A Phase 3 Study to Investigate the Safety, Tolerability, and Immunogenicity of Revaccinating Pregnant Participants During Subsequent Pregnancies and Persistence of Immunity of a Single Dose of a Bivalent Respiratory Syncytial Virus (RSV) Vaccine.

Descripción detallada

This study will include pregnant participants and their unborn babies (Cohort 1 and Cohort 2) and non-pregnant participants (Cohort 3), as described below: * Cohort 1- Pregnant participants who had previously received RSVpreF as part of a Pfizer clinical trial and have antibody data available from that trial will get a second dose of RSVpreF. * Cohort 2- Pregnant participants who received the RSVpreF in a previous pregnancy, either through commercial means with Abrysvo® or as part of a Pfizer clinical trial, will be randomly chosen to get either one dose of the RSVpreF or one dose of a placebo. This applies to those who do not have antibody data available from the earlier trial. For both groups of pregnant participants, the study will look at how safe and well-tolerated the vaccine is during pregnancy, as well as how the body responds to it. Infants will be monitored for six months after birth to check safety and antibodies level. • Cohort 3- Nonpregnant participants who received RSVpreF during a previous pregnancy as part of an earlier clinical trial will have blood samples taken to see how well their immune response has persisted over time. Participants will not receive any vaccine.