, a medical toxicologist at University Hospitals in Cleveland. “The thing that is not conveyed is that there are multiple different forms of mercury with very different toxicities.”
Mercury is ubiquitous in our environment, Marino says, and it arises from both natural and human sources. Volcanoes, forest fires and rock weathering all release mercury into the air, but the vast majority of the element comes from mining, the burning of coal and other fossil fuels and industrial waste.
How does ethylmercury differ from elemental mercury and methylmercury?
Microorganisms convert inorganic mercury in the environment into the compound methylmercury, which aquatic creatures inadvertently consume. Methylmercury accumulates up the food chain, so apex predators such as sharks, tuna and swordfish have the highest concentrations. That’s why the Food and Drug Administration and the Environmental Protection Agency recommend weekly limits to the consumption of certain fish, particularly such fish consumed by children and pregnant people.
The mercury in thimerosal, however, is ethylmercury, and that one missing letter makes a big difference.
Both molecules include the element mercury, a metal that, in its elemental form, is silver-colored, liquid at room temperature and well known for its use in old thermometers. And both molecules are organic, which means they include carbon atoms. Specifically, ethylmercury’s chemical formula is C2H5Hg+, and methylmercury’s is CH3Hg+. The different numbers of carbon and hydrogen atoms in those molecules mean they have very different properties. To get a sense of the difference a single atom can make, consider that our bodies need—and are mostly made up of—water, or H2O, but if you add another oxygen atom to that molecule, you get H2O2. The latter is hydrogen peroxide, something we definitely should not drink.
Methylmercury is more easily absorbed into neurological tissues and bioaccumulates, or builds up in the body, Marino says. It can cross the blood-brain barrier, and too much of it can result in symptoms ranging from “forgetfulness, irritability and depression all the way to dementia,” he says. The half-life of methylmercury is about 50 to 80 days, so it can remain in the body for nearly four months. But ethylmercury is not absorbed as readily into tissues as methylmercury is. Ethylmercury’s half-life is just three to seven days, so the body removes it within about a week and a half.
“Ethylmercury does not behave in the same way as methylmercury,” Marino says. While too much ethylmercury can also cause poisoning, “the body can clear the amount that’s in vaccines very rapidly. You would have to get hundreds, if not thousands, of vaccines at once” to cause any problems, he says.
When and why was thimerosal removed from vaccines?
In the late 1990s the U.S. government took measures to reduce human exposure to mercury, including the 1997 Food and Drug Administration Modernization Act. This law required the FDA to make a list of all foods and drugs containing mercury compounds and the amounts of those compounds. At the time, three childhood vaccines contained thimerosal: the diphtheria, tetanus, acellular pertussis (DTaP), hepatitis B and Haemophilus influenzae type b (Hib) vaccines.
Although no evidence up to that point had suggested the thimerosal in vaccines was harmful, only limited research existed at the time on the differences between ethylmercury and methylmercury. The total ethylmercury in childhood vaccines that infants could have been exposed to fell below the FDA’s recommended safety limits for methylmercury: 0.4 microgram per kilogram of body weight per day (0.4 μg/kg/d). But they slightly exceeded the EPA’s recommended limit of 0.1 μg/kg/d. (Agencies have different limits depending on their data sources and purpose.) But again, those were limits for methylmercury.
“The medical and scientific establishment used to think that ethylmercury behaved identically to methylmercury, and that’s now been soundly disproven,” Marino says.
During a meeting at the FDA in 1999, officials discussed whether to remove thimerosal from vaccines, given the federal agencies’ different safety thresholds, explains Neal Halsey, an infectious disease pediatrician and professor emeritus at Johns Hopkins University, who was chair of the American Academy of Pediatrics’ (AAP’s) committee on infectious disease at the time.
“I said there was no way I could endorse the use of vaccines that contained more mercury than any federal agency recommended,” Halsey says. No EPA or FDA guidelines existed on cumulative exposure to ethylmercury, and Halsey was concerned about premature infants and children of mothers who consumed a lot of fish because officials also lacked data on any possible additive effect of ethylmercury with methylmercury from diet. “There was no direct evidence of harm from thimerosal in vaccines,” Halsey says, “but we had enough evidence to say there was a theoretical risk.”
So, out of an abundance of caution, in 1999 the AAP and the U.S. Public Health Service issued a joint statement that recommended that thimerosal be removed from childhood vaccines. By 2001 thimerosal had been taken out of all vaccines except some influenza vaccines in multidose vials. These are the only vaccines that still use the preservative today. “Since those vials have multiple punctures and could potentially be used for days to weeks, it prevents contamination,” Marino says. Other preservatives come with their own risks, and flu shots from multidose vials are primarily administered to adults in the U.S. In the 2024-25 flu season, 96 percent of all U.S. flu vaccines and 98 percent of those in the Vaccines for Children program were thimerosal-free.
Other vaccines may contain trace amounts of thimerosal from the manufacturing process but no more than one microgram per dose, well below the limits considered safe by both the EPA and the FDA.
What have we learned about thimerosal since then?
Following the removal of thimerosal from childhood vaccines, researchers studied and learned more about the safety of ethylmercury. More than a dozen studies have concluded that thimerosal does not increase any neurodevelopmental risk or the risk of any other health concerns. Furthermore, in 2004 the Institute of Medicine, an independent nonprofit organization that is now called the National Academy of Medicine, published a comprehensive review of epidemiological research on thimerosal-containing vaccines that determined that they do not cause autism.
Both Halsey and Marino think that removing thimerosal from childhood vaccines was the right decision, but opinions in the medical community differ on how doing so affected public perception. “We want to reduce any exposure to mercury,” Marino says, “but I think, in retrospect, appeasing people’s fearmongering without any evidence may have backfired because now people think it was taken out because of problems when it was never shown to cause any issues.”
Halsey, however, says he believes public confidence would have plummeted if thimerosal was not removed before the research was clear. “The attention to thimerosal created a controversy,” he says. “But if we hadn’t taken the action, I think it would have been much worse.”