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. Until 1999, vaccines given to infants to protect them against diphtheria, tetanus, pertussis, Haemophilus influenzae type b (Hib), and Hepatitis B contained thimerosal as a preservative. 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.

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

The 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.

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

A review conducted by the Food and Drug Administration 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, but not the FDA, the Agency for Toxic Substances and Disease Registry, or the World Health Organization guidelines for methylmercury intake. 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, the American Academy of Pediatrics, and vaccine manufacturers agreed that thimerosal levels in vaccines should be reduced or eliminated as a precautionary measure, and the Food and Drug Administration committed to expediting the review of new vaccines that do not contain thimerosal.

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 amount 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.

Events that contributed to accomplishing this goal include the licensure of a thimerosal free Hepatitis B Vaccine (Recombinant) manufactured by Merck and Company in August 1999. FDA licensed another hepatitis B vaccine with only trace amounts of thimerosal, manufactured by GlaxoSmithKline in March 2000. A supplement for a new formulation of Aventis Pasteur's DTaP Vaccine with only a trace amount of thimerosal was approved in March 2001. Additionally, Wyeth-Lederle Vaccines and Pediatrics now only markets a single-dose, thimerosal-free formulation of its Haemophilus b (Hib) Conjugate Vaccine in the U.S. Thus, two hepatitis B vaccines are thimerosal free, four Hib vaccines are thimerosal free, and two DTaP vaccines are thimerosal free.

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.]

I've heard that children may be getting toxic levels of mercury from vaccines. Is that true?

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.

Tags: Childhood Immunizations

About the Author

www.cdc.gov
The Centers for Disease Control and Prevention (CDC) is one of the 13 major operating components of the Department of Health and Human Services (HHS), which is the principal agency in the United States government for protecting the health and safety of all Americans and for providing essential human services, especially for those people who are least able to help themselves.