Vaccine adjuvants: key tools for innovative vaccine design.

Vaccines represent the most efficient tool for preventing diseases caused by infectious pathogens. During the last century significant progress has been made in vaccine development, resulting in the eradication or control of several diseases. However, the emergence of new pathogens and the inadequate protection conferred by some existing vaccines render necessary new vaccination strategies. Newly arising immunization approaches, such as subunit vaccines and mucosal administration, make the use of novel adjuvants essential. However, only a limited number of adjuvants are available on the market. The present review is focused on vaccine adjuvants approved for human vaccines and promising candidates which are currently under development. In this regard, emerging immune stimulators and combinations are discussed, together with their strengths, limitations and regulatory framework.