Abstract
BACKGROUND: Ad26.RSV.preF is an adenovirus serotype 26 vector-based respiratory syncytial virus (RSV) vaccine encoding a prefusion conformation-stabilized RSV fusion protein (preF) that demonstrated robust humoral and cellular immunogenicity and showed promising efficacy in a human challenge study in younger adults. Addition of recombinant RSV preF protein might enhance RSV-specific humoral immune responses, especially in older populations. METHODS: This randomized, double-blind, placebo-controlled, phase 1/2a study compared the safety and immunogenicity of Ad26.RSV.preF alone and varying doses of Ad26.RSV.preF-RSV preF protein combinations in adults aged ≥60 years. This report includes data from cohort 1 (initial safety, n = 64) and cohort 2 (regimen selection, n = 288). Primary immunogenicity and safety analyses were performed 28 days postvaccination (cohort 2) for regimen selection. RESULTS: All vaccine regimens were well tolerated, with similar reactogenicity profiles among them. Combination regimens induced greater humoral immune responses (virus-neutralizing and preF-specific binding antibodies) and similar cellular ones (RSV-F-specific T cells) as compared with Ad26.RSV.preF alone. Vaccine-induced immune responses remained above baseline up to 1.5 years postvaccination. CONCLUSIONS: All Ad26.RSV.preF-based regimens were well tolerated. A combination regimen comprising Ad26.RSV.preF, which elicits strong humoral and cellular responses, and RSV preF protein, which increases humoral responses, was selected for further development. Clinical Trials Registration. NCT03502707.