Influenza vaccines are effective in protecting against illness and death caused by this seasonal pathogen.
Antibodies that block the function of either
hemagglutinin (HA) or
neuraminidase (NA) contribute to
vaccine efficacy, however vaccine potency is based only on HA content. NA
protein content in
vaccines varies from season to season due to differences in the relative amounts of HA and NA in
influenza A, H1N1 and H3N2, and influenza B viruses that are selected for each manufacturing campaign. This, as well as potential inherent differences in NA immunogenicity, may result in varying responses from year to year. Moreover, the antigenic stability of NA is likely to dictate whether similar antibody responses will be obtained to this
antigen throughout the shelf-life of the
vaccine. To address this factor, we subjected
NAs of
influenza A (subtypes N1 and N2) and B viruses to denaturing conditions to evaluate the stability of
enzyme activity. Each NA type/subtype had unique sensitivity to denaturing conditions. The N2
enzyme activity was more thermostable than that of N1 or
influenza B, while the NA activity of
influenza B was most resistant to
detergent. N1
enzyme activity was most resistant of the three
NAs to freeze-thaw cycling. In these experiments,
enzyme activity was indicative of the immunogenicity of NA, but was strain-dependent, with greater
neuraminidase inhibiting (NI) antibody titers elicited following immunization with the 2009 H1N1 pandemic virus A/California/7/2009, than the previously circulating seasonal H1N1 strain, A/Brisbane/59/2007. Robust NI antibody titers against both N1 and N2 components were induced following vaccination of mice with a trivalent inactivated
influenza vaccine. When stored under recommended conditions, the NA of both N1 and N2 subtypes remained immunogenic well after the
vaccine expiry date.