The path to a successful vaccine adjuvant--'the long and winding road'.

New generation vaccines will increasingly comprise highly purified recombinant proteins. Unfortunately, these antigens are often poorly immunogenic. Therefore, adjuvants will be required to enable these proteins to become effective vaccines. Although several novel adjuvants have recently emerged, including formulations comprising more than one adjuvant, the approval of vaccines containing novel adjuvants has been slow, particularly in the US. However, despite significant ongoing concerns, the necessary safety data is now emerging to show that new generation adjuvants can be safely used in diverse human populations. In combination with data showing the positive contributions of the adjuvants to the immune response, this safety data should allow several vaccines containing novel adjuvants to obtain licensure within the next few years.