This study is about a vaccine called CMV-MVA Triplex. It aims to help people who are getting a stem cell transplant avoid a virus called cytomegalovirus (CMV). The vaccine is given to donors, who are people giving stem cells. The vaccine helps the donor's body fight CMV, and this protection is passed to the recipient, or patient, during the transplant.

In this study, donors are divided into two groups. One group gets the vaccine, and the other gets a placebo, which is a shot that doesn't have the vaccine. After receiving the vaccine or placebo, donors go through a process to collect their stem cells. Meanwhile, patients are prepared for the transplant and receive the stem cells within nine weeks.

• Time Commitment: Donors follow up for a year; recipients have several follow-ups within a year post-transplant.
• Eligibility: Requires informed consent and specific health criteria for both donors and recipients.
• Risks: Participants should discuss potential risks with their healthcare providers.