Enzyme-linked assay (ELISA) test
For the detection of SARS-CoV-2 IgG antibodies, three commercial ELISA kits coated with recombinant S antigen of SARS-CoV-2 prototype, BA1 and BA.5 variant separately were used, namely the human Anti-SARS-CoV-2(Prototype)Spike protein IgG Antibody detection Kit, Human Anti-SARS-CoV-2(BA.1)Spike protein IgG Antibody detection Kit and Human Anti-SARS-CoV-2(BA.5)Spike protein IgG (Zhongyan Guobang (Beijing) Technology Co., LTD).
According to the instructions of the kits, all samples were diluted from 1:400-1:51200 with 2-fold dilutions. The diluted serum (100 μl/well) were added to each well pre-coated with recombinant S antigen and incubated for 1 h at 37°C. After washing, 100 μl HRP-anti-Human IgG working fluid was added to the corresponding wells, and plates were incubated for 45min at 37°C. After washing, 100 μl of Substrate Solution was added to each well. The reaction was stopped by adding Stop Solution at 50 μl/well, and the intensity of the Absorbance was measured at 450 nm~630nm.