Kinetics of SARS-CoV-2 hu-1, Omicron BA.4/5- and XBB.1.5-specific Abs in MM and controls with breakthrough infections. Kinetics of (A) ancestral virus hu-1 S1-specific IgG in BAUs/mL in infected MM patients (n = 15) and infected controls (n = 15) measured before vd1, one and six months after vd2, vd3, and vd4, and one month after vd5 of SARS-CoV-2 mRNA vaccine (BNT162b2 or mRNA-1273); dashed line—positive cut-off for S1-specific IgG at 35.2 BAUs/mL; (B) Omicron BA.4/5 RBD-specific IgG (as OD) in infected MM patients (n = 6) and infected controls (n = 15) six months after vd2, one and six months after vd3 and vd4, and one month after vd5; (C) Omicron XBB.1.5 RBD-specific IgG in infected MM patients (n = 12) and infected controls (n = 10) (as OD) at the same timepoints; black dashed lines in B and C, OD > 0.25 considered positive, horizontal line indicates GMC provided numerically above x-axis; kinetics of inhibition capacity (as % inhibition) of (D) Omicron BA.4/5 RBD-specific IgG in infected MM patients (n = 6) and infected controls (n = 15) six months after vd2, one and six months after vd3 and vd4, and one month after vd5; and (E) Omicron XBB.1.5-RBD-specific IgG in infected MM patients (n = 12) and infected controls (n = 10) at the same timepoints; inhibition levels >20% considered positive (black dashed line), inhibition levels >50% relevant (red dashed line), horizontal line indicates median provided numerically above x-axis; uninfected—empty symbols, infected—full symbols. Abbreviations: BAUs, binding antibody units; CO, control subjects, GMC, geometric mean concentration; IgG, immunoglobulin G; MM, multiple myeloma patients; mo, months; mRNA, messenger ribonucleic acid; OD, optical density; RBD, receptor-binding domain; S1, SARS-CoV-2 spike protein 1; vd, vaccine dose.