Impact of CMV-Specific Immune Reconstitution at the End of Letermovir Prophylaxis on the Development of Late Cytomegalovirus Infection in Hematopoietic Stem Cell Transplant Recipients (INMUNOEND)

Inclusion Criteria: * Age \>18 years. * CMV seropositivity (positive IgG) in the recipient at the time of SCT. * First allogeneic hematopoietic stem cell transplant recipient (bone marrow, peripheral blood, or cord blood). * Within the first 28 days post-SCT at the time of inclusion. * Indication for LTV prophylaxis within the first 28 days post-transplant up to 100 days post-SCT, according to the criteria established in each center. Exclusion Criteria: * CMV seronegativity (negative IgG) in the recipient at the time of transplant. * Previous allogeneic stem cell transplant (patients with a prior autologous transplant are allowed to be included). * History of CMV disease in the 6 months prior to inclusion. * Need for preemptive therapy in the month prior to inclusion in the study. * Received any of the following in the 14 days prior to inclusion: Ganciclovir, valganciclovir, foscarnet, acyclovir (at doses \>3200 mg orally per day or \>25 mg/kg IV per day), valacyclovir (at doses \>3000 mg orally per day), famciclovir (at doses \>1500 mg orally per day). * Received any of the following in the 30 days prior to screening: Cidofovir, CMV hyperimmune immunoglobulin, any CMV antiviral in the investigational phase. * Suspected or confirmed hypersensitivity reaction to the LTV formulation or any of its components. * Severe hepatic insufficiency (defined as Child-Pugh class C). * History of primary immunodeficiency prior to transplant. * Participation in a clinical trial involving the administration of CMV vaccines, other investigational CMV drugs, or monoclonal antibodies.