Nafamostat is a serine protease inhibitor used as an anticoagulant drug in the clinic that was recently shown to also inhibit influenza infection by blocking the cleavage of influenza hemagglutinin protein (HA0) into the subunits HA1 and HA2 by serine proteases (e.g., TMPRSS11D and TMPRSS2), which is essential for viral entry (8, 16). Here, TMPRSS11D is linked to influenza.