1. Two strains of influenza virus type A-2 were attenuated by means of low dilution passage of about 100 generations in embryonated eggs.
2. When human volunteers were inoculated intranasally with these attenuated strains, HI antibody was increased in a large proportion of them.
3. No difference in nature of attenuated virus could be found between these two strains.
4. Remarkable antibody response was observed when 0.01ml of the virus having approximately 10
9 EID
50 was inoculated to human volunteers.
5. The better results were obtained by inhalation method than nasal instillation method in producing antibody.
6. Too large inoculum resulted in clinical reaction such as fever of short duration in small proportion.
7. In most persons who had fever no increase of antibody tite. was detected.
8. Reisolation of virus from inoculated persons was unsuccessful.
9. Infection to others from inoculated persons was not observed.
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