Vector based vaccine candidates infect host cells including antigen presenting cells. Induction of immune response depends on antigen presented by host cells after uptake of the vector. Cellular and humoral immune responses are induced against vector and insert specific antigens through class I and class II MHC molecules, respectively, upon the antigen presentation. After boosting of immune response with the same vector, uptake of the vector by host cells gets hampered because of elimination of the vector by anti-vector immunity. This further affects antigen presentation and boosting of insert specific immune responses by the vector. Conversely, if a different vector is used for boosting, anti vector immunity induced by the prime cannot eliminate an antigenically different vector. Hence, the uptake as well as antigen presentation by the host cells is not affected, leading to boosting of insert specific immune responses.
