Poxvirus-based vaccine candidates for HIV: two decades of experience with special emphasis on canarypox vectors.

Poxvirus vectors have emerged as important vectors for licensed veterinary vaccines and candidate vaccines for humans. Vaccinia, highly-attenuated vaccinia strains and avipoxviruses have been assessed extensively in preclinical models, as well as in humans, to determine their immunogenicity and protective efficacy against HIV. The attenuated vaccinia strains and avipoxviruses have been shown to be safe and able to carry HIV genes and express their proteins to induce both antibodies and cellular immune responses. Preclinical studies show protection against HIV challenge. When using a live attenuated vector system, one must be cognizant of the potential for immune dampening because of vector-specific immunity. In this regard, avipoxviruses, such as canarypox, appear free of the inhibitory effects of vector immunity and repeated use. Unlike vaccinia-based vectors derived from classical vaccine strains, NYVAC and modified vaccinia Ankara may be less susceptible to this effect. In the coming 5 to 10 years, we will certainly know whether this class of vaccine candidates, either alone or in a prime-boost format with other vectors or proteins, will contribute to HIV disease management either from a preventive or therapeutic perspective. Additional Phase I and II studies, as well as human efficacy trials will provide new information. Furthermore, it is hoped that this body of data will contribute to a better understanding of the relevance of specific immunogenicity end points to protection and the predictive value of available animal models in HIV vaccine development.