Hemagglutination-inhibition (HI) assay HI assays were performed by standard methods [50] . Briefly, the assay was performed using, 0.5% v/v fowl or horse [30] red blood cells, 4 HA unit of reference H5N1 virus (A/HongKong/ 483/97, A/VietNam/1203/04, A/Anhui/1/2005, A/Indonesia/ 5/2005) and specific sera treated with receptor destroying enzyme. The HI titer was defined as the highest dilution of the serum able to inhibit hemagglutination.