In this study, doctors are testing a new vaccine called **CMV-MVA Triplex** to see if it helps protect patients who have a special type of stem cell transplant called **haploidentical hematopoietic stem cell transplant** (haploHCT). A stem cell transplant is a procedure where healthy blood-forming cells are given to a patient to replace their damaged ones. The vaccine is designed to boost the body's defense against a virus called **cytomegalovirus (CMV)**, which can cause serious infections in people with weakened immune systems, like those undergoing transplants. **Key Points:** - The study involves multiple visits and vaccinations for both donors and recipients over about a year. - Participants will have regular blood tests to check their response to the vaccine. - Safety and effectiveness of the vaccine will be closely monitored, with some potential risks or side effects. The study helps understand if this vaccine can be a safe and effective way to protect transplant patients from CMV, reducing complications and improving their recovery.