Evaluation of influenza virus mutants for possible use in a live virus vaccine

Two approaches to the attenuation of influenza A2 virus were studied: adaptation to a sub-optimum growth temperature and the production of temperature-sensitive mutants.A strain of A2/Hong Kong/68 virus was adapted to growth at 25 degrees C in calf kidney tissue culture, and a virus suspension was prepared for administration to volunteers after cloning by 2 terminal dilution purifications. The results indicated that the low-temperature-adapted strain had reduced infectivity for man, but was not attenuated since illness occurred when sufficient virus was administered to infect all volunteers.More encouraging results were obtained with 2 temperature-sensitive mutants of influenza A2/1965 virus. One of these mutants was unable to form plaques in calf kidney tissue culture at temperatures above 36 degrees C; the other showed restriction of plaque formation only at 38 degrees C and above. Both mutants were able to infect hamsters, but compared with the wild-type virus there was marked restriction of replication in the lungs. Prior infection of hamsters or mice with either mutant induced significant resistance to subsequent challenge with wild-type influenza A2 virus.