TMPRSS2 and viral infectious disease: Our cell fusion assay, first described by Simmons et al (2004), detects host‐cell‐spike interactions on a shorter time scale than the viral infection assay and has been used by several groups to investigate the mechanisms of cell entry of SARS‐CoV‐1, such as endosomal and protease involvement including TMPRSS2 (Matsuyama et al, 2005; Matsuyama et al, 2010).