Further studies of the neuraminidase content of inactivated influenza vaccines and the neuraminidase antibody responses after vaccination of immunologically primed and unprimed populations.

Purified concentrates of influenza A/USSR/90/77(H1N1)-like, A/Texas/1/77 (H3N2)-like, and B/Hong Kong/5/72-like viruses used for preparation of investigational and licensed vaccines in 1978 to 1979 were tested for their content of neuraminidase enzyme activity. Concentrates of H1N1 virus used to prepare vaccines for clinical investigations performed in the spring of 1978 had neuraminidase activity at that time which decreased during storage to almost undetectable levels (three lots) or by 50% (one lot) by the winter of 1978. Several other lots of concentrates prepared with H1N1 virus and used for vaccine formulation had no detectable neuraminidase enzyme activity when tested in the winter of 1978, at a time when they would be administered in vaccines. The range of specific activity for different lots of concentrates was about 40-fold for A/Texas/1/77, B/Hong Kong/5/72, and A/USSR/90/77 neuraminidases. Immunogenicity of investigational vaccines prepared with tested concentrates and administered between April and July 1978 was measured in volunteers aged 13 to >50 years. Frequency of neuraminidase antibody rises to two doses of H1N1-containing vaccine was 10% in unprimed subjects aged <26 years and about 18 to 36% in older persons. The frequency of neuraminidase antibody rises to one dose of H3N2-containing vaccine varied from 0 to 32% in different groups (mean, 18%). The frequencies of neuraminidase antibody responses were always much lower than the frequencies of hemagglutinin antibody responses. These observations confirm the existence of practical difficulties in achieving uniformity of the neuraminidase content in influenza vaccines and of ensuring good immunogenicity of vaccine neuraminidase even in primed populations.