Chloe Ramirez
Thimerosal, a mercury-based preservative, has been used for decades in the United States in multi-dose vials of vaccines to prevent the growth of harmful microbes inadvertently introduced into the vaccine during use. However, due to concerns over its safety, the use of thimerosal in vaccines has significantly declined. In 2025, the US Department of Health and Human Services recommended removing thimerosal from all influenza vaccines distributed in the United States, and as of the 2024-2025 flu season, approximately 91% of the vaccine supply was preservative-free. While the CDC's ACIP does not preferentially recommend vaccines without thimerosal, it is important to note that all vaccines routinely recommended for children 6 years of age and younger in the US are available without this preservative.
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What You'll Learn
- Thimerosal is a mercury-based preservative used in vaccines to prevent bacterial contamination
- Thimerosal has been largely phased out of vaccines for children in the US, but some flu vaccines still contain it
- There is no evidence that thimerosal causes autism or other developmental issues, despite some concerns
- Preservative-free flu vaccines are available and contain the same strains of the influenza virus as other shots
- The CDC does not preferentially recommend preservative-free vaccines for any populations
Thimerosal is a mercury-based preservative used in vaccines to prevent bacterial contamination
Thimerosal is a mercury-based preservative that has been used in vaccines since the 1930s to prevent bacterial contamination. It is added to multi-dose vials of vaccines to prevent the growth of harmful bacteria, fungi, and other microbes that may inadvertently be introduced when a syringe needle enters a vial. Thimerosal contains 49.6% mercury by weight, and in high doses, it can be toxic. However, the amount of mercury in a vaccine with Thimerosal is roughly the same as the amount of elemental mercury in a 3-ounce can of tuna fish.
Thimerosal has been the subject of controversy, with some people seeking preservative-free vaccines due to concerns about potential side effects. Claims have been made that Thimerosal causes autism, ADHD, and speech or language delays in children. However, clinical researchers have repeatedly debunked these claims, and studies by the CDC and FDA have found no association between Thimerosal exposure and neuropsychological delays. The only side effects of Thimerosal reported are minor local reactions at the injection site, such as redness and swelling.
Despite Thimerosal's proven safety and effectiveness, its use as a preservative in vaccines has declined due to the development of single-dose vaccines and the removal of Thimerosal from childhood vaccines. All vaccines routinely recommended for children 6 years of age and younger in the US are available without Thimerosal. For the 2024-2025 flu season, most of the projected vaccine supply will be Thimerosal-free or Thimerosal-reduced.
In summary, Thimerosal is a mercury-based preservative used in vaccines to prevent bacterial contamination. While it has been effective and generally safe, concerns about potential side effects have led to a reduction in its use, especially in childhood vaccines.
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Thimerosal has been largely phased out of vaccines for children in the US, but some flu vaccines still contain it
Thimerosal, also known as thiomersal, is a preservative that has been used in some drug products, including vaccines, since the 1930s. It is a mercury-based compound that prevents contamination by killing microbes and preventing their growth. In the human body, thimerosal is metabolized into ethylmercury, which is quickly eliminated from the blood.
In the United States, thimerosal has been largely phased out of vaccines for children. Since 2001, thimerosal has not been used in vaccines for children under the age of 6, and it is now available in formulations that do not contain thimerosal for children up to the age of 6. Additionally, vaccines that do not contain thimerosal as a preservative are available for adolescents and adults.
However, thimerosal is still used in some flu vaccines. It is found in multi-dose vials used for large-scale immunization efforts. The Advisory Committee on Immunization Practices (ACIP) has stated that the benefits of flu vaccination outweigh any risk from thimerosal exposure. ACIP does not recommend using the thimerosal-free flu vaccine over the thimerosal-containing flu vaccine.
Despite the proven safety of thimerosal, there have been concerns about its use in vaccines. Some people have claimed that thimerosal can cause autism, attention deficit hypersensitivity disorder, or speech or language delay. However, these claims have been repeatedly debunked by clinical researchers and a robust body of peer-reviewed scientific studies. The World Health Organization affirms that there is no evidence of toxicity in infants, children, or adults exposed to thimerosal-containing vaccines. The only side effects of thimerosal in vaccines are minor reactions at the injection site, such as redness and swelling.
For the 2024-2025 flu season in the United States, most of the projected vaccine supply is expected to be thimerosal-free or thimerosal-reduced. People who prefer to avoid thimerosal can ask their healthcare provider for a preservative-free flu vaccine or the FluMist nasal spray vaccine.
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There is no evidence that thimerosal causes autism or other developmental issues, despite some concerns
Thimerosal is an antimicrobial agent used in some vaccines to prevent the growth of bacteria, fungi, and other microorganisms that may contaminate a vial when a needle is inserted. It is a mercury-based compound. Thimerosal is used in multi-dose vials of the flu shot for delivery to multiple people. This helps ensure the purity of the vaccine from one person to the next.
