BackgroundRespiratory syncytial virus (RSV) is responsible for most respiratory tract infections and hospitalizations in infants and represents a significant economic burden for public health. The development of a safe, effective, and affordable vaccine is a priority for the WHO.MethodsWe conducted a double-blinded, escalating-dose phase 1 clinical trial in healthy males aged 18-50 years to evaluate safety, tolerability, and immunogenicity of a recombinant Mycobacterium bovis BCG vaccine expressing the nucleoprotein of RSV (rBCG-N-hRSV). Once inclusion criteria were met, volunteers were enrolled in three cohorts in an open and successive design. Each cohort included six volunteers vaccinated with 5 × 103, 5 × 104, or 1 × 105 CFU, as well as two volunteers vaccinated with the full dose of the standard BCG vaccine. This clinical trial (clinicaltrials.gov NCT03213405) was conducted in Santiago, Chile.FindingsThe rBCG-N-RSV vaccine was safe, well-tolerated, and no serious adverse events related to the vaccine were recorded. Serum IgG-antibodies directed against Mycobacterium and the N-protein of RSV increased after vaccination, which were capable of neutralizing RSV in vitro. Additionally, all volunteers displayed increased cellular response consisting of IFN-γ and IL-2 production against PPD and the N-protein, starting at day 14 and 30 post-vaccination respectively.InterpretationThe rBCG-N-hRSV vaccine had a good safety profile and induced specific cellular and humoral responses.FundingThis work was supported by Millennium Institute on Immunology and Immunotherapy from Chile (P09/016), FONDECYT 1190830, and FONDEF D11E1098.