A Clinical Study of Letermovir (MK-8228) in Children and Adolescents Who Receive a Kidney Transplant (KT) (MK-8228-077)

Researchers are looking for a way to prevent cytomegalovirus (CMV) in children and adolescents who receive a kidney transplant (KT) and weigh less than 40 kilograms (88.2 pounds). The goals of the study are to: * Learn what happens to letermovir in the body over time * Learn about the safety of letermovir and if participants tolerate it

Inclusion Criteria: The main inclusion criteria include but are not limited to the following: * Is a recipient of a primary or secondary allograft kidney * Is at least 4 weeks posttransplant and not more than 52 weeks posttransplant at the time of enrollment (Day 1) and is being managed per local standard of care * Has stable kidney function posttransplant * Has undetectable CMV deoxyribonucleic acid (DNA) from a plasma or whole blood sample collected within 14 days prior to enrollment * Must be able to take (as assessed by the investigator) letermovir tablets or oral pellets by mouth, or via gastrostomy or nasogastric tube (oral pellets only) * 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 (Day 1) until the intensive pharmacokinetics (IPK) sampling is completed * Weighs ≥2.5 and \<40 kg at enrollment (Day 1) Exclusion Criteria: The main exclusion criteria include but are not limited to the following: * Has CMV disease or suspected CMV disease between screening and enrollment * Is on dialysis or plasmapheresis at the time of enrollment * Has evidence of CMV viremia at any time from screening until the time of enrollment * Has Child-Pugh B or C hepatic insufficiency within 14 days before enrollment * Is a multi-organ transplant recipient (e.g., kidney-pancreas) * Has any uncontrolled infection on the day of enrollment * Requires mechanical ventilation, or is hemodynamically unstable, at the time of enrollment * Has received or is receiving protocol-specified prohibited medications