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Neuraminidase-specific antibody responses to inactivated influenza virus vaccine in young and elderly adults

Little information is available on the potential role of antibody to influenza virus neuraminidase (NA) in vaccine-induced immunity. In the present study, serologic responses to the N1Texas/91 and N2Beijing/92 NA components of trivalent inactivated influenza virus vaccine were measured by NA inhibition (NI) and enzyme-linked immunosorbent assay (ELISA), and the results for adults aged 18 to 45 (young) or > or = 65 (elderly) years were compared. The two age groups had comparable rates (32 to 50%) of NI response. In contrast, ELISA immunoglobulin G (IgG) antibody responses to N1 and N2 NAs occurred in 70 to 71 and 67 to 83%, respectively, of young subjects but in only 3 to 18 and 18 to 35%, respectively, of elderly subjects. prevaccination mean ELISA IgG and IgA NA antibody titers were generally lower for the young adults than they were for the elderly, whereas the corresponding NI titers were comparable. In young adults, plaque size-reducing NA antibody increases were positively associated with ELISA but not with NI antibody increases. There were no apparent age-related differences in the immunoglobulin isotype distribution of the anti-NA response, with IgG being the dominant class and IgG1 the dominant subclass of serum antibody. Anti-hemagglutinin antibody responses to H1Texas/91 and H3Beijing/92 were greater in magnitude and frequency than the corresponding NA-specific responses to N1Texas/91 and N2Beijing/92 when measured by hemagglutination inhibition and NI, respectively, but not when measured by ELISA. The discordance between NI and ELISA for measurement of NA-specific vaccine responses may reflect the relative insensitivity of NI in discriminating differences when initial antibody titers are low.