Thimerosal in Vaccines: Your Questions Answered
Key takeaways:
Thimerosal is a preservative made from mercury. It has been used in the U.S. since the 1930s.
Decades of studies have failed to find any long-term side effects from thimerosal in vaccines. Even so, the use of thimerosal in vaccines has dropped significantly over time.
Most vaccines are now thimerosal-free or have thimerosal-free options. Since 1999, all new vaccines introduced in the U.S. are made without thimerosal.
Vaccines are one of the most important advancements in preventing infectious disease. Before they were widely available, many people died from diseases that are now considered preventable. Thanks to advances in medicine and technology, we’re now able to put a stop to millions of deaths each year using vaccines.
That said, there have been plenty of vaccine myths and misconceptions over the years. And chances are, you’ve heard some about mercury in vaccines (known as thimerosal), and how it’s linked to long-term side effects. But is this actually true, and how many vaccines contain this ingredient?
Here we’ll examine thimerosal in vaccines, including what it is, if it’s safe, and how to find thimerosal-free options.
What is thimerosal?
Thimerosal is a preservative made from mercury. Mercury is a chemical element that’s found in earth’s air, crust, soil, and water. Everyone is exposed to mercury over the course of their lives. But there are several forms of mercury, each with varying levels of toxicity.
Elemental (metallic) mercury: If you’ve ever broken a thermometer and saw silver liquid spill out — that’s elemental mercury. You can also find it in certain dental fillings, fluorescent lighting, and other products. Most exposure to elemental mercury happens when it’s released from its container. But certain industries have reduced or phased out its use.
Methylmercury: When you hear about mercury exposure from eating fish — that’s methylmercury. This is the most common type of mercury people are exposed to, and it can be toxic if you’re exposed to a lot of it.
Ethylmercury: Thimerosal contains ethylmercury — not methylmercury. Your body can get rid of ethylmercury quickly. This prevents it from building up to harmful levels, making it less toxic than methylmercury.
In the U.S, thimerosal has been used as a preservative in multidose vaccines and medication vials. Multi-dose vials are those containing more than one dose. Preservatives are needed in this type of vial to prevent the growth of germs that could make you sick. That’s because the contents in the vial could become contaminated as doses are drawn out of it.
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But exactly how much thimerosal is in a vaccine? One dose of a multi-dose flu shot, for example, contains roughly the same amount of mercury as a 3-oz can of tuna. Again, this type of mercury is different from what’s found in fish.
Is thimerosal safe?
Short answer: Thimerosal is generally considered safe for use as a preservative.
There’s a long history of safety information on thimerosal dating back to the 1930s. And over the last 20 years, numerous studies have failed to show evidence of harm or long-term effects. Even so, thimerosal isn’t commonly used as a preservative in vaccines nowadays. We’ll discuss that more later.
Are there any long-term side effects?
To date, no studies have found evidence of long-term thimerosal side effects, including neurodevelopmental problems, like autism.
The only side effects linked to thimerosal in vaccines are injection site reactions, like swelling or redness, and allergic reactions.
Can you be allergic to thimerosal?
Yes, it’s possible to be allergic to thimerosal. But according to the CDC, thimerosal allergies are rare. Symptoms of an allergic reaction can be mild, like a rash. But they can also be life-threatening, including anaphylaxis.
An allergic reaction can happen shortly after receiving a vaccine. But it can also happen after you’ve left the vaccination site. Call 911 or get immediate medical help if you start experiencing shortness of breath or swelling of your face, lips, or throat.
What vaccines contain thimerosal?
Beginning in the 1930s, thimerosal was widely used as a preservative in the U.S. But is it still used in vaccines today?
As it turns out, the use of thimerosal in vaccines has dropped significantly. And several vaccines you may have received during childhood never contained it in the first place.
Currently, none of the childhood vaccines (except certain flu shots) contain thimerosal. And vaccines for adolescents and adults are available with thimerosal-free options.
Thanks to changes and new developments in vaccine manufacturing, thimerosal often isn’t needed. Many vaccines are now available in single-dose forms, like single-dose syringes, which don’t need a preservative. So, single-dose vaccines don’t contain thimerosal.
What are the options for thimerosal-free vaccines?
Many vaccines in the U.S. are now thimerosal-free or have thimerosal-free options.
Thimerosal was removed from childhood vaccines in 2001. Today, all recommended vaccines for children ages 6 years and younger are available with thimerosal-free options.
As mentioned above, some vaccines never contained thimerosal. These include:
Measles, mumps, and rubella (MMR)
Varicella (chickenpox)
Inactivated polio vaccine (IPV or IPOL)
Since 1999, all new vaccines introduced in the U.S. are free from thimerosal. For example, this means you won’t find thimerosal in any COVID-19 vaccine.
Thimerosal may still be found in multi-dose vials of seasonal influenza (flu) vaccines. But flu vaccines are also available as preservative-free single-dose syringes. For more information, talk to your healthcare provider or pharmacist about your thimerosal-free vaccine options.
Does the tetanus vaccine contain thimerosal?
One version of the tetanus vaccine does contain thimerosal. But it’s not because thimerosal is used as a preservative in the vaccine.
One manufacturer uses thimerosal in the manufacturing process of the single-dose Tetanus and Diphtheria Toxoids Adsorbed (Td) vaccine. A trace (very small) amount is still found in the final product. But this isn’t the only tetanus vaccine option that’s available. Tenivac is a brand-name version of the Td vaccine that doesn’t use or contain thimerosal.
The bottom line
Thimerosal has been used as a preservative for decades. To date, there’s no evidence that it causes long-term side effects when used in vaccines. Even so, most vaccines available today don’t contain any thimerosal.
Talk with your healthcare provider to learn more about which vaccinations may be appropriate for you. If you prefer, they can recommend formulations that don’t contain thimerosal.
Why trust our experts?
References
Centers for Disease Control and Prevention. (2001). Achievements in public health, 1900-1999 impact of vaccines universally recommended for children -- United States, 1990-1998. Morbidity and Mortality Weekly Report.
Centers for Disease Control and Prevention. (2020). About diphtheria, tetanus, and pertussis vaccination.
Centers for Disease Control and Prevention. (2020). Thimerosal and vaccines.
Centers for Disease Control and Prevention. (2021). Appendix B for the Pink Book-vaccine excipient summary.
Centers for Disease Control and Prevention. (2022). Metallic mercury - toxFAQs.
Food and Drug Administration. (2018). Thimerosal and vaccines.
Food and Drug Administration. (2018). Thimerosal in vaccines questions and answers.
United States Environmental Protection Agency. (2022). How people are exposed to mercury.
World Health Organization. (2019). Immunization.
