Serum SRH antibody level as a measure of the immunity against natural and artificial A/Victoria/3/75 infections.

A natural influenza A/Victoria/3/75 outbreak occurred in two communities where part of the residents had been vaccinated with a live recombinant vaccine strain, the RIT 4025 strain (A/Scotland/840/74 serotype). Using the single radial hemolysis technique to measure the antibody level, a relationship was established between the rate of infection and the pre-epidemic serum antibody titer. The higher the pre-epidemic antibody titer, the lower the rate of infection and illness. The protective titer 50% (PT 50) against infection was equal to 26.7 mm2. The same calculation was applied to a population of placebo recipients and RIT 4025 vaccinees challenged with a virulent A/Victoria/3/75 strain. Although the number of volunteers was much smaller, the same correlation was found for the infection detected by a significant antibody rise and for confirmed illness.