Why is thimerosal used as a preservative in some vaccines?

Thimerosal is used as a preservative in some multi-dose vials of vaccines to prevent contamination. Preservatives are not required for vaccines in single-dose vials. As a preservative, thimerosal is added at the end of the production process to the bulk or final container to prevent contamination after multi-dose vials are opened. Today, with the exception of some flu vaccines, none of the vaccines used in the U.S. to protect preschool aged children against 12 infectious diseases contain thimerosal as a preservative. Thimerosal still may be used in the early stages of manufacturing of certain vaccines, but is removed through a purification process, with only trace, or insignificant, amounts remaining.

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If thimerosal was used in vaccines for many years, why did it become a concern?

The Food and Drug Administration (FDA) Modernization Act of 1997 called for the FDA to review and assess the risk of all mercury containing food and drugs. As part of this effort, the FDA conducted a review of mercury content in vaccines.

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What recommendations did the Federal government make in 1999 with respect to thimerosal in vaccines?

A review conducted by the FDA concluded that the use of thimerosal as a preservative in vaccines might result in the intake of mercury during the first 6 months of life that exceeds the Environmental Protection Agency (EPA), but not the FDA, the Agency for Toxic Substances and Disease Registry (ATSDR), or the World Health Organization (WHO) guidelines for methylmercury intake (Ball et al., 2001). Thimerosal contains ethylmercury. Methylmercury is a related compound and has been more thoroughly researched than ethylmercury. Thus, federal safety standards are based on information we have about methylmercury.

FDA's review found no evidence of harm caused by doses of thimerosal in vaccines, except for minor local reactions. Nevertheless, in July 1999 the Public Health Service agencies (PHS), the American Academy of Pediatrics (AAP), and vaccine manufacturers agreed that thimerosal levels in vaccines should be reduced or eliminated as a precautionary measure, and the Food and Drug Administration (FDA) committed to expediting the review of new vaccines that do not contain thimerosal.

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What progress has been made since July 1999 in removing thimerosal from vaccines routinely recommended for infants?

Substantial progress has been made in the effort to reduce thimerosal exposure from vaccines. Today, with the exception of some flu vaccines, none of the vaccines used in the U.S. to protect preschool aged children against 12 infectious diseases contain thimerosal as a preservative. The vaccines with trace amounts of thimerosal licensed to date contain less than 0.5 micrograms of mercury per dose, that is, a given dose of vaccine contains less than 1 part per million.

Prior to the recent initiative to reduce or eliminate thimerosal from childhood vaccines, the maximum cumulative exposure to mercury via routine childhood vaccinations during the first six months of life was 187.5 micrograms. With the newly formulated vaccines, the maximum cumulative exposure during the first six months of life will now be less than three micrograms of mercury; this represents a greater than 98 percent reduction in the amount of mercury a child would receive from vaccines in the first six months of life. [Influenza (flu) vaccine is not given until six months or older.]

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Are children getting toxic levels of mercury from vaccines?

No. There is no evidence of harm caused by the minute doses of thimerosal in vaccines, except for minor effects like swelling and redness at the injection site due to sensitivity to thimerosal. Most importantly, since 1999, newly formulated thimerosal preservative-free vaccines have been licensed. With the newly formulated vaccines, the maximum cumulative exposure during the first six months of life will now be less than three micrograms of mercury. No children are receiving toxic levels of mercury from vaccines.

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What research is being conducted by the Federal Government regarding the safety of vaccines containing thimerosal?

There is no evidence to suggest that thimerosal in vaccines causes any health problems in children and adults beyond local reactions like redness and swelling at the injection site. However, efforts to remove thimerosal from the U.S. vaccine supply have been accompanied by research investigations to better assess the potential health effects of exposure to thimerosal containing vaccines:

• The National Institute of Allergy and Infectious Diseases at the National Institutes of Health funds thimerosal research that focuses on better understanding of what happens to thimerosal once it is introduced into the body and how this compares to current knowledge of methylmercury pathways.

• Pharamacokinetics is the study of how an agent is absorbed, distributed, metabolized (broken down), and excreted. For more information go to www.niaid.nih.gov/factsheets/thimerosal.htm.

• The FDA has been actively addressing the issue of thimerosal as a preservative in vaccines. For more information, go to http://www.fda.gov/cber/vaccine/thimerosal.htm.

• The CDC's Center for Environmental Health and the National Center for Health Statistics are doing a study looking at all mercury exposures and working with the National Health and Nutrition Examination Survey (NHANES).

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Does thimerosal cause autism?

There is no conclusive evidence that any vaccine or vaccine additive increases the risk of developing autism or any other behavior disorder. Rather, evidence is accumulating of the lack of any harm resulting from exposure to vaccine containing-thimerosal as a preservative. In a 2004 report, the Institute of Medicine (IOM) concluded that there is no association between autism and vaccines that contain thimerosal as a preservative. Nonetheless, given the level of concern among parents and others regarding vaccines and autism, the CDC is committed to investigating this issue to the fullest extent possible, using the best scientific methods available.

