Challenges in developing a pediatric RSV vaccine.

RSV bronchiolitis is the leading cause of infant hospitalization in industrialized countries. There is an unmet need to prevent RSV lower respiratory tract infection in young infants. Although many vaccinology approaches, including live attenuated, viral and bacterial vectored and adjuvanted subunit vaccines have been evaluated in rodent and primate models there is currently no approved RSV vaccine. A vaccine candidate for RSV-naive infants must provide immunogenicity in the presence of maternally acquired antibodies, avoid enhanced disease and have minimal reactogenicity. Because live RSV infection does not potentiate for enhanced disease and elicits systemic and mucosal immune responses, live RSV vaccine candidates are currently preferred. Two live attenuated RSV vaccine candidates, rA2cpts248/404/1030/DeltaSH, a temperature sensitive RSV with a deletion of the SH gene, and rb/h PIV3/RSV F2 which has RSV F vectored into a bovine/human chimeric parainfluenza type 3 genome, have recently advanced into clinical studies.