Immune Responses To Twice-annual Influenza Vaccination In Older Adults In Hong Kong

Abstract

Purpose This observational study was to assess the immunogenicity of the southern hemisphere (SH) seasonal influenza vaccine (SIV) among older adults in Hong Kong and its effect on the immunogenicity of subsequent northern hemisphere (NH) SIV in 2015-16. Methods In 2014/15, Hong Kong experienced a substantial A(H3N2) winter epidemic with a mismatched vaccine. Local authorities procured and administered to older adults the 2015 SH SIV which included an updated and matching A/Switzerland/9715293/2013(H3N2) strain. We compared immune parameters in pre- and post-vaccination sera from older adults ≥75 years of age who received one versus two influenza vaccines per year. Results We enrolled 978 older adults with 470 vaccinations for summer 2015 and 827 vaccinations for winter 2015/16. Recipients of SH vaccination had boosts in haemagglutination inhibition (HAI) titers against all three vaccine strains. When receiving influenza vaccination for the subsequent winter, the southern hemisphere vaccine recipients had higher pre-vaccination HAI titers but lower post-vaccination HAI titers, compared to those who had not received the SH vaccine. Furthermore, cellular immunity was impacted by bi-annual vaccination, with reduced influenza-specific CD4 T cell responses in the second season of vaccination. Conclusions We observed blunting of immune responses in the twice-annual vaccination group compared to 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.