Nonclinical phenotypic and genotypic analyses of a Phase 1 pediatric respiratory syncytial virus vaccine candidate MEDI-559 (rA2cp248/404/1030ΔSH) at permissive and non-permissive temperatures

► RSV vaccine MEDI-559 has 39 nt silent differences compared to a previous candidate. ► Variants of temperature-sensitive (ts) MEDI-559 can occur at elevated temperatures. ► Most variants involved changes at one or two ts markers called 248 and 1030. ► One variant maintained all ts and cp markers and had 2 novel changes in polymerase. ► All variants were highly attenuated in cotton rats.