This clinical study is testing two ways to prevent a virus called cytomegalovirus (CMV) from causing illness in adults who have just received a kidney transplant from a donor who had CMV. **CMV** is a common virus that can be harmful for transplant patients. Participants will be divided into two groups: one group will receive *antiviral prophylaxis* (AP) with a drug like Valganciclovir or Letermovir, while the other will be monitored regularly and only treated (preemptive therapy, PET) if they show signs of the virus. The study will last for up to 5.5 years, with close follow-up on health and safety. **Key Points:** - Participants will be randomly assigned to one of the two treatment groups. - The study will last up to 5.5 years with regular check-ups. - Women of childbearing age and men must use birth control during parts of the study. **Eligibility:** Adults 18+ who received a kidney from a CMV-positive donor, and must meet specific health criteria. Participants cannot join if they have certain health conditions or allergies or are in another clinical trial.