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 vaccine (from the US National Institutes of Health) has been shown to be safe and immunogenic in RSV-seronegative children in Phase I studies. 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 this 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.

Respiratory syncytial virus (RSV) is the most common cause of lower respiratory tract. infection (LRTI) among infants and young children, resulting in annual epidemics worldwide. Despite more than 50 years of attempted vaccine development, there are no licensed vaccines. While RSV prevention exists in the form of a specific RSV IgG (palivizumab) requiring 5 once monthly injections, it is licensed only for infants who experience the greatest morbidity and mortality from RSV: preterm infants born ≤ 35 weeks GA, children with chronic lung disease of prematurity, children with hemodynamically significant congenital heart disease. EDP-938 is a novel, orally administered, non-fusion replication inhibitor of respiratory syncytial virus (RSV) that is being developed as a potential treatment for RSV infection.