The purpose of this study is to evaluate the efficacy and safety of a personalized strategy for discontinuing Letermovir (a drug used to prevent Cytomegalovirus [CMV] infection) based on the recovery of the patient's own immune system.

Cytomegalovirus (CMV) is a common and serious complication after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Currently, Letermovir is typically given as a standard prevention for about 100 days post-transplant. However, some patients may recover their CMV-specific immunity earlier, while others may need longer protection.

In this study, researchers will use a dynamic monitoring technology (QuantiFERON-CMV) to detect the level of CMV-specific T-cells in patients. Participants will be randomly assigned to either the experimental group or the control group:

Experimental Group: Letermovir discontinuation will be guided by T-cell recovery. If the test shows that the patient's CMV-specific T-cells have recovered, Letermovir may be stopped earlier than the standard 100 days.

Control Group: Patients will receive the standard Letermovir prophylaxis for approximately 100 days, regardless of T-cell status.

The study aims to determine if this immune-guided strategy can effectively prevent CMV infection while potentially reducing the duration of medication and associated costs, without increasing the risk of CMV disease.

Inclusion Criteria:

1. Recipients of allogeneic hematopoietic stem cell transplantation (allo-HSCT).
2. CMV serostatus of the recipient is positive (R+).
3. Aged 18 years or older.
4. Expected survival > 6 months.
5. Provision of signed informed consent.

Exclusion Criteria:

• 1.Active CMV infection or CMV disease at the time of screening.

  2.Known hypersensitivity to Letermovir or its excipients.

  3.Severe hepatic or renal impairment.

  4.Pregnant or breastfeeding women.