ATTENUATED AND MODIFIED VACCINE VIRUSES: DO THEY ACTAS A SOURCE OF SOME OTHER VIRAL DISEASES?

For effective control of several viral diseases, live virus vaccines are given preference over killed or inactivated vaccines for their higher immunogenic potentials. Live virus vaccine strains are developed by attenuation by passaging in nonnatural host species, tissues, embryonated eggs, etc., or by transferring viral genes/genomes to some other vector virus. The attenuated viruses fail to infect the species of their choice; though can elicit immunity when used as vaccines. The vector viruses are modified by the addition of a section of the genome of the disease-producing viruses in their genome to offer the desired immunity in vaccinated humans or animals. During mass vaccinations, the attenuated live viruses or vector viruses spread in the biosphere. It is suspected that many erstwhile low-profile disease agents genetically related to the viruses used in live vaccine production are causing more severe disease symptoms, changing tissue or organ of predilection, and even changing or broadening host species specificity. Three representative viral diseases affecting three species (human, goat, and cattle) are discussed as examples to enlighten the possible linkage between the spread of the attenuated or vector viruses in the biosphere and the increased virulence of related disease-producing viruses.