What are vaccines based on recombinant vectors?

— When the live microorganism is still too virulent and purified proteins are poorly immunogenic, these proteins are expressed by another live microorganism, which is non pathogenic, even for immunocompromized recipients — Genes encoding vaccine candidate antigens are introduced in an attenuated virus or bacterium that becomes a vector. It replicates within the host and expresses the gene product against which an immune response is desired — Examples of vectors : — Bactera: Vibrio cholerae, Salmonella typhiTy21A, Shigella flexneri and Listeria monocytogenes (all attenuated) — Viral : vaccinia, canarypox virus, adenovirus, measles