TMPRSS2 and COVID-19: A benefit of using A1AT compared to low-molecular-weight protease inhibitors against COVID-19 is the primarily extracellular action of A1AT (32), which will only inhibit extracellular and plasma membrane-bound serine proteases, such as TMPRSS2, whereas inhibitors such as camostat mesylate can penetrate the cell, potentially increasing the risk of side effects.