ELISA plate was coated by N protein, 100 µL/cell, at various concentrations. The direct ELISA analysis was performed by loading 100 µL per well of a anti C-ter His tag HRP conjugated mAb at a concentration of 1:2000. The plate was incubated for 1 hours at 37℃, then washed 5 time. Detection was performed using TMB substrate for 10 minutes at room temperature in the dark. The plate was stopped with 2M sulfuric acid. Absorbances were read on a spectrophotometer at 450 nm.