Safety and Immunogenicity of a ChAd155-Vectored Respiratory Syncytial Virus (RSV) Vaccine in Healthy RSV-Seropositive Children 12-23 Months of Age

Background: Safe and effective respiratory syncytial virus (RSV) vaccines remain elusive. This was a phase I/II trial (NCT02927873) of ChAd155-RSV, an investigational chimpanzee adenovirus-RSV vaccine expressing 3 proteins (fusion, nucleoprotein, and M2-1), administered to 12-23-month-old RSV-seropositive children followed up for 2 years after vaccination. Methods: Children were randomized to receive 2 doses of ChAd155-RSV or placebo (at a 1:1 ratio) (days 1 and 31). Doses escalated from 0.5 × 1010 (low dose [LD]) to 1.5 × 1010 (medium dose [MD]) to 5 × 1010 (high dose [HD]) viral particles after safety assessment. Study end points included anti-RSV-A neutralizing antibody (Nab) titers through year 1 and safety through year 2. Results: Eighty-two participants were vaccinated, including 11, 14, and 18 in the RSV-LD, RSV-MD, and RSV-HD groups, respectively, and 39 in the placebo groups. Solicited adverse events were similar across groups, except for fever (more frequent with RSV-HD). Most fevers were mild (≤38.5°C). No vaccine-related serious adverse events or RSV-related hospitalizations were reported. There was a dose-dependent increase in RSV-A Nab titers in all groups after dose 1, without further increase after dose 2. RSV-A Nab titers remained higher than prevaccination levels at year 1. Conclusions: Three ChAd155-RSV dosages were found to be well tolerated. A dose-dependent immune response was observed after dose 1, with no observed booster effect after dose 2. Further investigation of ChAd155-RSV in RSV-seronegative children is warranted. Clinical Trials Registration: NCT02927873.