Extended Data Fig. 1: Single cell sorting and memory B cell frequencies.

a, b, The gating strategy shows from left to right: CD19⁺ B cells; CD19⁺CD27⁺IgD^-; CD19⁺CD27⁺IgD^-IgM^-/IgM⁺; CD19⁺CD27⁺IgD^-IgM^-Sprotein⁺; CD19⁺CD27⁺IgD^-IgM⁺Sprotein⁺ for a healthy donor (used as negative control for S protein staining) and a vaccinated subject. c, The graph shows the frequency of CD19⁺CD27⁺IgD^-IgM^- and IgM⁺ in seronegative (n=5) and seropositive donors (n=5). d, The graph shows the frequency of CD19⁺CD27⁺IgD^-IgM^- and IgM⁺ able to bind the SARS-CoV-2 S protein trimer (S protein⁺) in seronegative (n=5) and seropositive (n=5) donors. Geometric mean and standard deviation are denoted on the graphs. A nonparametric Mann–Whitney t test was used to evaluate statistical significances between groups. Two-tailed p-value significances are shown as *p < 0.05, **p < 0.01, ***p < 0.001, and ****p < 0.0001. e, The table summarizes the frequencies of the cell population above described for all subjects enrolled in our study.

Source data