Study of an RSV-hMPV-PIV3 Trivalent Vaccine Candidate VXB-251 in Older Adults

This study is being done to find out how safe and effective a new combined vaccine candidate, called VXB-251, is for older adults. The vaccine candidate is designed to protect against three common viruses that can cause respiratory tract infections: * RSV (respiratory syncytial virus) * hMPV (human metapneumovirus) * PIV3 (parainfluenza virus type 3) Two components of this vaccine (RSV and hMPV) have already been tested in people before, as part of another study for a two-in-one vaccine. However, this is the first time that the PIV3 component and all three components together (RSV, hMPV, and PIV3) are being tested in people. The vaccine candidate will be given as a single intramuscular injection. The study will also test unlicensed comparator vaccines and a placebo (a substance that looks like the real vaccine but doesn't contain any active ingredients) that target none, one or two of these viruses to see whether combining all three components affects safety or how well the immune system responds.

This is a multicenter randomized, placebo- and comparator-controlled, dose-ranging study to be conducted in Australia in older adults, aged 60 to 83 years, to evaluate the safety, reactogenicity, and immunogenicity of a trivalent RSV/hMPV/PIV3 vaccine candidate, VXB-251. All investigational medicinal products (IMPs) will be administered as a single 0.5 mL intramuscular injection on day 1. Recruitment will be in 2 stages: Stage 1 (N=10). Two cohorts, each of 5 participants, will be sequentially enrolled at a 4:1 ratio to receive: * Cohort 1: either a medium dose of the vaccine candidate or the placebo control, * Cohort 2: either a high dose of the vaccine candidate or the placebo control At each enrolling site, at least 1 hour must elapse between IMP injection in the first sentinel and next IMP injection to monitor for hypersensitivity reactions and other fast-onset adverse events (AEs). The investigator or delegate will decide if and when the next sentinel can be vaccinated. A Safety Monitoring Committee (SMC) will make recommendations on escalation from 1 sequential cohort to the next and progress from Stage 1 to Stage 2 based on 1-week safety and reactogenicity available data from the prior cohort. Stage 2 (N=230). Participants will be concurrently assigned on day 1 (Visit 2) at an unequal ratio into 1 of 8 study groups and receive one of the following: * VXB-251 trivalent (RSV/hMPV/PIV3) vaccine candidate, low dose (N=30) * VXB-251 trivalent (RSV/hMPV/PIV3) vaccine candidate, medium dose (N=26) * VXB-251 trivalent (RSV/hMPV/PIV3) vaccine candidate, high dose (N=26) * VXB-241 bivalent (RSV/hMPV) unlicensed comparator (N=30) * VXB-213 monovalent (RSV) unlicensed comparator (N=30) * VXB-221 monovalent (hMPV) unlicensed comparator (N=30). * VXB-232 monovalent (PIV3) unlicensed comparator (N=30) * Placebo (N=28) In Stage 1, the study will be open-label across cohorts and observer-blind within each cohort. In Stage 2, the study will be observer-blind. The study duration for each participant will be 1 year.