The potential use of twice-annual repeated influenza vaccination in older adults

Abstract

Older adults are advised to receive inactivated influenza vaccination (IIV) annually in Hong Kong. However, vaccine protection may not span 12 months, and twice-annual vaccination may improve protection in locations with year-round activity. We are conducting a randomized controlled trial of once-annual versus twice-annual influenza vaccination in adults 70-79 years of age in Hong Kong to study the potential use of twice-annual influenza vaccination in older adults in tropical countries. From 2016/17 winter to 2021 summer, all participants will receive northern hemisphere quadrivalent IIV in autumns/winters, and participants will be randomized to receive either the southern heisphere trivalent/ quadrivalent IIV or placebo in springs/summers, for a total of 10 rounds of vaccination. We will then compare the mean-fold rises and post-vaccination GMTs against all vaccine strains between the two groups 30 days post-vaccination in each round. Additional blood draws and active surveillance for respiratory illnesses during influenza seasons will enable the assessment of protection by different immune measures. The trial will allow the comprehensive assessment of the strength, breadth and duration of immune responses under twice-annual vaccination and improve our understanding of the immunologic consequences of repeated vaccination.