Notably, only SARS-CoV-2 and SARS-CoV RBDs were recognized by SARS-CoV RBD-specific, but not MERS-CoV RBD-specific, polyclonal antibodies (Fig. 1c), whereas only MERS-CoV RBD was recognized by MERS-CoV RBD-immunized polyclonal antibodies (Fig. 1d), suggesting the cross-reactivity of SARS-CoV RBD-specific antibodies with SARS-CoV-2 RBD protein.