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151 subjects, vaccinated by oral route with live influenza viruses attenuated in the USSR (bivalent A/England/42/72-B/Hong Kong/5/72 vaccine) and 66 non-vaccinated cohabitants have been studied. The vaccination did not cause any important side effect. The vaccine showed good immunizing activity: HI antibodies appeared in 91% of the subjects initially devoid of antibodies against A type and in 70% against B type. No viral diffusion occurred from the vaccinated to the cohabitants. During an A/Port Chalmers/1/73 epidemic the morbidity (3.3%) among the orally vaccinated patients turned out significantly lower than that among the non-vaccinated cohabitants (12.1%). The protection index was 72.6%.

A/Port Chalmers 1/73 and A/Victoria 3/75 live vaccines were well tolerated after oral administration and showed good immunogenic properties in subjects devoid of immune-defence or having low antibody titer. A/Victoria 3/75 vaccine was prepared before the epidemiological appearance of the corresponding virus which demonstrates that the preparation of a vaccine with new strains can be made in a comparatively short time. Our experiments and controls allow us to establish that the oral administration of influenza virus, grown on chicken kidney embryo, elicits the appearance of HI antibodies in a high number of susceptible subjects. A 60% seroconversion rate obtained after administration of two doses represents a favourable result, the more so as the vaccine can be considered absolutely safe in very young subjects.