Correlated Studies of a Recombinant Influenza-Virus Vaccine. IV. Protection against Naturally Occurring Influenza in Military Trainees

An inactivated influenza vaccine made from a recombinant virus, X-31, derived from HK/Aichi/68 and A0/PR8/34 influenza viruses, was tested in a population of 9,616 military trainees at Fort Ord, California, during the winter and spring of 1970. Of the population, 1,682 (18%) were immunized with 556 chick-cell-agglutinating units of X-31 vaccine and 7,934 (82%) served as controls. In a smaller subgroup in which antigenicity of the vaccine was studied, 71% of those vaccinated exhibited a significant hemagglutination-inhibition antibody response. During the period of surveillance, Hong Kong influenza was diagnosed in 102 (1.3%) of 7,934 control subjects. Only five cases (0.3%) were detected in 1,682 subjects previously given X-31 vaccine. The calculated protection ratio was 4.3, and the reduction in clinical infection was 73%. These studies demonstrate the feasibility of producing an effective vaccine from a recombinant virus. This protection compares favorably with that afforded by earlier standard influenza vaccines.