Design and identification of vaccine candidates based on DNA and VTT vectors. A Schematic representation of the recombinant VTT vaccine expressing the MERS-S gene (the vaccine termed VTT-S); the white areas on the MERS-S gene represent HR1 and HR2 respectively, the green and black areas represent RBD and transmembrane, and the grey represents the histidine tag. B VTT-S vaccine was indirectly identified by fluorescence microscopy. Scale bar: 100 μm. C Schematic representation of the recombinant DNA vaccines with expressing the MERS-S1 gene or MERS-S△TD (the vaccines termed DNA-S1 and DNA-S△TD, respectively); the grey area at the end of S1 signifies the histidine tag, while the blue area at the end of S△TD represents the flag tag. D Vaccine candidates based on DNA and VTT vectors were identified by Western blot. Lane 1 represented 293F cell, line 2 represented 293F which transfected DNA-p3.0, line 3 represented 293F which transfected DNA-S1, line 4 represented 293F which transfected DNA-S△TD, line 5 and 7 represented marker, line 6 represented CEFs which infected VTT-S vaccine, line 8 represented CEFs and line 9 represented CEF which infected VTT (752-1). E Immunization strategies of vaccine candidates based on DNA and VTT vectors.