2.6 HI assay.
The HI assay was performed following the standard methods. Briefly, 25 μl of 2-fold serially diluted antisera or mAb were mixed with 25 μl of 4 HA units of tested virus. After incubating at room temperature for 30 min, 50 μl of 0.5% chicken red blood cells were added to each well and incubated at room temperature for 40 min. The HI titer was defined as the reciprocal of the highest serum dilution that completely inhibited hemagglutination.