• Percentage of participants experiencing ≥1 adverse events (AEs) [ Time Frame: Up to 24 weeks (from Week 14 post-transplant to Week 38 post-transplant) ]
  An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention.
• Percentage of participants withdrawing from study drug due to an adverse event (AE) [ Time Frame: Up to 14 weeks (from Week 14 post-transplant to Week 28 post-transplant) ]
  An AE is any untoward medical occurrence in a clinical study participant, temporally associated with the use of study intervention, whether or not considered related to the study intervention.
• Percentage of participants with clinically significant CMV infection from Week 14 post-transplant through Week 38 post-transplant [ Time Frame: Up to 24 weeks (from Week 14 post-transplant to Week 38 post-transplant) ]
  The percentage of participants with clinically-significant CMV infection from Week 14 to Week 38 will be determined in each arm.
• Percentage of participants with clinically significant CMV infection from Week 14 post-transplant through Week 48 post-transplant [ Time Frame: Up to 34 weeks (from Week 14 post-transplant to Week 48 post-transplant) ]
  The percentage of participants with clinically-significant CMV infection from Week 14 to Week 48 will be determined in each arm.
• Time to onset of clinically significant CMV infection from Week 14 post-transplant to Week 28 post-transplant [ Time Frame: Up to 14 weeks (from Week 14 post-transplant to Week 28 post-transplant) ]
  The time to onset of clinically-significant CMV infection from Week 14 to Week 28 will be determined in each arm.
• Time to onset of clinically significant CMV infection from Week 14 post-transplant to Week 48 post-transplant [ Time Frame: Up to 34 weeks (from Week 14 post-transplant to Week 48 post-transplant) ]
  The time to onset of clinically-significant CMV infection from Week 14 to Week 48 will be determined in each arm.
• Percentage of participants with pre-emptive therapy (PET) for CMV viremia from Week 14 post-transplant to Week 28 post-transplant [ Time Frame: Up to 14 weeks (from Week 14 post-transplant to Week 28 post-transplant) ]
  The percentage of participants with PET for CMV viremia from Week 14 to Week 28 will be determined in each arm.
• Percentage of participants with pre-emptive therapy (PET) for CMV viremia from Week 14 post-transplant to Week 48 post-transplant [ Time Frame: Up to 34 weeks (from Week 14 post-transplant to Week 48 post-transplant) ]
  The percentage of participants with PET for CMV viremia from Week 14 to Week 48 will be determined in each arm.
• Percentage of participants with all-cause mortality from Week 14 post-transplant to Week 28 post-transplant [ Time Frame: Up to 14 weeks (from Week 14 post-transplant to Week 28 post-transplant) ]
  The percentage of participants with all-cause mortality from Week 14 to Week 28 will be determined.
• Percentage of participants with all-cause mortality from Week 14 post-transplant to Week 48 post-transplant [ Time Frame: Up to 34 weeks (from Week 14 post-transplant to Week 48 post-transplant) ]
  The percentage of participants with all-cause mortality from Week 14 to Week 48 will be determined.
• Time to all-cause mortality from Week 14 post-transplant to Week 28 post-transplant [ Time Frame: Up to 14 weeks (from Week 14 post-transplant to Week 28 post-transplant) ]
  The time to all-cause mortality from Week 14 to Week 28 will be determined.
• Time to all-cause mortality from Week 14 post-transplant to Week 48 post-transplant [ Time Frame: Up to 34 weeks (from Week 14 post-transplant to Week 48 post-transplant) ]
  The time to all-cause mortality from Week 14 to Week 48 will be determined.

• Drug: Letermovir
  LET tablet or intravenous infusion at a total daily dose of 240 mg (when given with cyclosporin A) or 480 mg (when given alone).
  Other Name: PREVYMIS™, MK-8228
• Drug: Placebo
  Placebo was administered as tablets matched to LET or as inactive (saline) intravenous infusion.

• Experimental: Letermovir
  Participants who received HSCT transplant and 100 days of LET prophylaxis were randomized to an additional 100 days of LET (480 mg once daily alone or 240 mg once daily for participants on cyclosporin A) treatment.
  Intervention: Drug: Letermovir
• Placebo Comparator: Placebo
  Participants who received HSCT transplant and 100 days of LET prophylaxis were randomized to an additional 100 days of placebo treatment.
  Intervention: Drug: Placebo

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 contraception during the treatment period and for ≥28 days after the last dose of study drug.

Exclusion Criteria:

• has a history of CMV end-organ disease or preemptive treatment therapy for CMV after HSCT prior to randomization
• has a history of >14 days total of LET interruption during the first 100 days post-transplant prior to randomization
• has suspected or known hypersensitivity to active or inactive ingredients of LET formulations
• has severe hepatic insufficiency defined as Child-Pugh Class C within 14 days prior to randomization.
• has serum aspartate aminotransferase (AST) or alanine aminotransferase (ALT) >5× the upper limit of normal (ULN) within 14 days prior to randomization
• has end-stage renal impairment with a creatinine clearance less than 10 mL/min, as calculated by the Cockcroft-Gault equation using serum creatinine within 14 days prior to randomization
• has both moderate hepatic insufficiency AND moderate-to-severe renal insufficiency
• has an uncontrolled infection on the day of enrollment
• requires mechanical ventilation or is hemodynamically unstable at the time of enrollment
• has a documented positive result for a human immunodeficiency virus antibody (HIV-Ab) test at any time prior to screening, or for hepatitis C virus antibody (HCV-Ab) with detectable HCV ribonucleic acid (RNA), or hepatitis B surface antigen (HBsAg) within 6 months prior to screening.
• has active solid tumor malignancies with the exception of localized basal cell or squamous cell skin cancer or the condition under treatment (eg, lymphomas)
• has received any prohibited medications within 7 days prior to screening
• has received cidofovir or CMV immunoglobulin with 30 days prior to screening
• is currently participating or has participated in a study with an unapproved investigational compound, monoclonal antibody, or device within 28 days or 5× half-life of the investigational compound or monoclonal antibody, whichever is longer, of initial dosing in this study
• has previously participated in this study or any other study involving LET, or is currently participating in any study involving administration of a CMV vaccine or another CMV investigational agent, or is planning to participate in a study of a CMV vaccine or another CMV investigational agent during the course of this study
• is pregnant or expecting to conceive, is breastfeeding, or plans to breastfeed from the time of consent through 28 days after the last dose of study therapy
• is expecting to donate eggs starting from the time of consent through 28 days after the last dose of study therapy
• has clinically relevant drug or alcohol abuse within 12 months of screening that may interfere with participant treatment, assessment, or compliance with the protocol as assessed by the investigator
• has a history or current evidence of any condition, therapy, lab abnormality, or other circumstance that might confound the results of the study, interfere with the participant's participation for the full duration of the study, or would be put at undue risk as judged by the investigator, such that it is not in the best interest of the participant to participate in this study