Waning COVID-19–specific antibodies in allo-HSCT recipients following the second vaccine (Vacc) dose. Levels of IgG antibodies in serum against the RBD in S1 were determined 1 month after the first dose of vaccination (n = 50) and 1 (n = 50) and 4 to 5 months (n = 45) after the second COVID-19 vaccine dose. Patients responsive to S1-specific peptides, reflecting virus-specific T-cell reactivity, at 1 month after the second vaccine dose are shown with filled rings. Patients without a detectable S1 IFN-γ Τ-cell response 1 month after second vaccine dose are shown with open rings. Encircled are the 11/45 (24%) samples with below 100 BAU/ml anti-RBD S1 IgG levels 4-5 months after the second immunization.