Clinical and Immunologic Evaluation of NeuraminidaseSpecific Influenza A Virus Vaccine in Humans

Groups of schoolchildren were immunized with an inactivated recombinant influenza virus vaccine specific for the neuraminidase antigen of Port Chalmers influenza A virus (HeqlN2Ch), a conventional biphasic Port Chalmers strain of influenza virus vaccine (H3ChN2ch), or a placebo. Immunization with either virus vaccine was found to be safe and had no major adverse effects. Immunization with the HeqlN2Ch vaccine resulted in no specific hemagglutination-inhibiting antibody response to H3ch antigen, although a specific neuraminidase antibody response to N2ch antigen was observed in >90% of the vaccinees. A subsequent natural outbreak of influenza virus resulted in serologically proven infection with H3Ch virus in 26% of vaccinees receiving H3ChN2Ch virus vaccine, 47% of those receiving HeqlN2ch1 virus vaccine, and 44% of those receiving a placebo. However, the protective efficacy against illness was 74.3% for the H3ChN2Ch vaccine and only 51.4% for the HeqlN2ch vaccine. Regardless of the type of vaccine employed, vaccinees with serologic evidence of infection and clinical illness were found to have very low titers of hemagglutination-inhibiting and neuraminidase antibody. However, vaccinees with serologically proved infection but without clinical illness were found to have titers of antibody to neuraminidase before infection that were fourto eightfold higher than titers in vaccinees who were infected and who had clinical illness.