To evaluate the efficacy of CMV-specific T cell immunity test in prolonged usage of letermovir for avoiding late-onset csCMVi after all-HSCT.

Reactivation of cytomegalovirus (CMV) leads to significant morbidity and mortality following allogeneic hematopoietic stem cell transplantation (allo-HSCT). Letermovir (LTV) has substantially reduced the risk of clinically significant CMV infection (csCMVi) in CMV seropositive recipients of allo-HSCT. LTV discontinuation after day 100 (d100) has been reported to increase the risk of late-onset csCMVi, causing by impaired reconstitution of CMV-specific T immunity. The investigator sought to decrease the probability of CS-CMVi after letermovir withdrawal. Restoration of CMV-specific T cells is imperative for effective control of CMV reactivation following allo-HSCT. Letermovir has been found impending recovery of CMV-specific T immunity. The investigators' retrospective study has proved that lower CMV-specific CD4+ T cells (<2.01 cells/µL) at week 8 increased the risk of late-onset CMV reactivation (50.0%) compared to the higher ones (7.69%, p=0.04) in letermovir prophylaxis. Thus, the guidance of CMV-specific cell immunity is recommended in letermovir prophylaxis.

Therefore, the investigator conduct a multicenter, randomized, controlled study based on retrospective research to further explore and validate the efficacy of CMV-specific T cell immunity test guiding the prolonged usage of letermovir.

Inclusion Criteria:

• first allogeneic hematopoietic stem cell transplantation;
• 18-70 years old;
• use cytomegalovirus prophylaxis with letemovir after allo-HSCT;
• CMV Ig G D+/R+;

Exclusion Criteria:

• Allergy, known hypersensitivity to letermovir tablet or injection components;
• CMV DNAemia within six months before transplantation or previous CMV disease;
• Presence of organ failure and inability to tolerate allogeneic hematopoietic stem cell transplantation;
• Second transplantation;
• Combination of immunodeficiency diseases;
• Those judged by the investigator to be unsuitable for participation in this trial.