The cotton rat (Sigmodon hispidus) is semi-permissive for RSV infection and has been used to model RSV vaccine-associated enhanced respiratory disease (VERD) and to evaluate candidate RSV vaccines for immunogenicity, efficacy and safety.41,42 We selected the mRNA vaccine candidates encoding sF, mF, sDS-Cav1, and mDS-Cav1 for further characterization in this model to allow additional characterization of the transmembrane and secreted forms of the DS-Cav1 antigen, in comparison with unmodified RSV F transmembrane and secreted forms. This evidence concerns the gene SDS and respiratory system disorder.