Comparison of the protective efficacies of the live vaccine RIT 4025 and an inactivated vaccine against a natural heterologous A/Victoria/3/75 infection.

A clinical trial was initiated in South Africa before the winter season of 1976. The study involved 253 volunteers divided into three groups of vaccinees and one control group. Two groups of vaccinees were inoculated with either one or two doses at 2 weeks' interval (10(7.2) EID 50/dose) of the RIT 4025 live recombinant strain [A/Scotland/840/74 (H3N2) serotype] and one group received one injection of an inactivated vaccine [A/Port Chalmers/1/73 (H3N2), 360 i.u., A/Scotland/840/74 (H3N2), 300 i.u. and B/Hong Kong/8/73, 300 i.u./dose]. The serum antihaemagglutinin antibody responses against the heterologous A/Victoria/3/75 strain as measured by the single radial haemolysis test were satisfactory and not statistically different in all groups of vaccinees. On the other hand, the antineuraminidase antibody response was better in the group receiving the killed vaccine. At the end of the influenza season, A/Victoria/3/75 infections were confirmed serologically. Only 12% of the infections were symptomatic. The infection rate was significantly reduced in the live vaccine groups, whereas in the killed vaccine group the percentage of infection was lower but not significantly different from that in the placebo group.