Live recombinant vectors entered the AIDS vaccine field with the realization that live attenuated HIV vaccines posed too great a safety risk, and that subunit vaccines elicited antibodies which lacked the breadth or potency needed to induce sterilizing immunity. Vectored vaccines provided a means to bring the cellular arm of the immune system into play by mimicking natural viral infection. By delivering antigens within host cells, processing and presentation could occur for induction of cellular immune responses. This recombinant vector approach, either alone or combined with other strategies, has produced impressive results. Recombinants have been generated from DNA and RNA viruses and bacteria. With few exceptions, each vector poses some risk, yet each possesses unique features that make it attractive. In addition to safety, key considerations in vector selection have included previous success as a vaccine against the wild-type agent or other pathogens; ability to induce potent, persistent immune responses; ability to target mucosal inductive sites and antigen presenting cells; lack of integration into the host genome; presence of pre-existing immunity in people; ease of mucosal administration; cloning capacity; ease of engineering and production; and stability of the final product. Here we up-date the status of several live recombinant vectors that have shown good potential in pre-clinical studies. Some have progressed to human clinical trials, and others will shortly. The abundance of vectors, coupled with the complexity arising from use of combination regimens with other vaccine types and heterologous vectors, will necessitate selection of the most promising candidates for large-scale efficacy trials in people. The sooner comparative studies can be designed and implemented in which live recombinant vectors containing the same inserted genes are evaluated head-to-head, the closer we will be to an eventual vaccine.
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