Humoral and T-cell–mediated immune responses to vaccination in healthy controls and allo-HSCT recipients. S1-specific immune responses were determined in healthy controls (H.C.) and allo-HSCT recipients before vaccination (pre), after the first (Vac1), or second vaccine dose (Vac2). IgG antibody levels in serum against the RBD in S1 (A), and IFN-γ production in supernatant plasma following stimulation of whole blood with S1 peptides (B). The number of samples below the level of detection (LOD) for each assay (anti-RBD S1 IgG < 14 BAU/AU/mL and S1 IFN-γ < 10 pg/mL) is shown in brackets under each bar. Statistical comparisons were calculated by empirical distribution of 100 000 permutations of the average differences. ns, not significant.