Letermovir Treatment in Pediatric Participants Following Allogeneic Haematopoietic Stem Cell Transplantation (HSCT) (MK-8228-030)

The primary objective of this study is to evaluate the pharmacokinetics (PK) of letermovir (LET) in pediatric participants. Participants will be enrolled in the following 3 age groups: Age Group 1: From 12 to \<18 years of age (adolescents); Age Group 2: From 2 to \<12 years of age (children); and Age Group 3: From birth to \<2 years of age (neonates, infants and toddlers). All participants will receive open label LET for 14 weeks (\~100 days) post-transplant, with doses based on body weight and age.