To investigate biological characteristics of the IVpi-189 progeny virus derived from the culture of influenza A virus as a live-attenuated vaccine candidate.Persistent infection of a cultured cell line with influenza A virus(MDCK-IVpi) was established by incubating continuously influenza virus-infected cells at a lower temperature.The infectious progeny virus derived from MDCK-IVpi cells at the 189rd subculture was designated as the IVpi-189 strain of influenza virus.The cytopathic effect induced by IVpi-18...