FURIN and COVID-19: Furthermore, the amino acid sequences connecting S1 and S2 subunits in the hCoV spike are variable, and the ‘PRRA’ furin cleavage motif of SARS-CoV-2 plays critical roles in enhancing infectivity and COVID-19 pathogenesis (Coutard et al., 2020; Hatmal et al., 2020; Johnson et al., 2021).