““Vaccination with a live-attenuated virus vaccine can be compared with the cross-species transmission of viruses that cause zoonotic infection. If the vaccine strain is an RNA virus, it can be expected that the vaccine inoculum will be made up of a population of genotypes representing the quasispecies which is created during replication and amplification of the virus in host cells. Furthermore, virus replication in the vaccine recipient creates genetic diversity, providing an opportunity for reversion of the attenuated phenotype. This is a real but rarely realized disadvantage of live-attenuated virus vaccines.””
