A first-in-human Phase 1/1b randomized trial evaluating the safety and immunogenicity of a respiratory syncytial virus (RSV) virus-like particle subunit vaccine (IVX-121) in healthy young and older adults

Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract disease in all age groups, with substantial burden in older adults. Here, we report the safety and immunogenicity of IVX-121, an investigational virus-like particle vaccine against RSV. This Phase 1/1b trial (EudraCT number: 2020-003633-38) randomized 90 healthy young (18-45 y of age; Phase 1) and 130 older adults (60-75 y of age; Phase 1b) to receive a single intramuscular injection of IVX-121 at low (25 µg), medium (75 µg), or high (250 µg) dose, with/without the adjuvant aluminum hydroxide (Al[OH]₃), or placebo. Outcomes were assessed through Day 180. Older adult participants who had received IVX-121 or placebo could continue into an extension study in which safety was assessed for an additional 6 months before receiving IVX-121 (75 µg, unadjuvanted, open-label) on Day 365; safety and immunogenicity were assessed up to Day 546. Adverse reactions were generally mild, and there were no vaccine-related serious adverse events. All IVX-121 dose levels and formulations boosted preexisting RSV neutralizing antibody (nAb) responses. Although not statistically powered to compare treatment groups, there was no clear dose response and the adjuvant Al(OH)₃ did not appear to improve immunogenicity. For both age groups, IVX-121 induced nAbs against RSV A and B, which remained above baseline levels for at least 6 months. A second dose of IVX-121 in older adults elicited a modest increase in nAbs against RSV A but not RSV B. Overall, IVX-121 had a favorable safety profile and induced immune responses against RSV in young and older adults.