The recent developments in modern vaccinology are mainly based on: (i) cloning of microbial genes into recombinant vectors containing genetic information for expression of desired neutralizing immunogens; (ii) alternatives of attenuated vectors with deleted genes permitting the insertion of several foreign genes expressing antigens exposed to the host immune system during the abortive replication of such vectors; (iii) combined vaccines with the aim to protect against many diseases with a limited number of administrations; (iv) evidence demonstrating the ability of animals to respond...