This study investigates the impact of letermovir prophylaxis on viral infections (including CMV, EBV, BKV, HHV-6/7, RSV, ADV, HSV, etc.) following allogeneic hematopoietic stem cell transplantation in pediatric patients with EBV-associated T/NK-cell lymphoproliferative diseases and refractory/relapsed EBV-related hemophagocytic lymphohistiocytosis. Additionally, we examine its effects on other transplantation complications, including engraftment failure, graft-versus-host disease (GvHD), disease relapse, thrombotic microangiopathy (TMA), overall survival (OS), post-transplant lymphoproliferative disorder (PTLD) incidence, and immune reconstitution.

Inclusion Criteria:

• Diagnosed with EBV-positive T/NK lymphoproliferative disease (EBV-T/NK LPD) according to ICC 2022 criteria, or diagnosed with refractory/relapsed EBV-associated hemophagocytic lymphohistiocytosis (EBV-HLH) according to the 2004-HLH diagnostic criteria;
• Undergoing first allogeneic hematopoietic stem cell transplantation (allo-HSCT) at the study center;
• Age < 18 years;
• CMV seropositive (IgG+) prior to transplantation;
• Presence of at least one high-risk factor for CMV infection: haploidentical transplantation, HLA-mismatched transplantation, receipt of ATG (including ATLG/ALG) in conditioning, sustained corticosteroid use post-conditioning, donor/recipient CMV serostatus mismatch, or positive NGS result pre-transplant.

Exclusion Criteria:

• History of CMV end-organ disease within 6 months prior to enrollment;
• Severe hepatic dysfunction (defined as Child-Pugh Class C);
• End-stage renal impairment with creatinine clearance < 10 mL/min (calculated by Cockcroft-Gault equation);
• Prior allogeneic hematopoietic stem cell transplantation;
• Expected survival ≤ 3 months;
• Received radiation therapy during conditioning;
• Initiation of letermovir prophylaxis after day 28 post-transplant;
• Letermovir dosage or administration not in accordance with the prescribing information;
• Lack of signed informed consent.