Plasma ELISA. (a and b) Graph shows area under the curve (AUC) for plasma IgM (a) and plasma IgA (b) antibody binding to SARS-CoV-2 Wuhan-Hu-1 RBD 1.5 mo (m) and 6 mo after vaccination for n = 18 samples. Lines connect longitudinal samples. (c and d) Graph shows AUC for plasma IgM (c) and plasma IgA (d) binding to RBD in convalescent infected individuals 1.3 mo after infection (Robbiani et al., 2020), and mRNA vaccinees after prime or 1.3 mo after second vaccination (Vax2; Cho et al., 2021) compared with Ad26.COV2.S vaccinees 1.5 mo after vaccination (left) or convalescent infected individuals 6.2 mo after infection (Gaebler et al., 2021) and mRNA vaccinees 5 mo after Vax2 (Cho et al., 2021) compared with Ad26.COV2.S vaccinees 6 mo after vaccination (right). All experiments were performed at least in duplicate and repeated twice. Red bars and values represent geometric mean values. Statistical significance in panels a and b was determined by Wilcoxon matched-pairs signed rank test, and in panels c and d was determined by two-tailed Kruskal–Wallis test with subsequent Dunn’s multiple comparisons.