Immunization with the receptor-binding domain of SARS-CoV-2 elicits antibodies cross-neutralizing SARS-CoV-2 and SARS-CoV without antibody-dependent enhancement

The ongoing coronavirus disease 2019 (COVID-19) pandemic is a serious public health crisis, which is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The major structural protein of SARS-CoV-2 is spike (S) protein, and its ectodomain is divided into two subunits, S1 and S2, which are responsible for receptor binding and membrane fusion, respectively. Like the closely related severe acute respiratory syndrome coronavirus (SARS-CoV), SARS-CoV-2 uses human angiotensin-converting enzyme 2 (ACE2) as entry receptor 1 . S protein binds ACE2 through its receptor-binding domain (RBD) located within S1 subunit.