The antiviral effects of 3-amidinophenyalanine (Phe(3-Am))-derived inhibitors of host proteases such as matriptase and transmembrane serine protease 2 (TMPRSS2) are based on the inhibition of viral fusion with the host cell, which is essential for the replication and spread of certain influenza and coronaviruses. This evidence concerns the gene TMPRSS2 and influenza.