Logan Reed
Thimerosal, also known as thiomersal, is an organic compound containing approximately 50% mercury by weight. It has been used as a preservative in vaccines to prevent bacterial and fungal contamination. In 1999, the U.S. Public Health Service, the American Academy of Pediatrics, and vaccine manufacturers agreed to reduce or eliminate thimerosal from vaccines as a precautionary measure. This decision was based on concerns about potential risks associated with mercury exposure, especially in children. Since then, vaccines routinely recommended for children in the U.S. have been available in formulations without thimerosal, and efforts have been made to remove mercury from all vaccines. While some studies have linked thimerosal to autism and neurodevelopmental disorders, the scientific consensus is that thimerosal does not cause autism or neuropsychological delays.
| Characteristics | Values |
|---|---|
| Thimerosal | A mercury-based preservative used in vaccines to prevent bacterial and fungal contamination |
| Thimerosal composition | Approximately 50% mercury by weight |
| Thimerosal in children's vaccines | All vaccines routinely recommended for children 6 years of age and younger in the U.S. are available without thimerosal. However, some types of flu shots may contain thimerosal. |
| Thimerosal safety | The FDA, CDC, and other scientific and medical bodies have found no association between thimerosal exposure and neuropsychological delays or autism. |
| Thimerosal removal | In 1999, the CDC, American Academy of Pediatrics, and vaccine manufacturers agreed to reduce or eliminate thimerosal in vaccines as a precautionary measure. In 2025, the U.S. Health and Human Services Secretary adopted a recommendation to remove thimerosal from all influenza vaccines distributed in the U.S. |
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What You'll Learn
- Thimerosal/Thiomersal is an organic compound that contains mercury
- Thimerosal has been used as a preservative in vaccines to prevent microbial growth
- In 1999, the FDA requested plans to remove thimerosal from US-licensed vaccines
- Thimerosal was removed from all US influenza vaccines in 2025
- Studies have found no link between thimerosal exposure and autism
Thimerosal/Thiomersal is an organic compound that contains mercury
Thimerosal, also known as thiomersal, is a mercury-containing organic compound (an organomercurial) that has been used as a preservative in a number of biological and drug products, including many vaccines, since the 1930s. It is added to vials of vaccine that contain more than one dose (multi-dose vials) to prevent the growth of harmful microbes inadvertently introduced into the vaccine during use. This helps to prevent contamination by germs, which could cause severe local reactions, serious illness, or even death. Thimerosal contains ethylmercury, which is different from methylmercury, the type of mercury found in certain kinds of fish that can be toxic to people at high exposure levels.
Ethylmercury is cleared from the human body more quickly than methylmercury, making it less likely to cause harm. Studies have shown that ethylmercury does not accumulate in the blood and is eliminated from the body through the gastrointestinal system. While thimerosal is approximately 50% mercury by weight, the low doses used in vaccines have not been found to cause harm, except for minor reactions like redness and swelling at the injection site.
In the United States, concerns about pediatric exposure to mercury through vaccine administration have led to efforts to reduce or eliminate thimerosal from vaccines. In 1999, the Public Health Service, the FDA, and other health organizations agreed that thimerosal should be reduced or eliminated from vaccines as a precautionary measure. As a result, vaccines recommended for children under 6 years of age in the US no longer contain thimerosal as a preservative or contain only trace amounts, with the exception of certain inactivated influenza vaccines.
While some people have linked thimerosal-containing vaccines to autism, stating that increased exposure to thimerosal could explain the higher prevalence of autism in recent years, multiple studies have found no association between thimerosal exposure and neuropsychological delays or an increased risk for autism spectrum disorder (ASD). The benefits of flu vaccination, for example, are stated to outweigh any risk from thimerosal exposure.
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Thimerosal has been used as a preservative in vaccines to prevent microbial growth
Thimerosal is a mercury-based preservative that has been used for decades in the United States in multi-dose vials of vaccines and medicines. It is added to vials of vaccines that contain more than one dose to prevent the growth of harmful microbes inadvertently introduced into the vaccine during use. Thimerosal is approximately 50% mercury by weight and has been widely used as a preservative in biological and drug products since the 1930s.
Thimerosal has been used to prevent microbial growth, including bacteria and fungi, which can contaminate vaccines and cause severe local reactions, serious illness, or even death. Its use as a preservative in vaccines has declined due to the development of vaccines supplied in single-dose presentations that do not require preservatives. Vaccines that do not contain thimerosal as a preservative are available for adolescents and adults.
In 1999, the Public Health Service, the FDA, and other health organizations, along with the American Academy of Pediatrics (AAP) and the American Academy of Family Physicians (AAFP), concluded that it was prudent to reduce childhood exposure to mercury from all sources, including vaccines. Thimerosal was removed from childhood vaccines in the United States in 2001, except for certain influenza (flu) vaccines.
Studies have assessed the possible association between ethyl mercury exposure from thimerosal-containing vaccines and neuropsychological functioning, but research shows no association between thimerosal exposure and neuropsychological delays. The Immunization Safety Review Committee of the Institute of Medicine rejected the hypothesis of a causal relationship between thimerosal-containing vaccines and autism.
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In 1999, the FDA requested plans to remove thimerosal from US-licensed vaccines
Thimerosal, also known as thiomersal, is approximately 50% mercury (Hg) by weight. It is a mercury-based preservative that has been used for decades in the United States in multi-dose vials of medicines and vaccines. It is added to some vaccines to prevent germs (like bacteria and fungi) from growing in them.
