Despite the high disease burden of respiratory syncytial virus (RSV) in older adults, there is no approved vaccine. We evaluated the experimental RSV vaccine, Ad26.RSV.preF, a replication-incompetent adenovirus 26 vector encoding the F protein stabilized in prefusion conformation.

This phase 1 clinical trial was performed in healthy adults aged ≥60 years. Seventy-two participants received 1 or 2 intramuscular injections of low-dose (LD; 5 × 10¹⁰ vector particles) or high-dose (HD; 1 × 10¹¹ vector particles) Ad26.RSV.preF vaccine or placebo, with approximately 12 months between doses and 2-year follow-up for safety and immunogenicity outcomes.

Solicited adverse events were reported by 44% of vaccine recipients and were transient and mild or moderate in intensity. No serious adverse events were related to vaccination. After the first vaccination, geometric mean titers for RSV-A2 neutralization increased from baseline (432 for LD and 512 for HD vaccine) to day 29 (1031 for LD and 1617 for HD). Pre-F–specific antibody geometric mean titers and median frequencies of F-specific interferon γ–secreting T cells also increased substantially from baseline. These immune responses were still maintained above baseline levels 2 years after immunization and could be boosted with a second immunization at 1 year.

Ad26.RSV.preF (LD and HD) had an acceptable safety profile and elicited sustained humoral and cellular immune responses after a single immunization in older adults.