Factors affecting humoral immune responses to seasonal influenza vaccination in older adults in Hong Kong

Abstract

Background: In the winter of 2014-15, emergence of a drifted non-matched influenza A(H3N2) strain led the local Centre for Health Protection to procure and administer the 2015 southern hemisphere (SH) seasonal inactivated influenza vaccine (IIV) which included an updated and matching H3N2 strain, in additional to the northern hemisphere (NH) formulation that is used annually. We investigated factor affecting the humoral immune responses to NH IIV in 2015/16. Methods: In summer 2015, we enrolled older adults aged ≥75 years who were receiving SH IIV and followed them through to winter 2015-16 when they received NH IIV and collected pre- and post-vaccination sera. For comparison we enrolled a separate group of older adults who received NH IIV in winter 2015-16 without prior receipt of the 2015 SH IIV. Antibody titres against vaccine strains were measured by haemagglutination inhibition assays. Multivariate log-linear regression models were fitted to explore potential factors associated with vaccine responses in both groups of older adults winter 2015-16 in terms of post-vaccination antibody titres and the titer rises from pre-vaccination to post-vaccination. Results: We enrolled 419 participants in the twice-annual and 408 participants in the once-annual vaccination group. Significantly lower post-vaccination titres and titre rises against influenza A strains but higher geometric mean post-vaccination titres against B/Victoria were found among older age groups. Female participants showed higher geometric mean post-vaccination titres and titre rises against A(H1N1), and ever-smokers showed higher post-vaccination titres against both influenza A strains. We observed blunted immune responses in participants with recent prior vaccination which varied somewhat by strain. Conclusions: We observed some evidence of different humoral immune responses to NH IIV in 2015/16 between different ages, sexes, behaviours, and vaccination histories. The response differences were likely to be associated with the pre-vaccination titres via previous vaccination or exposure to the virus, while different age, sex, and behaviours may also link to the variation in innate and adaptive immune responses among individuals. However, we believe that these factors do not stand alone in affecting the antibody response in older adults.