Comparative Efficacy of Neuraminidase-Specific and Conventional Influenza Virus Vaccines in Induction of Antibody to Neuraminidase in Humans

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Abstract
Groups of college students received either conventional A/England/42/72 (H3N2) vaccine (X-37), an antigenic hybrid (HeqlN2) vaccine (X-38) containing the same neuraminidase (and thus effectively neuraminidase-monospecific), or a placebo injection. The vaccines contained 798 and 643 chick cell-agglutinating units per dose, respectively, and equivalent immunogenic units of N2 as defined in antigenic extinction tests in rabbits. All subjects had antibody to N2 before immunization, and mean initial titers were comparable in both vaccine groups. Homotypic hemagglutination- inhibition response to vaccine hemagglutinin was slightly more frequent (77%) but of lower magnitude in the students vaccinated with X-38 than in those vaccinated with X-37. Significant antibody response to N2 was observed in 25% of those vaccinated with X-37 and in 69% of those vaccinated with X-38. Mean antibody response to N2 was twofold greater in those vaccinated with X-38. Heterotypic hemagglutination-inhibition was seen in 56% of those receiving X-38 vaccine. In preliminary plaque-inhibition titrations this heterotypic antibody did not have neutralizing activity. Testing of antibody response to N2 with earlier neuraminidase antigens demonstrated “original antigenic sin” from earlier priming. The superiority of the “neuraminidase-specific” X-38 (HeqI N2) vaccine as an immunogen for antibody to neuraminidase may reflect different processing of N2 when it is associated with a hemagglutinin to which the study population has not been previously exposed.