Immune response induced by the third dose of mRNA-1273 vaccine. Spike-specific IgG analyzed by ELISA in plasma collected 0, 28, 90, 180, and 187 days after the first dose of the mRNA-1273 vaccine (arrows) in each of the 9 low responder HCT recipients. Single HCT recipients were identified with a number and a color, as reported. Spike-specific IgG titers are expressed as fold increase compared with baseline (A). Surrogate virus neutralization test was performed before (day 180) and after (day 187) the third mRNA-1273 vaccine dose. Data are reported as ACE2/RBD binding inhibition percentage for each subject. A threshold (dotted line) was placed at 30% inhibition percentage to discriminate between positive and negative samples, according to the instructions of the manufacturer (B). Spike-specific B-cell response following mRNA-1273 vaccination. Percentages of RBD⁺Spike⁺ B cells among CD19⁺ cells were assessed in each subject immediately before (day 180) and after (day 187) the third dose (C). Statistical significance in (B) and (C) was assessed with the Wilcoxon test for paired samples. Bivariate analysis of the spike-specific IgG and RBD⁺Spike⁺ B cells. A linear model is fitted on the data (straight line), and 95% CI is reported (dashed lines); the correlation was performed with Spearman’s method (D).