In agreement with what we found in some subjects with a history of endemic HCoV infection, the patient with severe COVID-19 had detectable antibodies that cross-reacted with peptides from SARS-CoV and nonhuman CoVs encoding the furin-like S2′ cleavage site and heptad repeat 2 region of the S protein, peptides from the C-terminal region of the HCoV-HKU1 N protein downstream of the dimerization domain, as well as 2 antigenic sites of the MERS-CoV pp1a (Supplemental Figure 8). This evidence concerns the gene PROS1 and COVID-19.