Safety and Immunogenicity of the Respiratory Syncytial Virus Vaccine RSV/ΔNS2/Δ1313/I1314L in RSV-Seronegative Children
Respiratory syncytial virus (RSV) is the most important cause of severe acute lower respiratory illness (LRI) in infants and children worldwide, and the relative importance of RSV has increased as the burden of bacterial pneumonia has declined with vaccine implementation. According to global estimates, RSV caused approximately 33 million cases of LRI and approximately 118 000 deaths in children <5 years of age in 2015. More than 80% of all RSV-associated LRIs (RSV-LRIs) and more than half of the RSV-associated deaths in low- and middle-income countries were estimated to occur in infants ≥6 months old, highlighting the importance of developing RSV vaccines for active immunization of infants and children.
Live attenuated intranasal RSV vaccines are attractive for pediatric immunization because they mimic mild natural infections and induce durable cellular, humoral, local and systemic immunity. Furthermore, candidate live attenuated RSV vaccines have not caused the vaccine-associated enhanced RSV disease that was observed in children who received formalin-inactivated RSV  and that also seemed to be associated with administration of RSV subunit vaccines in experimental animals.