Figure 1. Different influenza A/H1N1 antibody titers in old males and females after QIVs vaccination. Violin plots are shown for immune responses of influenza ((A) A/H1N1, (B) A/H3N2, (C) BYAM, (D) BVIC)-reactive antibody in subject sera before and after vaccination based on HAI assay, p-value <0.05 indicated statistical significance. The dotted line represents the threshold of protective antibody titer ≥40 against each influenza vaccine strain.