Composition and mode of action of adjuvants in licensed viral vaccines

The immunogenicity and efficacy of vaccines is largely governed by nature and the amount of antigen(s) included. Specific immune-stimulating substances, so-called adjuvants, are added to vaccine formulations to enhance and modulate the induced immune response.Adjuvants are very different in their physicochemical nature and are primarily characterized by their immune-enhancing effects. In this report, adjuvants that are components of vaccines licensed in the EU will be presented and their mode of action will be discussed.Aluminum salts have been used for almost acentury as vaccine adjuvants. In recent years numerous novel immune-stimulating substances have been developed and integrated into licensed human vaccines. These novel adjuvants are not only intended to generally increase the vaccine-induced antibody titers, but are also aimed at modulating and triggering aspecific immune response. The search for innovative adjuvants was considerably stimulated during development of pandemic influenza vaccines. By using squalene-containing oil-in-water adjuvants (namely AS03 and MF59), pandemic influenza vaccines were developed that were efficacious despite asignificant reduction of the antigen content.The development of novel adjuvants is ahighly dynamic and essential area in modern vaccine design. Some years ago, vaccines for prevention of HPV-induced cervix carcinoma and hepatitisB were licensed that contained the toll-like receptor4 agonist 3-O-desacyl-monophosphoryl lipidA (MPL), adetoxified LPS version, as the adjuvant. Quite recently, aherpes zoster vaccine was licensed in Europe with acombination of MPL and the saponin QS21 as adjuvant. This combination of immune enhancers is also used in the formulations of the same manufacturer's malaria and hepatitisB vaccine.