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Frequently Asked Questions About Thimerosal (Ethylmercury)

What is thimerosal? Is it the same as mercury?

  • Thimerosal is a mercury-containing organic compound and has been used for decades in the United States and other countries. It’s use as a preservative in a number of biological and drug products, including many vaccines, to help prevent potentially life threatening contamination with harmful microbes.
  • Mercury is a metal found naturally in the environment and affects the human body differently than thimerosal.

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What is the difference between ethylmercury and methylmercury? How are they different?

  • When learning about thimerosal and mercury it is important to understand the difference between two different compounds that contain mercury: ethylmercury and methylmercury. They are totally different materials.
  • Methylmercury is formed in the environment when mercury metal is present. If this material is found in the body, it is usually the result of eating some types of fish or other food. High amounts of methylmercury can harm the nervous system. This has been found in studies of some populations that have long-term exposure to methylmercury in foods at levels that are far higher than the U.S. population. In the United States, federal guidelines keep as much methylmercury as possible out of the environment and food, but over a lifetime, everyone is exposed to some methylmercury.
  • Ethylmercury is formed when the body breaks down thimerosal. The body uses ethylmercury differently than methylmercury; ethylmercury is broken down and clears out of the blood more quickly.  Low-level ethylmercury exposures from vaccines are very different from long-term methylmercury exposures, since the ethylmercury does not stay in the body.

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Does thimerosal cause autism?

  • No. Research does not show any link between thimerosal and autism.

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Is thimerosal safe for people?

  • Yes. Thimerosal has been used safely in vaccines for a long time (since the 1930s) and has a proven track record of being safe. A variety of scientists have been studying the use of vaccines that have thimerosal in them for many years. They haven’t found any actual evidence that thimerosal causes harm.

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Why was thimerosal removed from vaccines given to children?

  • Although no evidence suggests that there are safety concerns with thimerosal, vaccine manufacturers have stopped using it as a precautionary measure.The only vaccine that still includes thimerosal as a preservative is the multi-dose inactivated influenza vaccine. There are other formulations of flu vaccine that do not include thimerosal.
  • In 1999, the Food and Drug Administration (FDA) was required by law to assess the amount of mercury in all the products the agency oversees, not just vaccines. The U.S. Public Health Service decided that as much mercury as possible should be removed from vaccines, and thimerosal was the only source of mercury in vaccines.
  • The decision to remove it was a made as a precautionary measure to decrease overall exposure to mercury among young infants.
  • Thimerosal was removed from all childhood vaccines in 2001 with the exception of inactivated flu vaccine in multi-dose vials. However, thimerosal has been removed from all single-dose preparations of flu vaccine for children and adults. There has never been thimerosal in live attenuated flu vaccine or recombinant flu vaccine. No acceptable alternative preservative has yet been identified for multi-dose flu vaccine vials.

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Why is thimerosal used in some vaccines?

  • Because it prevents the growth of dangerous microbes, thimerosal is used as a preservative in multi-dose vials of flu vaccines, about a third of flu vaccine doses that are available.
    • Each time a new needle is inserted into the multi-dose vial, it is possible for microbes to get into the vial.
    • The preservative, thimerosal, prevents contamination in the multi-dose vial when individual doses are drawn from it. Receiving a vaccine contaminated with bacteria can be very dangerous.
  • For two, non-flu childhood vaccines, thimerosal is used to prevent the growth of microbes (bacteria and other environmental contaminants) during the manufacturing process.
    • When thimerosal is used this way, it is removed later in the process.
    • Only trace (very tiny) amounts remain.
    • Today, the only childhood vaccines that have trace amounts of thimerosal are: one DTaP vaccine and one DTaP-Hib combination vaccine.
  • Among flu vaccines that might be given to children, one manufacturer's single-dose formulation (tradename Fluvirin), which is approved for use among children 4 years and older, has trace amounts of thimerosal. All other single-dose formulations of flu vaccine have no thimerosal.

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Do all flu vaccines contain thimerosal?

  • No. Influenza (flu) vaccines are currently available in both thimerosal-containing and thimerosal-free versions. The total amount of flu vaccine without thimerosal as a preservative has increased over time. This year, about 2/3 of flu vaccine that is manufactured for the U.S. will be thimerosal-free.

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Why is thimerosal still in some flu vaccines that children may receive?

  • To produce enough flu vaccine for the entire country, some of it must be put into multi-dose vials. These vials have very tiny amounts of thimerosal as a preservative. This is necessary because each time an individual dose is drawn from a multi-dose vial with a new needle and syringe, there is the potential to contaminate the vial with harmful microbes (toxins). So, this preservative is needed to prevent contamination of the vial (as a safeguard) when individual doses are drawn from it, and keep the children safety who are receiving the flu shot from the multi-dose vial. Children can safely receive flu vaccine that contains thimerosal.
  • Flu vaccine that does not contain thimerosal is available in single-dose vials or single-dose syringes. One formulation of single-dose inactivated flu vaccine, Fluvirin, contains trace amounts of thimerosal.

What keeps today’s childhood vaccines from becoming contaminated if they do not contain thimerosal as a preservative?

  • The childhood vaccines that used to contain thimerosal as a preservative are now put into single-dose vials or syringes, so no preservative is needed. In the past, these vaccines were put into multi-dose vials, which could become contaminated when new needles were used to get vaccine out of the vial for each dose.

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Was thimerosal used in all childhood vaccines?

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How can I find out if thimerosal is in a vaccine?

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How does thimerosal work in the body?

  • Thimerosal contains “ethylmercury,” which is a completely different form of mercury than elemental mercury or methylmercury, which are found in the environment and some kinds of fish. Elemental mercury and methylmercury stay in the human body and at high levels can make people sick. But ethylmercury (that is found in thimerosal) does not stay in the body a long time and clears out of the blood quickly, so it does not build up and reach harmful levels. In fact, when thimerosal enters the human body, it breaks down to ethylmercury and thiosalicylate, which are easily eliminated.

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What are the possible side-effects of thimerosal?

  • Most people don’t have any side effects from thimerosal, but some people will have mild reactions like redness and swelling at the place where the shot was given, which only last 1 to 2 days. It’s very unlikely you will have an allergic reaction to thimerosal. Research shows that most people who are allergic to thimerosal will not have a reaction when thimerosal is injected under the skin (Wattanakrai, 2007; Heidary 2005).
  • Anyone who believes they have been injured by a vaccine should contact the National Vaccine Injury Compensation Program.

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Does thimerosal use in vaccines interfere with brain activity?

  • The study, “Thimerosal Exposure in Early Life and Neuropsychological Outcomes 7-10 Years Later,” which was published on July 15, 2011, in the Journal of Pediatric Psychology, looked for a possible association between exposure to thimerosal-containing vaccines before birth or in the first seven months of life and neuropsychological function at ages 7-10 years old.
    • This study used the same data from the study by Thompson et al published in the September 27, 2007 New England Journal of Medicine. The original study evaluated 1,047 children ages 7-10 and their biological mothers and concluded that ethyl mercury exposure from thimerosal-containing vaccines and immunoglobulins does not affect neuropsychological functioning at ages 7-10 years.
  • The authors concluded that the weight of the evidence in this study does not support a causal association between early exposure to mercury from thimerosal-containing vaccines and immune globulins administered prenatally or during infancy and neuropsychological functioning at the age of 7–10 years for any of the other neuropsychological outcomes assessed.

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The Science

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Other Helpful Resources

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