Despite its proven safety, many people are unconvinced that thimerosal is safe. This controversy began in 1997 when a group of scientists in Great Britain published a study suggesting that the measles, mumps, and rubella (MMR) vaccine caused children to develop autism. This notion has been repeatedly debunked by clinical researchers. Several well-conducted studies have found no evidence of harm caused by the low doses of thimerosal in vaccines. In fact, thimerosal has been removed from or reduced in all vaccines routinely recommended for children 6 years of age and under, except for the influenza vaccine. However, even after thimerosal was removed from childhood vaccines, autism rates continued to rise, which is the opposite of what would be expected if thimerosal caused autism.
Additionally, studies in Denmark and Sweden, where thimerosal was removed from vaccines in 1992, found no link between thimerosal-containing vaccines and autism. A 2010 study by the Centers for Disease Control and Prevention (CDC) also showed that prenatal and infant exposure to vaccines and immunoglobulins containing thimerosal does not increase the risk for autism spectrum disorder (ASD). Similarly, an Italian study found that immunization in infancy with thimerosal-containing vaccines does not decrease neuropsychological performance later in childhood.
While some people may have mild side effects like redness and swelling at the injection site, thimerosal is very safe and does not cause autism or other developmental issues. If you are concerned about thimerosal, you can ask your healthcare provider for a preservative-free flu vaccine or the FluMist nasal spray vaccine.
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Preservative-free flu vaccines are available and contain the same strains of the influenza virus as other shots
Flu shots contain various ingredients that work together to ensure that the vaccine is safe and effective. The specific ingredients vary slightly among vaccines, but they often include deactivated influenza viruses, chemicals that boost the body's response to the vaccine, preservatives to prevent contamination, and stabilizers. The flu vaccine contains many ingredients, all of which are safe and necessary to keep flu vaccines shelf-stable and effective. The viruses that cause the flu, known as influenza viruses, are constantly changing. To ensure the flu vaccine remains effective, researchers and manufacturers work together to update the vaccine every year.
Different flu vaccines have slightly different ingredients. For example, an injection usually contains tiny amounts of deactivated, and therefore not harmful, flu viruses. A nasal spray, on the other hand, contains live viruses that have been weakened and are therefore not harmful. The nasal spray flu vaccine, such as FluMist, is preservative-free, but it contains a live weakened virus and should not be used in people with compromised immune systems, children under two years old, adults over 50, pregnant people, or those with severe allergies or other medical conditions.
Flu shots, with or without preservatives, are contraindicated for use in specific situations. Unless flu vaccination is contraindicated for an individual, there is no safety-related reason to avoid getting a flu shot with preservatives. However, if someone prefers to avoid thimerosal, they can ask their healthcare provider if a preservative-free flu vaccine or the FluMist nasal spray vaccine is an option for them.
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The CDC does not preferentially recommend preservative-free vaccines for any populations
Thimerosal, a mercury-based compound, is used as a preservative in some vaccines. It is used to prevent the growth of bacteria, fungus, and other microorganisms that might contaminate a vial when a needle is inserted. Despite its proven safety record, some people seek out preservative-free vaccines because they believe that thimerosal causes autism. This claim has been repeatedly debunked by clinical researchers, and the only known side effects of thimerosal are minor reactions at the injection site, such as redness and swelling.
The Advisory Committee on Immunization Practices (ACIP) develops recommendations on how to use vaccines to control disease in the United States. These recommendations become official CDC policy once they are adopted by the CDC's Director. According to the CDC's ACIP, the committee does not preferentially recommend vaccines that do not contain thimerosal for any populations. However, ACIP has recommended that children, pregnant women, and all adults receive single-dose flu immunizations that do not contain thimerosal.
For the 2024-2025 flu season, most of the projected vaccine supply produced will be thimerosal-free or thimerosal-reduced. This is a continuation of a trend that has been ongoing for nearly two decades. The federal government is currently considering recommending that people only receive flu shots made without thimerosal. However, some health centers still use multi-dose vials because they are more practical, cheaper, and take up less space in refrigerators.
While the CDC does not preferentially recommend preservative-free vaccines for any population, individuals who wish to avoid thimerosal can ask their healthcare provider if a preservative-free flu vaccine or a FluMist nasal spray vaccine is available.
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Frequently asked questions
Yes, all vaccines routinely recommended for children 6 years of age and younger in the U.S. are available in formulations that do not contain preservatives such as thimerosal. However, thimerosal is still used in multi-dose vials of the flu vaccine for delivery to multiple people to ensure the purity of the vaccine from one person to the next.
Thimerosal is a mercury-based preservative that has been used for decades in the United States in multi-dose vials (vials containing more than one dose) of medicines and vaccines. Thimerosal has been used to prevent the growth of harmful microbes inadvertently introduced into the vaccine during use.
There is no evidence of harm caused by the low doses of thimerosal in vaccines, except for minor reactions like redness and swelling at the injection site.
