The combination of SARS-CoV-2 S and ACE2 of host cells is similar to the combination of SARS-CoV and ACE2, indicating that they have the same mechanism to entry into host cells [97]. The S protein of metastable prefusion conformation undergoes a series of structural rearrangements to combine with the viral membrane of host cells [111,120]. This process consists of the S1 subunit binding to the host cell receptor, triggering of the prefusion trimer's instability, and shedding of the S1 subunit, resulting in a highly stable post-fusion conformation of the S2 subunit [121]. During the binding of the subunit S1 to its cognate receptor, it is important to note that the S1 subunit exists in 2 different states, a “down” conformation and an “up” conformation state, which corresponds to a receptor-inaccessible state and an unstable receptor-accessible state, respectively [[122], [123], [124], [125]]. Unfortunately, there are significant conformational differences between SARS-CoV and SARS-CoV-2 such that the commercially available monoclonal antibodies against SARS-CoV do not react with SARS-CoV-2 [97].