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Background The aim was to investigate safety and immunogenicity of vaccine formulations against respiratory syncytial virus (RSV) containing the stabilized prefusion conformation of RSV fusion protein (RSVPreF3). Methods This phase I/II, randomized, controlled, observer-blind study enrolled 48 young adults (YA; 18–40 years) and 1005 older adults (OA; 60–80 years) between January and August 2019. Participants were randomized into equally sized groups to receive two doses of unadjuvanted (YA and OA) or AS01-adjuvanted (OA) vaccine or placebo two months apart. Vaccine safety and immunogenicity were assessed until one (YA) or 12 months (OA) after second vaccination. Results The RSVPreF3 vaccines boosted humoral (RSVPreF3-specific IgG and RSV-A neutralizing antibody) responses, which increased in an antigen-concentration-dependent manner and were highest post-dose one. Compared to pre-vaccination, the geometric mean frequencies of polyfunctional CD4+ T-cells increased after each dose and were significantly higher in adjuvanted than unadjuvanted vaccinees. Post-vaccination immune responses persisted until end of follow-up. Solicited adverse events (AEs) were mostly mild-to-moderate and transient. Despite a higher observed reactogenicity of AS01-containing vaccines, no safety concerns were identified for any assessed formulation. Conclusions Based on safety and immunogenicity profiles, the AS01E-adjuvanted vaccine containing 120 μg of RSVPreF3 was selected for further clinical development. Trial registration ClinicalTrials.gov NCT03814590; URL: https://clinicaltrials.gov/ct2/show/NCT03814590

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