Antiviral drugs typically address subtypes of viruses and thus may not be effective against different subtypes. For example, Influenza A viruses have subtypes on the basis of two proteins on the surface of the virus: hemagglutinin (HA) and neuraminidase (NA). There are 18 known HA subtypes and 11 known NA subtypes. Vilotos addresses viral development by targeting replication enzymes, yielding greater efficacy across all virus subtypes.