• Area under the concentration-time curve of plasma letermovir for oral formulation [ Time Frame: Day 7: Pre-dose, 1, 2.5, 8, and 24 hours post-dose ]
  Area under the curve from time 0 to 24 hours post-dose (AUC0-24) of plasma letermovir during intensive pharmacokinetics (PK), for participants receiving oral formulation.
• Maximal concentration of plasma letermovir for oral formulation [ Time Frame: Day 7: Pre-dose, 1, 2.5, 8, and 24 hours post-dose ]
  Maximal concentration (Cmax) of plasma letermovir during intensive PK, for participants receiving oral formulation.
• Minimum concentration of plasma letermovir observed before next dose for oral formulation [ Time Frame: Day 7: Pre-dose ]
  Minimum concentration of plasma letermovir observed before next dose (Ctrough) during intensive PK, for participants receiving oral formulation.
• Area under the concentration-time curve of plasma letermovir for intravenous formulation [ Time Frame: After 5 consecutive days of administration of intravenous formulation: Pre-dose, 1, 2.5, 8, and 24 hours post-dose (up to 14 weeks) ]
  Area under the curve from time 0 to 24 hours post-dose (AUC0-24) of plasma letermovir, for participants receiving intravenous (IV) formulation.
• Concentration at the end of infusion of plasma letermovir for IV formulation [ Time Frame: After 5 consecutive days of administration of IV formulation: 1 hour post-dose (up to 14 weeks) ]
  Concentration of plasma letermovir at the end of infusion (Ceoi), for participants receiving IV formulation.
• Minimum concentration of plasma letermovir observed before next dose for IV formulation [ Time Frame: After 5 consecutive days of administration of IV formulation: Pre-dose (up to 14 weeks) ]
  Minimum concentration of plasma letermovir observed before next dose (Ctrough) during intensive PK, for participants receiving IV formulation.
• Minimum concentration of plasma letermovir observed before next dose during sparse PK for oral formulation [ Time Frame: Pre-dose on Weeks 2, 4, 6, 8, 12, 14 ]
  Minimum concentration of plasma letermovir observed before next dose (Ctrough) during sparse PK, for participants receiving oral formulation.
• Minimum concentration of plasma letermovir observed before next dose during sparse PK for IV formulation [ Time Frame: Pre-dose on Weeks 2, 4, 6, 8, 12, 14 ]
  Minimum concentration of plasma letermovir observed before next dose (Ctrough) during sparse PK, for participants receiving IV formulation.

• Participants with an adverse event [ Time Frame: Up to Week 48 post-transplant (up to 52 weeks) ]
  Percentage of participants with one or more adverse event (AE).
• Participants who discontinued study medication [ Time Frame: Up to Week 14 post-transplant (up to 18 weeks) ]
  Percentage of participants who discontinued study medication due to an AE.
• Participants with clinically significant CMV infection through Week 14 post-transplant [ Time Frame: Up to Week 14 post-transplant (up to 18 weeks) ]
  Percentage of participants with clinically significant CMV infection (CS-CMVi) through Week 14 post-transplant
• Participants with clinically significant CMV infection through Week 24 post-transplant [ Time Frame: Up to Week 24 post-transplant (up to 28 weeks) ]
  Percentage of participants with clinically significant CMV infection (CS-CMVi) through Week 24 post-transplant
• Score on a palatability scale for participants receiving oral granules. [ Time Frame: On the first and eighth day of administration of oral formulation up to Week 14 post-transplant (up to 18 weeks) ]
  Palatability was measured by a facial hedonic scale (FHS), a five point facial expression scale depicting various degrees of pleasure, where a score of 1 indicated disliked very much; ranging to a score of 5 which indicated liked very much.

• Drug: Letermovir oral granules
  Granules administered orally based on age, weight, and whether participant takes cyclosporin A as a concomitant medication.
  Other Names:

  • MK-8228
  • AIC246
  • AIC001
• Drug: Letermovir tablet
  Tablet administered orally based on age, weight, and whether participant takes cyclosporin A as a concomitant medication.
  Other Names:

  • MK-8228
  • AIC246
  • AIC001
• Drug: Letermovir intravenous
  Letermovir administered intravenously based on age, weight, and whether participant takes cyclosporin A as a concomitant medication.
  Other Names:

  • MK-8228
  • AIC246
  • AIC001

Inclusion Criteria:

• All participants 12 to <18 years old must have documented positive CMV serostatus (CMV IgG seropositive) for the recipient (R+). Participants from birth to <12 years old must have documented positive CMV serostatus (CMV IgG seropositive) for the recipient (R+) and/or the donor (D+) and the time of screening.
• Is the recipient of a first allogeneic HSCT (bone marrow, peripheral blood stem cell, or cord blood transplant).
• Has undetectable CMV DNA from a plasma or whole blood sample collected within 5 days prior to enrollment.
• Is within 28 days post-HSCT at the time of enrollment.
• Females are not pregnant, not breastfeeding,and is not a woman of childbearing potential (WOCBP); or is a WOCBP who agrees to follow the contraceptive guidance during the treatment period and for at least 28 days after the last dose of study intervention.
• Participants from 2 to <18 years of age must not be on concomitant Cyclosporin A (CsA), and must be able to take LET tablets or the oral granules (either by mouth or via G tube/NG tube), provided the participant does not have a condition that may interfere with the absorption of oral medication (e.g. vomiting, diarrhea, or a malabsorptive condition) from the day of enrollment until the intensive PK sampling is completed in these participants

Exclusion Criteria:

• Has received a previous allogeneic HSCT (Note: receipt of a previous autologous HSCT is acceptable).
• Has a history of CMV end-organ disease within 6 months prior to enrollment.
• Has evidence of CMV viremia at any time from either signing of the ICF or the HSCT procedure, whichever is earlier, until the time of enrollment.
• Has suspected or known hypersensitivity to active or inactive ingredients of LET formulations.
• Has severe hepatic insufficiency within 5 days prior to enrollment.
• Is on hemodialysis or has end-stage renal impairment.
• 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 (HIVAb) test at any time prior to enrollment, or for hepatitis C virus antibody (HCV-Ab) with detectable HCV RNA, or hepatitis B surface antigen (HBsAg) within 90 days prior to enrollment.
• Has active solid tumor malignancies with the exception of localized basal cell or squamous cell skin cancer or the condition under treatment (e.g. lymphomas).
• Has a preexisting cardiac condition a) for which the patient is currently being treated or b) which required hospitalization within the last 6 months or c) that may be expected to recur during the course of the trial.
• Has received within 7 days prior to screening any of the following: ganciclovir; valganciclovir; foscarnet; acyclovir; valacyclovir; famciclovir.
• Has received within 30 days prior to screening of any of the following: cidofovir; CMV immunoglobulin; any investigational CMV antiviral agent/biologic therapy; Rifampin and other strong inducers (such as phenytoin, carbamazepine, St John's wort (Hypericum perforatum), rifabutin and phenobarbital) and moderate inducers such as nafcillin, thioridazine, modafinil and bosentan.
• Has received LET at any time prior to enrollment in this study.
• Is currently participating or has participated in a study with an unapproved investigational compound or device within 28 days, or 5X half-life of the investigational compound (excluding monoclonal antibodies), whichever is longer, of initial dosing in this study.
• Has previously participated in this study or any other study involving LET.
• Has previously participated 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 intervention.
• Is expecting to donate eggs starting from the time of consent through 28 days after the last dose of study intervention.
• 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.