The "STEP-wise" future of adenovirus-based HIV vaccines

The HIV pandemic continues to be a public health crisis with over 30 million people currently living with the disease and, depending on the estimate, another 2 - 2.8 million infected annually. The disappointing results of the first Phase II study of a highly immunogenic adenovirus-vectored vaccine, named the STEP trial, was a wake up call to both the clinical and preclinical HIV vaccine fields. A vaccine designed only to elicit T cells and including a single HIV gene insert, will not be sufficient to reduce transmission or lower viremia in people. Additionally, future use of adenovirus-based vectored vaccines needs to be carefully planned with respect to vector type, gene inserts, route of immunization and risk factors among subject volunteers. The initial observation of a transient, increased risk of infection in Ad5 seropositive, uncircumcised men who have sex with men (MSM) is still unexplained, and may yet be considered simply a random event. The vaccine field has not given up on adenoviruses and there is continued interest in pursuing these highly immunogenic vectors, either in combination approaches with DNA, use of rare serotypes with low seroprevalence, or those derived from simian origin. Finally, evaluation of replicating adenovirus vectors known to be capable of inducing potent cellular, humoral, and mucosal immunity will be vital to meeting our future goal of an effective HIV vaccine.