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[Studies on live influenza virus vaccine. I. Attenuated influenza A2 Hong Kong virus and its biological and immunological characteristics (author's transl)].

The present study was undertaken to investigate the development of an attenuated influenza virus strain for live vaccine.The attenuation of the A2/Hong Kong type Abe strain of influenza virus was made possible by the adaptation of the strain to such low temperature as 28°C after 20 serial passages in embryonated eggs free from avian leukovirus. During the process of adaptation, plaque purification was performed twice and 5 serial passages were also conducted by the limiting dilution technique.The cold-adapted Abe strain of influenza virus showed an impaired capacity of reproducing at elevated temperature (38°C) and acid pH (5.7 to 5.9), as compared with the original strain. It was adsorbed more slowly to chick embryo fibroblast tissue culture cells (CEF), eluted more rapidly from chick red blood cells, and more resistant to inhibitor of horse serum than the original strain.It had no virulence for either mice or hamsters, but elicited a good response to serum hemagglutination-inhibition (HI) antibody and satisfactory nasopharyngeal IgA formation.Reproduction of challenge virus was inhibited completely in the lungs of mice immunized with live virus vaccine. On the other hand, a virus dose of 102.5 EID50 was detected from the lungs of mice immunized with killed vaccine and 105.5 EID50 from those of untreated control mice. These characteristics were maintained even after 3 serial passages in embryonated eggs at 34°C.