TMPRSS2 and infection: Shi et al. propagated porcine epidemic diarrhea virus (PEDV) strain NJ supplemented with 5 μg/mL trypsin [32]; Wicht et al. propagated and titrated PEDV strain CV777 containing 15 μg/ml trypsin to investigate the impact of trypsin on PEDV S [22], and Shirato et al. propagated PEDV strain MK in Vero cells adding 2.5 μg/ml of trypsin and compared the effect of proteases (trypsin and TMPRSS2) on PEDV MK strain infection with 1.25 μg/ml of trypsin [18], indicating the different PEDV strains might account for the differences in trypsin concentrations.