Correlates of adjuvanticity: A review on adjuvants in licensed vaccines

After decades of slow progress, the last years have seen a rapid acceleration of the development of adjuvanted vaccines which have lately been approved for human use. These adjuvants consist of different components, e.g. aluminium salts, emulsions such as MF59 and AS03, Toll-like receptor (TLR) agonists (CpG ormonophosphoryl lipid A (MPL) adsorbed on aluminium salts as in AS04) or combination of immunopotentiators (QS-21 and MPL in AS01). Despite their distinctive features, most of these adjuvants share some key characteristics. For example, they induce early activation (although at different levels) of innate immunity which then translates into higher antibody and cellular responses to the vaccine antigens. In addition, most of these adjuvants (e.g. MF59, AS03, AS04) clearly induce a wider breadth of adaptive responses able to confer protection against, for example, heterovariants of the influenza viruses (MF59, AS03) or against human papillomavirus strains not contained in the vaccine (AS04). Finally, the use of some of these adjuvants has contributed to significantly enhance the immune response and the efficacy and effectiveness of vaccines in the elderly who experience a waning of the immune responsiveness to infection and vaccination, as shown for MF59- or AS03-adjuvanted influenza vaccines and AS01-adjuvanted herpes zoster vaccine. These results, together with the track record of acceptable safety profiles of the adjuvanted vaccines, pave the way for the development of novel vaccines at the extremes of age and against infections with a high toll of morbidity and mortality. Here, we review the mechanisms associated with the performance of those adjuvanted vaccines in animal models and in humans through recent advances in systems vaccinology and biomarker discovery. We also provide some perspectives on remaining knowledge gaps but also on opportunities that could accelerate the development of new vaccines.