The characteristics to be considered in choosing a viral vector are dictated by which of these types of therapy are intended. The available viral vectors differ widely. Viral vectors also differ in the size of the gene or genes that can be packaged for transmission. Additional considerations in choosing a vector are its compatibility with the chosen application route. Making the proper choice concerning the mode of physical delivery can be as important as choosing the right biological vector.
All viral vectors used for gene therapy are perforce recombinant.
Such viruses are referred to as replication-defective. Because the helpers do not contain the nucleic acid sequences for the virus coat material to package around them, they are not made into virus particles. The recombinant virus is subsequently purified from the cultures and away from the helpers. It requires the production of reproducibly high virus titers. At the same time, the cells must be kept free of exogenous pathogens.