Efficacy of Extended Letermovir Prophylaxis to Prevent CMV Reactivation in High-Risk Chinese Adults Undergoing Allogeneic HSCT

Inclusion Criteria: 1. The patients have decided to undergo an initial allogeneic hematopoietic stem cell transplantation (allo-HSCT). 2. The patients are ≥18 years old. 3. The patients are CMV seropositive prior to transplantation. 4. The patients have at least one high-risk factor for CMV reactivation, including: (1) Haploidentical transplantation, HLA-mismatched transplantation, or unrelated donor transplantation. (2) The primary source of stem cells is cord blood. (3) A conditioning regimen including total body irradiation (TBI). (4) A GVHD prophylaxis regimen containing alemtuzumab or high-dose anti-thymocyte globulin (ATG). 5\. The patients are able to comply with the study visit schedule, understand and agree to adhere to all protocol requirements, and have voluntarily signed the informed consent form to participate in the study. 6\. The patients have no plans for reproduction from the date of consent until 90 days after the last dose of the study treatment. Exclusion Criteria: 1. Patients who have previously received allogeneic hematopoietic stem cell transplantation (allo-HSCT). 2. Patients with evidence of CMV viremia at any time prior to enrollment. 3. Patients with a history of CMV end-organ disease within 6 months prior to enrollment. 4. Patients with suspected or known allergy to letermovir or any active or inactive components of similar drugs. 5. Patients with severe hepatic impairment (defined as Child-Pugh Class C). 6. Patients with end-stage renal disease with a creatinine clearance \< 10 mL/min. 7. Patients requiring mechanical ventilation or experiencing hemodynamic instability at the time of enrollment. 8. Patients who received any investigational drug therapy within 28 days prior to enrollment. 9. Patients who received or plan to receive any of the following treatments within 28 days prior to enrollment or during the study: cidofovir, CMV immune globulin, or any experimental CMV antiviral drugs/biological therapies. 10. Patients who previously participated or are currently participating in any study involving a CMV vaccine or other CMV investigational drugs, or who plan to participate in such studies during this trial. 11. Patients who are pregnant or breastfeeding at the time of enrollment or planning to become pregnant within 90 days after the last dose of study medication. 12. Patients who test positive for human immunodeficiency virus antibodies (HIV-Ab) at any time prior to randomization, or who test positive for hepatitis C virus antibodies (HCV-Ab) with detectable HCV RNA, or for hepatitis B surface antigen (HBsAg) within 90 days prior to randomization. Laboratory testing for HIV, HBV, or HCV is allowed using locally acceptable methods. 13. Patients with active solid malignancies, except for localized basal cell or squamous cell carcinoma of the skin or a condition currently under treatment (e.g., lymphoma).