Kinetics of S1-specific IgG, S-specific memory B cells, and plasmablasts in high- and low-responder MM vaccinees. Kinetic of S1-specific IgG (in BAUs/mL), S-protein-specific memory B cells (as % of total memory B cells), and plasmablasts (PBs; as percentages of total CD19+B cells) determined before vd1 and either one week (S-specific memory B and PBs) or one month (S1-specific IgG) after vd2, one and six months after vd3 and vd4, and one month after vd5 of SARS-CoV-2 mRNA vaccine in (A) MM Ab high responders (n = 8; S1-specific IgG >2500 BAUs/mL one month after vd2 and >295 BAUs/mL six months after vd2) and (B) MM Ab low responders (n = 10; S1-specific IgG < 1600 BAUs/mL one month after vd2 and <295 BAUs/mL six months after vd2), data points represent arithmetic mean with SEM, and (C) in one exemplary MM low-responder patient with breakthrough infection in the time period between one and six months after vd4. Grey boxes in A and B indicate the difference in B memory cell responses between high- and low-responders; rose-coloured boxes in B and C indicate differences in B memory cell and Ab kinetics in MM low responders without and with infection. Abbreviations: Ab, antibody; BAUs, binding antibody units; IgG, immunoglobulin G; inf, infection; MM, multiple myeloma; PBs, plasmablasts; RBD, receptor-binding domain; S1, SARS-CoV-2 spike protein 1; SEM, standard error of the mean; vd, vaccine dose.