Inclusion Criteria:  prior allogeneic hematopoietic stem cell transplantation 3 months to 5 years earlier (any donor type) age > or = 18 years at inclusion. written informed consent  Exclusion Criteria:  HIV seropositivity Pregnancy Active malignant disease at inclusion Current grade III-IV acute Graft Versus Host Disease (GVHD) In vitro T-cell depletion of the graft if vaccination within the 6 months after transplantation. Rituximab administration in the 6 months prior to study inclusion Prior documented COVID-19 infection