Infants and young children are at increased risk for respiratory syncytial virus (RSV) infections because of their maturing immune system and lack of prior exposure to RSV. A genetically stable live-attenuated RSV ΔNS2/Δ1313/I1314L vaccine (from the US National Institutes of Health) has been shown to be safe and immunogenic in RSV-seronegative children in Phase I studies. Further research is ongoing in RSV-seropositive children. Sanofi conducted a Phase I/II clinical study (VAD00001) with a similar RSV ΔNS2/Δ1313/I1314L vaccine (referred to as RSVt vaccine) to assess the vaccine safety, immunogenicity, and infectivity of the candidate. The results in the VAD00001 study showed an acceptable safety profile and a promising immunogenicity and infectivity profile for the RSVt vaccine candidate. A Phase II study is ongoing to evaluate vaccine virus transmissibility to close contacts of study participants (VAD00014 study). Study VAD00004 will be initiated as part of the Phase III development of the RSVt vaccine. The objective of the study is RSVt clinical efficacy, while also further investigating the safety and immunogenicity of the RSVt vaccine in a global context. Vaccine efficacy against lower respiratory tract disease (LRTD) and upper respiratory tract disease (URTD) will be assessed separately, which will provide evidence of protection against RSV respiratory disease if efficacy against both LRTD and URTD is demonstrated.