XK and susceptibility to pneumonia measurement: One strain, A/NTU4223/2009 isolated from a mild case on July 15,2009 had co-substitutions in HA (S203T, D222G) and NA (V106I, V166I, N248D), and the other strain, A/NTU340/2010 isolated from a pneumonia case on January 19, 2010 had co-substitutions in HA (S203T, K163E) and NA (I34T, V106I, E119K, N248D, G382E, D416N).