95 Review www.expert-reviews.com ISSN 1476-0584 © 2011 Expert Reviews Ltd 10.1586/ERV.10.154 Vaccination remains an important public health tool for disease control. Live vaccines typically induce potent immune responses and complete protection. However, live vaccines have been associated with a number of safety concerns, including reversion to virulence, resulting in disease and other adverse effects ranging from simple headache or fever (influenza, hepatitis A and B, and yellow fever) to more severe enceph- alitis (measles mumps rubella), intussusception (rotavirus), vaccine-associated paralysis (polio) or even death (smallpox) [1] . Primarily for safety reasons, a major trend in vaccinology has been to move towards the use of subunit vaccines, which are composed of highly purified proteins that can be targeted by the immune system. However, the major drawback of subunit vac- cines is that they are poorly immunogenic, and often result in poor or suboptimal vaccine efficacy. In order to be effective, subunit vac- cines require coadministration with adjuvants. With very few exceptions, currently available vaccines contain a single adjuvant. Although some vaccines with a single adjuvant provide optimal protection, in many cases, the adjuvant has a number of limitations, including induc- tion of immune responses of low potency or of inappropriate quality. This is especially the case for human vaccines where there is a limited choice of clinically approved adjuvants. Alum, which was until recently the only clinically used adjuvant in humans, typically induces Th2- but not Th1-type immune responses, and therefore does not induce sufficient protection from some infections such as influenza. Even some promis- ing experimental adjuvants have their own limi- tations. Using multiple adjuvants in combina- tion can overcome these limitations. Indeed, there is evidence demonstrating that selected adjuvant combinations can be complementary or even synergistic. This approach may be ben- eficial for improving a number of vaccines. For example, addition of a Toll-like receptor (TLR) agonist to an alum-containing vaccine may sig- nificantly improve the efficacy of such a vac- cine. As discussed later, a number of vaccines containing various combinations of adjuvants have been evaluated. This approach is promis- ing and may improve a number of currently available vaccines whose efficacy is less than optimal. This approach may be particularly beneficial for vaccines against specific popula- tions, such as newborns and the elderly. In neo- nates, vaccination generally induces a default Th2 response and adjuvants that promote Th1 responses are required. Immune responses decrease with advancing age, partly as a result of thymic involution, which results in reduced output of naive T cells [2] . Strong adjuvants may provide an approach to boost immune responses in the elderly. However, strong adju- vant formulations have the potential to induce undesirable side effects such as inflammation at the injection site. Thus, rigorous safety evalu- ations will be required for formulations based on adjuvant combinations. George Mutwiri †1,2 , Volker Gerdts 1 , Sylvia van Drunen Littel-van den Hurk 1,3 , Gael Auray 1 , Nelson Eng 1 , Srinivas Garlapati 1 , Lorne A Babiuk 4 and Andrew Potter 1 1 Vaccines and Infectious Disease Organization/International Vaccine Center, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5E3, Canada 2 School of Public Health, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5E3, Canada 3 Microbiology and Immunology, University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5E3, Canada 4 University of Alberta, 3–7 University Hall, Edmonton, AB, T6G 2J9, Canada † Author for correspondence: Tel.: +1 306 966 1511 Fax: +1 306 966 7478 george.mutwiri@usask.ca Adjuvants are critical components of many vaccines. The majority of existing vaccines contain a single adjuvant. Owing to their inherent limitations, no single adjuvant is capable of inducing all the protective immune responses required in the many different vaccines. Consequently, investigators are exploring the potential of using formulations with multiple adjuvants in a vaccine. An emerging paradigm is that careful selection of adjuvant combinations can result in complementary and even synergistic enhancement of immune responses to vaccines. This approach is promising and presents tremendous opportunities for vaccinologists to tailor immune responses to specific vaccines. In this article, adjuvant combinations at different stages of development will be reviewed. KEYWORDS:฀adjuvants฀•฀delivery฀systems฀•฀immunostimulators฀•฀vaccines Combination adjuvants: the next generation of adjuvants? Expert Rev. Vaccines 10(1), 95–107 (2011) For reprint orders, please contact reprints@expert-reviews.com