Extension of Letermovir (LET) From Day 100 to Day 200 Post-transplant for the Prevention of Cytomegalovirus (CMV) Infection in Hematopoietic Stem Cell Transplant (HSCT) Participants (MK-8228-040)

Inclusion Criteria: * have documented positive CMV serostatus (CMV immunoglobulin G \[IgG\] seropositive) for recipient (R+) at the time of transplant * has a history of allogeneic HSCT (bone marrow, peripheral blood stem cell, or cord blood transplant) within \~100 days prior to randomization * has undetectable CMV deoxyribonucleic acid (DNA) or detectable/not quantifiable CMV DNA from a plasma sample collected within 14 days prior to randomization * has received LET as primary prophylaxis that started within 28 days of HSCT and continued through Week 14 post-transplant (± 1 week) prior to randomization * is at high risk of CMV disease, defined as meeting one or more of the following criteria: * has a related donor with at least 1 mismatch at 1 of the specified 3 human leukocyte antigen (HLA) gene loci (HLA-A, B, or DR) * has an unrelated donor with at least one mismatch at one of the specified four HLA gene loci (HLA-A, B, C, and DRB1) * has a haploidentical donor * has umbilical cord blood as the stem-cell source * has ex-vivo T-cell-depleted grafts * has received anti-thymocyte globulin * has received alemtuzumab * has graft versus host disease (GVHD) or other conditions, requiring the use of systemic prednisone (or equivalent) at a dose of ≥1 mg/kg of body weight per day within 6 weeks of randomization * for female participants, is not pregnant or breastfeeding, and is either not a woman of childbearing potential (WOCBP) or is a WOBCP who agrees to use acceptable contra…