This study is looking at a vaccine called RSVpreF for adults with weakened immune systems. A vaccine is a shot that helps your body fight off certain diseases. RSV stands for respiratory syncytial virus, which can make people very sick. The study will check if getting the RSVpreF vaccine again after 1, 2, or 3 years is safe and works well.

People in the study will get one shot in their arm. Everyone in the study is at least 18 years old and has a weakened immune system. The study will last about 12 months and requires at least 4 visits to the study site.

• The study lasts 12 months with at least 4 visits.
• All participants receive one RSVpreF shot.
• Participants must have a weakened immune system.

Not everyone can join. People with certain past health issues, like severe vaccine reactions or specific treatments, cannot participate. For more details about joining, contact the study team.