Although subunit vaccines based on the full-length S protein may elicit potent immune responses and/or protection, studies have found that antibodies induced by some of these vaccines mediate enhancement of viral infection in vitro, as in the case of SARS-CoV (Kam et al., 2007; Jaume et al., 2012), raising safety concerns for the development of full-length S protein-based subunit vaccines against SARS-CoV and MERS-CoV. This evidence concerns the gene PROS1 and viral infectious disease.