Antigen dosage, immunization doses, and intervals may significantly affect the immunogenicity of MERS-CoV subunit vaccines. Notably, a MERS-CoV RBD (S377-588-Fc) subunit vaccine immunized at 1 μg elicited strong humoral and cellular immune responses and neutralizing antibodies in mice although the one immunized at 5 and 20 μg elicited a higher level of S1-specific antibodies (Tang et al., 2015). In addition, among the regimens at one dose and two doses at 1-, 2-, and 3-week intervals, 2 doses of this protein boosted at 4 weeks resulted in the highest antibodies and neutralizing antibodies against MERS-CoV infection (Wang Y. et al., 2017).