About Vaccine Adjuvants
It is widely believed that adjuvants are important, and in some cases critical, for the success of most modern vaccines, particularly for new types of vaccines that have highly purified or synthetic antigens. Although aluminum salts are the most commonly used type of adjuvant for human vaccines, they are weak adjuvants that have complex mechanisms that favor induction of antibodies rather than cellular immunity. Aluminum salts do have a long record of relative safety, but they are also often responsible for local reactions at the site of injection, particularly for reactions that are associated with subcutaneous administration. Adjuvant selection for human vaccines still relies strongly on direct empirical testing of candidate adjuvants for safety and efficacy in humans. However, principles of innate immunity have been developed that provide some guidance for rational selection of adjuvants. New forms of vaccine adjuvants that have been proposed for various vaccines feature oil-based emulsions; bacterial products, such as lipid A, heat-labile E. coli enterotoxin, or CpG nucleotides; viral products, such as virus-like particles; plant products, such as saponin derivatives; biodegradable particles, such as liposomes; molecular adjuvants; and synthetic adjuvants.