As described above, MERS-CoV subunit vaccines containing different antigens or fragment lengths, particularly those based on the RBD, have apparently variable immunogenicity and/or protective efficacy, and a critical neutralizing domain that contains an RBD fragment corresponding to residues 377–588 of S protein elicits the highest neutralizing antibodies among several fragments tested (Ma et al., 2014b; Zhang et al., 2015).