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Thimerosal and Vaccines Q&A

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 used 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.

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.

Is there a link between thimerosal and autism?

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

Why was the decision made to remove thimerosal from most vaccines?

  • Although no evidence suggests there are safety concerns with thimerosal, most 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. No acceptable alternative preservative has yet been identified for multi-dose flu vaccine vials. 

Why is thimerosal used in some vaccines?

  • Thimerosal is used as a preservative in multi-dose vials of flu vaccines to prevent the growth of dangerous microbes.
    • 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. 

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. 

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.

Was thimerosal used in all childhood vaccines?

  • No. Some other vaccines, including the measles, mumps, and rubella vaccine (MMR), do not and did not ever contain thimerosal or any preservative. Varicella (chickenpox), inactivated polio (IPV), and pneumococcal conjugate vaccines have also never contained thimerosal.
  • There is no thimerosal used in the vaccines on the childhood immunization schedule. 

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.

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.

For similar information and more information about the science of vaccines, visit