HI antibody stratified by age (per-protocol immunogenicity sub-cohort).
(A). seroconversion rate, (B). seroprotection rate. CI, confidence interval; QIV, Inactivated quadrivalent influenza vaccine; TIV-Vic, inactivated trivalent influenza vaccine Victoria lineage B strain; TIV-Yam, inactivated trivalent influenza vaccine Yamagata lineage B strain. Seroconversion rate defined as the proportion with antibody titer < 1:10 at baseline and with post-vaccination titer of ≥ 1:40, or pre-vaccination titer of ≥ 1:10 and a ≥ 4-fold post-vaccination increase in titer; Seroprotection rate defined as defined as proportion with post-vaccination titer ≥ 1:40.