A comparative study of attenuated influenza viruses.

Influenza A and influenza B viruses were adapted to growth at 25 degrees C. When given to volunteer subjects, the viruses were attenuated but remained infective and antigenic. The minimum immunizing dose of an egg-adapted virus appeared to be 10(5.0) EID(50). Cloning by plaque selection at 25 degrees C gave seed cultures of relatively low infectivity titres. These titres were increased when necessary by passage at 33 degrees C. No reversion to virulence was observed.Viruses attenuated in the United Kingdom and the USA were compared in volunteer trials with vaccine strains that had already been used in the USSR for mass immunization. Results were broadly similar. Currently available methods of attenuation and work with temperature-sensitive mutants are reviewed.