Immunity to challenge in volunteers vaccinated with an inactivated current or earlier strain of influenza A(H3N2)

Volunteers were inoculated with vaccine made from the 30c mutant, A/Port Chalmers/73 or B/Hong Kong/8/73. Preliminary experiments showed that the 30 c strain was antigenically quite close to A/HK/8/68. Volunteers given 30c developed haemagglutination inhibiting antibodies against the 'current' 1973 serotypes (as well as to the vaccine virus) but the titres were less than those after the A/PC/73 vaccine. Volunteers were then challenged with a live attenuated virus, WRL 105, with A/Finland/4/74 antigens, by intranasal inoculation. The rates of infection were 43% after B/Hong Kong/8/73, 20% after 30c and 5% after A/PC/73. This indicated that the 30c gave some protection but that the vaccine prepared from the current strain gave more.