For more information on autism and vaccines go to http://www.cdc.gov/od/science/iso/mmr_autism.htm.

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Could thimerosal have been a factor in Great Britain's rise in rates of autism in children born in the 1980's and early 1990's?

It is unlikely that the rapid increase in autism cases in the UK was due to thimerosal. The only vaccine in the UK's childhood immunization program that contains thimerosal is DTP. All other vaccines (OPV, BCG, MMR, Hib, menC) added to the program since the 1950's are and have always been thimerosal free. In addition, if thimerosal in vaccines were causing autism, we would expect to see a simultaneous increase in both thimerosal exposure and autism cases. However, from the 1950's on, there was no increase in the amount of thimerosal UK children were receiving from vaccines, yet there was a jump in autism cases in the 1980's.

Do those sensitive to mercury have problems with thimerosal-containing vaccines?

Some individuals experience local skin reactions such as redness and swelling that may suggest a delayed-type of minor allergic reaction following injection with products containing thimerosal. A prior history of a minor reaction to thimerosal in a vaccine is not considered a contraindication to further vaccination with thimerosal-containing vaccines. Severe allergic reactions to any vaccine is a contraindication to further vaccination with the vaccine.

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Do measles, mumps, and rubella (MMR) vaccines contain thimerosal?

No, MMR vaccine does not and never did contain thimerosal. Varicella (chickenpox), inactivated polio (IPV), and pneumococcal conjugate vaccines have also never contained thimerosal. 

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Why are chemicals and other substances added to vaccines?

Many foods and medicines, including vaccines, have chemicals added to them to prevent the growth of germs, reduce spoilage, and prevent it from losing its potency over time. Some additives are used in the production of vaccines. Vaccines may include suspending fluid (e.g., sterile water, saline, or fluids containing protein); preservatives and stabilizers (e.g., albumin, phenols, and glycine); and adjuvants, or enhancers, that help the vaccine improve its ability to protect against disease.

For more information on additives, go to: www.cdc.gov/vaccines/vac-gen/additives.htm.

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How can I find out what chemical additives are in specific vaccines?

Ask your healthcare provider or pharmacist for a copy of the vaccine package insert. The package insert lists ingredients in the vaccine and discusses any known adverse reactions.

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Which childhood vaccines do not contain thimerosal?

Today, with the exception of some influenza vaccines, none of the vaccines used to protect preschool children against 12 infectious diseases contain thimerosal as a preservative. (Those with a concentration of less than 0.0002% contain what is considered “trace” or insignificant, amounts.) Certain Influenza (flu) vaccines and tetanus-diphtheria vaccines (Td) given to children age 7 and older contain thimerosal as a preservative. For more information on thimerosal content in some currently manufactured U.S. licensed vaccines, go to www.fda.gov/cber/vaccine/thimerosal.htm.

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Why does some flu vaccine contain thimerosal when the preservative has been removed from other pediatric vaccines?

Influenza (flu) vaccine is a new addition to the recommended childhood immunization schedule. In 2004, the CDC added the vaccine to the childhood immunization schedule, recommending that children 6-23 months of age be vaccinated against flu each year. Inactivated influenza vaccine for children 6-23 months of age is currently available both with thimerosal as a preservative and preservative-free.

The removal of thimerosal as a preservative from influenza vaccine is a complicated process. The total amount of flu vaccine without thimerosal as a preservative will be increased as vaccine manufacturing capabilities are expanded. In the meantime, it is important to keep in mind the benefits of influenza vaccination outweight the theoretical risk, if any, for exposure to thimerosal.

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Why weren't thimerosal-containing vaccines taken off the market?

Scientific data have not established that vaccines containing thimerosal, used as a preservative, create an imminent or substantial hazard to public health or are in violation of FDA laws or regulations, and therefore do not justify such a recall. A mandatory recall requires that the product present “an imminent or substantial hazard to the public health.” The U.S. FDA is responsible for voluntary and mandatory recalls of drug and vaccine products. The FDA continuously monitors the safety of these products.

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Should immunization providers stop using licensed pediatric vaccines that contain thimerosal?

No. Immunization providers should use the vaccines available in their stock. The use of vaccines should continue according to the currently recommended schedule. The risks of not vaccinating children on time are significant, whereas the risks of thimerosal-containing vaccines have not been proven scientifically. Furthermore, availability of any vaccine containing thimerosal preservative is rare.

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What is the availability of the preservative-free hepatitis B vaccine?

All hepatitis B vaccines intended for use in infants and children are free of thimerosal as a preservative, and an adequate supply of these vaccines is available for all infant and childhood vaccinations. This vaccine should be administered to all newborn infants and is a major cornerstone in the prevention of a potentially fatal disease in children and adults.

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Are older lots of pediatric vaccines which contain thimerosal as a preservative still on the shelves in doctor's offices and being used in the U.S.?

With the exception of some influenza vaccines and tetanus-diptheria (Td) vaccines (given to children age 7 and older), the last lots of recommended childhood vaccine which contain thimerosal as a preservative expired by early 2003. If providers have such expired vaccines in their stocks, they should discard them.

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