Circulating memory B cells (MBCs) in H7N7 live attenuated influenza vaccine (LAIV)–primed (A), H2N3 LAIV–primed (B), and unprimed (C) cohorts before and after H7N7 inactivated influenza vaccination. Antigen-specific immunoglobulin G (IgG) MBC frequencies were determined in peripheral blood mononuclear cells (PBMCs) collected on days 0, 14, and 28. MBC analysis was based on in vitro stimulation to generate IgG antibody-secreting cells (ASCs), followed by enumeration of antigen-specific and total IgG ASCs. Antigen-specific MBC frequencies are shown as the percentage of total IgG MBCs. MBCs specific for H7 from NL/00, H7 head only from SH/13, chimeric cH4/7, and H1 from CA/09 are shown for each cohort. MBCs specific for H7 from NL/03, H3 from WI/05, and chimeric cH9/1 are shown in Supplementary Figure 3. PBMCs for all sampling times were not available for all subjects. To illustrate kinetic patterns, data points from consecutive samplings are connected for individual subjects. The numbers of data points per sampling time are 5–6 for the H7N7 LAIV–primed cohort, 5 for the H2N3 LAIV–primed cohort, and 7–9 for the unprimed cohort. Abbreviation: IIV, inactivated influenza vaccine.