Respiratory syncytial virus (RSV) is the most important pathogen for lower respiratory tract infection in infants, but there are no approved vaccines. RSV live attenuated vaccines (LAV) are the most clinically advanced candidates but have two main problems, lack of potency and genetic instability. This study will compare two novel RSV LAVs with improved potency and stability in relevant pre-clinical models, in preparation for taking a lead candidate to clinical trial.