Adjuvants play an essential role in enhancing host immune responses to MERS-CoV subunit vaccines, including those based on the RBD, and different adjuvants can promote host immune responses to variant levels (Lan et al., 2014; Zhang et al., 2016). For example, while a MERS-CoV RBD subunit vaccine (S377-588 protein fused with Fc) alone induced detectable neutralizing antibody and T-cell responses in immunized mice, inclusion of an adjuvant enhanced its immunogenicity. Particularly, among the adjuvants (Freund’s, aluminum, Monophosphoryl lipid A, Montanide ISA51 and MF59) conjugated with this RBD protein, MF59 could best potentiate the protein to induce the highest-titer anti-S antibodies and neutralizing antibodies, protecting mice against MERS-CoV infection (Zhang et al., 2016). Moreover, a recombinant RBD (rRBD) protein plus alum and CpG adjuvants elicited the highest neutralizing antibodies against pseudotyped MERS-CoV infection, whereas the strongest T-cell responses were induced by this protein plus Freund’s and CpG adjuvants (Lan et al., 2014).