The life cycle of the virus includes early-stage events and later-stage events (Figure 1a,b). In the first stage, the virus utilizes its spike (S) protein to bind to angiotensin converting enzyme 2 (ACE2) on the host cell membrane [13,14]. The virus enters the host cell after the spike S protein-ACE2 complex is proteolytically activated by transmembrane protease serine 2 (TMPRSS2) (see (b) in Figure 1), which eventually permits the virus-host cell fusion and the release of the viral RNA genome [15]. Alternatively, the bound virus spike S protein can also be proteolytically activated by furin [16]. Further processing is promoted by cathepsins in (endo)lysosomes to ultimately aid in the viral envelope fusion with the host membranes and the release of the viral genome (see (a) in Figure 1) [17].