2.12. Avidity of antibody
The avidity of ORF8-specific IgG was measured using the ammonium thiocyanate elution method.[21] Briefly, the plates were pre-coated with ORF8 and the samples were blocked with PBS-milk, followed by incubation of the diluted mice sera at 37 °C for 1 h. Then, the plate was incubated with 100 μL of ammonium thiocyanate (0-0.75 M) at room temperature for 15 min. After a washing cycle, the determination was repeated. The avidity index (AI) was expressed as ammonium thiocyanate concentration needed to decrease the absorbance by 50%.