Avidity (ELISA)
To test IgG antibody avidity against SARS-CoV-2 full spike protein and RBD protein, threefold serial dilutions of 1/20 diluted mice sera, were added to ELISA plates coated over night with 1.0µg/ml RBD and spike proteins, respectively. After incubation at RT for 1h, the plates were washed once in PBS-0.01% Tween, and then washed 3x with 7M urea in PBS-0.05%Tween or with PBS-0.05% Tween for 5min every time. After washing with PBS-0.05%Tween, goat anti-mouse IgG conjugated to Horseradish Peroxidase (HRP) (Jackson ImmunoResearch, West Grove, Pennsylvania) was added 1/2000 and incubated for 1h at RT. Plates were developed and read at OD450 nm.