ACE2 and COVID-19: Several studies have established that levels of serum antibodies that bind to the S1 domain of the SARS-CoV-2 spike protein (S), which contains the angiotensin-converting enzyme 2 (ACE2) receptor binding domain (RBD), and the in vitro virus neutralizing ability of serum antibodies are correlates of protection against the development of severe COVID-19 following infection [5,17,18].