The Technical Expert Working Group of the World Health Organization’s (WHO) Global Influenza Surveillance and Response System (GISRS) for Surveillance on Antiviral Susceptibility (WHO-AVWG) has established a set of criteria to define the NA inhibitor susceptibility of influenza viruses based on the fold-change in IC50 value compared with the median value for viruses from the same type/subtype/lineage [8]. Here, XK is linked to influenza.