Immune Responses to Twice-Annual Influenza Vaccination in Older Adults in Hong Kong

Many health authorities recommend influenza vaccination of older adults to reduce disease burden. We hypothesized that in tropical and subtropical areas with more prolonged influenza seasons, twice-annual influenza vaccination might provide older adults with improved immunity against influenza. In 2014-2015, Hong Kong experienced a substantial A(H3N2) winter epidemic with a mismatched vaccine. Local authorities procured and administered to older adults the 2015 southern hemisphere influenza vaccine, which included an updated and matching A/Switzerland/9715293/2013(H3N2) strain. We compared immune parameters in pre- and postvaccination sera from older adults ≥75 years of age who received 1 vs 2 influenza vaccines per year. We enrolled 978 older adults with 470 vaccinations for summer 2015 and 827 vaccinations for winter 2015-2016. Recipients of southern hemisphere vaccination had higher geometric mean titers (GMTs) by the hemagglutination inhibition assay against all 3 vaccine strains. When receiving influenza vaccination for the subsequent winter, the southern hemisphere vaccine recipients had higher prevaccination GMTs but lower postvaccination GMTs, compared to those who had not received the southern hemisphere vaccine. Furthermore, cellular immunity was impacted by biannual vaccination, with reduced influenza-specific CD4 T-cell responses in the second season of vaccination. We observed some reductions in immune responses in the twice-annual vaccination group compared with the once-annual vaccination group, in the context of unchanging vaccine strains, while protection was likely to have been improved during the summer and autumn for the twice-annual vaccination group due to the continued circulation of the A/Switzerland/9715293/2013(H3N2) virus.