A review of vaccine development and research for industry animals in Korea

Lee, Nak-Hyung; Lee, Jung-Ah; Park, Seung-Yong; Song, Chang-Seon; Choi, In-Soo; Lee, Joong-Bok

Although live viral vaccines are produced in several ways, the most common method for creating vaccine strains is made through passing viruses in cell cultures, embryos, or suitable materials. For instance, a selected virus strain is serially passed in chicken embryos, resulting in better replication in chick cells but with a lost ability to replicate in animals cells of the target host. Also, the live vaccine viruses can be generated by inducing random mutations on viral genome and followed by selecting a non-virulent mutant incapable of causing clinical diseases. An alternative to creating attenuated viral strains is that viruses are serially passed in a nonadapted host until they can effectively propagate, and the loss of their pathogenicity in the original host is confirmed. All of these methods involving passing virus in suitable matter can create a new version of the virus that can still be recognized by animal immune systems but cannot replicate well in a vaccinated host. This produces the necessary immune response in the host if the host is challenged with infection by the original pathogenic virus. Most importantly, protection efficacy derived from a live attenuated vaccine typically outlasts that provided by a killed or inactivated vaccine. Nevertheless, these vaccines still have a residual virulence or a risk of reversion to a virulent phenotype [11] . A single point mutation on certain gene may tend to induce attenuation of virus but may lead to back mutation, resulting in the wild type virulent virus. Considering the relatively high mutation rate of RNA viruses, it is taken into consideration that live vaccine viruses would need to have multiple mutations on various genes of the viral genome when developing an attenuated vaccine strain. Despite these drawbacks of live vaccines, live vaccines play an important role in preventing and eradicating viral diseases in industry animals. Interestingly, a potent adjuvant is not necessary for the formulation of live vaccines because live vaccine viruses are capable of infecting target cells and provoking immune responses to injected viruses. Additionally, live vaccines can easily be administered by various routes, such as injection, drinkable water, or instillation into the nasal cavity or eyes.