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An adjuvant is a substance that, when added to a vaccine, greatly enhances its protection against infection. The term “adjuvant” comes from the Latin word adjuvare, meaning “to help.”

Alum, a mixture of aluminum salts, was the first vaccine adjuvant to be widely used in the United States. It was the only vaccine adjuvant in use until 2009, when the Food and Drug Administration approved Cervarix, a human papillomavirus vaccine that contains an adjuvant called AS04. This adjuvant is a mixture of alum and a bacterial lipid (fat) molecule that has been modified so that it does not cause disease.

How Do Adjuvants Work?

Researchers have found that adjuvants work by stimulating early immune responses to foreign substances. Adjuvants kick-start the immune system and enable the active components in a vaccine―called antigens―stimulate a broad response that leads to long-term protection.

Benefits of Vaccine Adjuvants

Adjuvants have several important benefits:

  • Adding an adjuvant reduces the amount of the active component required in a vaccine. This has two important consequences—reducing the cost per vaccine and making more doses available for public use, which is especially important during an epidemic or pandemic.
  • A person may need fewer doses of a vaccine containing an adjuvant because the immune response is stronger and lasts longer.
  • People with compromised immune systems, such as the elderly or the very young, benefit from vaccines with adjuvants because their immune systems require an extra boost to provide protection.
  • Adjuvants are especially effective in boosting the immune-stimulating effects of newer vaccines, such as those made with purified antigens.

NIAID and Adjuvant Research

In May 2011, NIAID developed a Strategic Plan for Research on Vaccine Adjuvants to guide adjuvant discovery, development, and translational research. This plan summarizes the status of NIAID-sponsored adjuvant research, identifies gaps in knowledge and capabilities, and defines NIAID’s goals for the continued discovery, development, and application of adjuvants for vaccines against infectious diseases.

Read the NIAID Strategic Plan for Research on Vaccine Adjuvants.

Other Vaccine Ingredients

Vaccines also may contain substances to prevent contamination during manufacturing, to maintain a vaccine’s potency at less-than-optimal temperatures, or to keep multi-dose vials of vaccine sterile after they are opened. One such ingredient is thimerosal, which since the 1930s has been added to some vaccines and other products because it is effective in killing bacteria and preventing bacterial contamination. Read more about thimerosal in vaccines.

One product of the degradation or metabolization of thimerosal is ethyl mercury, an organic derivative of mercury. See a mercury exposure comparison chart.

Read about NIAID-supported studies on Mercury Levels in Infants Receiving Routine Immunizations and Pharmacokinetics and Tissue Distribution of Thimerosal, Ethyl Mercury, and Methyl Mercury in Animals.

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Last Updated March 28, 2012