The Use of Cytomegalovirus Cell Mediated Immunity to Optimize the Duration of Letermovir Prophylaxis in Hematopoietic Cell Transplant Recipients

Inclusion criteria 1. Allogeneic HCT recipients with positive CMV serostatus 2. On letermovir prophylaxis at day 90 post transplant (+/- 7 days) 3. At high risk for CMV reactivation after day +100: 1. Prior or active graft versus host disease requiring systemic steroids 2. Mismatch stem cell donor (includes haploidentical, mismatch unrelated donor (MMUD), match related donor with at least one mismatch at one of the three specified HLA gene loci (HLA-A, HLA-B, or HLA-DR) and cord donor recipients) 3. Received T cell depletion or anti thymoglobulin during conditioning 4. CMV reactivation prior to day 100 post transplant 5. On steroids at any dose within 2 weeks of enrollment Exclusion criteria 1. Patients under the age of 18 2. Patients are discharged from our institution and unwilling to come back for follow up 3. Patients are actively undergoing treatment for CS-CMVi at time of screening. Prior CS-CMVi is not an exclusion from study. 4. Patients are allergic or intolerant to letermovir or have history of letermovir resistant CMV infection. 5. Not able to procure letermovir for extended prophylaxis beyond day +100.