BACKGROUND: This trial evaluated co-administration of the AS01E-adjuvanted respiratory syncytial virus (RSV) prefusion F protein-based vaccine (adjuvanted RSVPreF3) and an Omicron XBB.1.5-based COVID-19 mRNA vaccine in ≥50-year-olds. METHODS: This phase 3, open-label, multi-center trial randomized ≥50-year-olds 1:1 to co-administration (Co-Ad group) or sequential administration approximately one month apart (Control group) of adjuvanted RSVPreF3 and the COVID-19 mRNA vaccine. Primary objectives were to demonstrate non-inferiority of humoral immune responses at one month post-vaccination in terms of RSV-A, RSV-B, and SARS-CoV-2 neutralizing titers, with the upper limit of the 95% confidence interval (CI) of adjusted geometric mean titer (GMT) group ratios (Control/Co-Ad) ≤1.50. Secondary objectives included assessment of reactogenicity up to four days and safety up to six months after vaccination. RESULTS: Overall, 833 participants were vaccinated (Co-Ad: 417; Control: 416). Non-inferiority was achieved for RSV-A (GMT ratio: 1.12 [95% CI: 0.97-1.28]) and RSV-B (GMT ratio: 1.08 [95% CI: 0.94-1.23]), and marginally missed for SARS-CoV-2 Omicron XBB.1.5 (GMT ratio: 1.31 [95% CI: 1.13-1.51]). Within four days post-vaccination, the most frequently reported solicited events were administration-site pain, myalgia, and fatigue. Most solicited events were of mild-to-moderate intensity, and the overall median duration was ≤three days and comparable between groups. Unsolicited and serious adverse events were generally balanced between groups. CONCLUSIONS: Adjuvanted RSVPreF3 co-administered with a COVID-19 mRNA vaccine maintained an acceptable safety profile in ≥50-year-olds. The marginal miss of the non-inferiority criterion for SARS-CoV-2 does not suggest clinically relevant interference. Therefore, the results support the co-administration of these vaccines.