In July 1999, the American Academy of Pediatrics (AAP) and the US Public Health Service (PHS) issued a joint statement recommending the removal of thimerosal from vaccines as soon as possible. The statement also recommended a temporary suspension of the birth dose of the hepatitis B vaccine for children born to low-risk mothers until a thimerosal-free alternative was available. By August 1999, at least one formulation of each vaccine on the recommended childhood immunization schedule for children aged six years or younger was available without thimerosal.
Since 1999, the use of mercury-containing preservatives has declined due to the development of new products formulated into single-dose presentations that do not require preservatives. All vaccines routinely recommended for children 6 years of age and younger in the US are now available in formulations that do not contain thimerosal.
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Thimerosal was removed from all US influenza vaccines in 2025
Thimerosal, a mercury-based preservative, has been used for decades in multi-dose vials of medicines and vaccines in the United States. In 1999, the Public Health Service, the FDA, the NIH, the CDC, the HRSA, the American Academy of Pediatrics, and the American Academy of Family Physicians agreed that thimerosal should be reduced or eliminated from vaccines as a precautionary measure. This was due to concerns about childhood exposure to mercury, even though there was no clear evidence of harm caused by the low doses of thimerosal in vaccines.
In 2025, the US Department of Health and Human Services (HHS) announced that it was recommending the removal of thimerosal from all influenza vaccines distributed in the country. This recommendation was made by the Centers for Disease Control and Prevention's Advisory Committee on Immunization Practices (ACIP), which is composed of leading medical and public health experts. The ACIP voted 5-1, with one abstention, to recommend that only single-dose flu vaccines be administered to children, pregnant women, and adults.
HHS Secretary Robert F. Kennedy Jr. signed the recommendation, formally adopting it into federal health policy. This decision fulfilled a long-standing pledge to remove mercury from vaccines and aligned US policy with that of Europe. Vaccine manufacturers confirmed they could replace multi-dose vials with mercury-free alternatives, ensuring uninterrupted supply for the Vaccines for Children (VFC) program and adult vaccine supplies.
The removal of thimerosal from all US influenza vaccines in 2025 marked a significant step toward enhancing public confidence in vaccines and advancing safer immunization practices.
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Studies have found no link between thimerosal exposure and autism
Thimerosal is a mercury-based preservative that has been used for decades in the United States in multi-dose vials of medicines and vaccines. It is a different kind of mercury that does not stay in the body and is unlikely to cause sickness. While some studies have suggested a link between thimerosal-containing vaccines and autism, extensive research has proven that thimerosal is a safe ingredient in vaccines and does not cause neurological problems or autism.
The CDC, FDA, and many academic institutions have reviewed the safety of thimerosal-containing vaccines in children. In 2004, the Institute of Medicine reviewed over 200 scientific studies and concluded that there was no association between thimerosal-containing vaccines and autism. The CDC has also issued statements on autism and thimerosal, noting that evidence from several studies does not support an association between increased exposure to thimerosal and the higher prevalence of autism.
Additionally, the FDA has concluded that the evidence does not support an association between thimerosal-containing vaccines and autism. The ACIP, a part of the CDC, has stated that the benefits of flu vaccination outweigh any risk from thimerosal exposure.
Several studies have specifically assessed the association between thimerosal exposure and neuropsychological functioning, including objective measurements. These studies have found no association between thimerosal exposure and neuropsychological delays. For example, a CDC study found no association between prenatal and infant exposure to thimerosal-containing vaccines and an increased risk for autism spectrum disorder (ASD). Similarly, an Italian study found that immunization in infancy with thimerosal-containing vaccines did not decrease neuropsychological performance later in childhood. Another CDC study found no association between early exposure to thimerosal in vaccines and neuropsychological problems in children between the ages of 7 and 10 years.
In summary, while there have been concerns about the potential link between thimerosal-containing vaccines and autism, extensive research has found no evidence to support this association. The CDC, FDA, and other reputable organizations have concluded that thimerosal is safe and does not cause autism or other neurological problems.
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Frequently asked questions
Thimerosal, a mercury-based preservative, has been used for decades in the United States in multi-dose vials of vaccines. However, since 1999, there has been a push to reduce or eliminate its use in vaccines as a precautionary measure. As of 2025, the US Health and Human Services Secretary has taken action to remove thimerosal from all influenza vaccines distributed in the country. All vaccines routinely recommended for children 6 years of age and younger in the US are available in formulations that do not contain thimerosal.
In 1999, the CDC and the American Academy of Pediatrics (AAP) determined that under the existing vaccination schedule, some children could be exposed to a cumulative level of mercury that exceeds federal guidelines. Additionally, the Environmental Protection Agency (EPA) decreased the suggested maximum allowable amount of methylmercury exposure in 1997. While there is no evidence of harm caused by the low doses of thimerosal in vaccines, there was scientific uncertainty at the time, and reducing childhood exposure to mercury from all sources was deemed prudent.
No, multiple studies and reviews by major scientific and medical bodies have found no association between thimerosal-containing vaccines and autism or other neurodevelopmental disorders. The increased recognition and diagnosis of autism in recent years may be a contributing factor to the apparent rise in its prevalence.
Thimerosal contains ethylmercury, which is processed differently in the human body compared to methylmercury, the type of mercury found in the environment. Ethylmercury is less likely to accumulate in the body and cause harm. While there may be minor reactions like redness and swelling at the injection site, there is no data or evidence of any significant harm caused by the low doses of thimerosal used in vaccines.
