Cross-reactive hemagglutinin (HA) binding activity of plasmablast-derived polyclonal antibodies (PPAbs) after immunization with 2010 inactivated influenza vaccine (IIV) and live attenuated influenza vaccine (LAIV). A pool of PPAbs was assembled by combining equal amounts of immunoglobulin G (IgG) from each PPAb sample in the IIV or LAIV groups. Each PPAb pool was serially diluted, starting at a concentration of 9 ng/mL for LAIV and 4.1 ng/mL for IIV of IgG, and analyzed with an enzyme-linked immunosorbent assay (ELISA) for binding to recombinant HAs from indicated influenza virus strains. Relative binding activity to each HA (numbers in each panel) was defined as the ratio of its area under curve (AUC) to that of the relevant 2010 vaccine strain (circle in each panel, assigned a value of 1).