During vigorous purification procedures, the integrity and immunogenicity of viruses produced in cultured cells are largely destroyed. Consequently, most whole virus vaccines are relatively impure in comparison with other products, similarly derived from cultured cells.

Despite this, whole virus vaccines are derived from large scale animal cell cultures. However, there arc examples, where it is not possible either to culture the whole viruses or to make a safe product from them.

Therefore and otherwise, using recombinant DNA technology it became possible to obtain viral subunits from animal cell lines. Vaccines derived from these proteins are purer, safer and more efficacious and are already being used for clinical trials.

Similarly, Vaccinia virus (originally used for small pox vaccines) may be used as a vector for foreign viral genes, which may express on infection of host cultured cells by Vaccinia.

This system has been tried and needs to be tried in future for live vaccine production. Using this system, recombinant 'Vaccinia' virus vaccines have actually been developed. One such rabies virus vaccine has been field tested in Europe and U.S.A (using cattle).