A Confirmatory, Dual Center Nonhuman Primate Study on the Efficacy of Novel Oropharyngeal Spray Immunization With an Adenoviral Vector Vaccine Against RSV-Important Lessons Learned.

In a randomized, blinded, dual center study across two European primate research facilities, we evaluated the immunogenicity and protective efficacy of a DNA prime-oropharyngeal adenoviral boost vaccination strategy against respiratory syncytial virus (RSV). Rhesus macaques received an RSV fusion protein (F) DNA vaccine via electroporation, followed by two mucosal administrations of a recombinant adenovirus vector encoding the same antigen. Mock-vaccinated animals and a comparator group mimicking natural immunity induced by primary RSV infection were included as controls. Systemic and mucosal immune responses were monitored longitudinally. We detected robust cellular and humoral immune responses, but with some variability between the two centers. Nevertheless, RSV-vaccinated animals demonstrated markedly reduced viral replication in the upper and lower respiratory tract after RSV challenge infection. Unexpected RSV-specific immunity in the control group at one single study site prevented confirmation of the predefined primary endpoint. Our results support the potential of mucosal adenoviral boosting following DNA priming to induce protective immunity against RSV, while highlighting challenges associated with multi-center preclinical vaccine studies.