A Study to Assess the Safety, Tolerability, and Immunogenicity of RSVpreF in Adults at High Risk of Severe RSV Disease

The purpose of this study is to learn about the safety and immunogenicity of a study vaccine (called RSVpreF) in several adult groups. Respiratory Syncytial Virus (RSV) is a common type of virus (germ) that can cause severe illness, where medical help is needed. RSV can lead to airway diseases in all ages. Vaccines help your body make antibodies which help fight against diseases. This is called an immune response. This study will measure how much antibody participants make after receiving RSVpreF (immunogenicity). The study consists of 2 groups (Substudy A and Substudy B). Substudy A is seeking approximately 675 participants who are: * Between 18 and 60 years of age. * Considered having a high likelihood of severe RSV disease due to certain long-term medical conditions. Such medical conditions do not include immunocompromising conditions. Participants will need to come to the study clinic at least 2 times. At the first clinic visit, participants will receive 1 shot of RSVpreF or placebo in the arm by chance. A placebo looks like the study vaccine but contains no active ingredients. At each clinic visit, a blood sample will be taken. A third (final) visit can be either completed in clinic or via telephone contact. This study is about 6 months long for each participant. Substudy B is seeking approximately 200 participants who are: * At least 18 years of age. About half of the participants will be at least 60 years of age. * Considered having a weakened immune system (immunocompromised). Participants will need to come to the study clinic at least 3 times. All participants will receive a shot of RSVpreF at the first study clinic visit. The second study clinic visit will be 1 month later. All participants will receive a second shot of the study vaccine at this second study clinic visit. Blood samples will be taken at the 3 study clinic visits. A fourth (final) visit can be either completed in clinic or via telephone contact. This study is about 7 months long for ea