ELISA for SARS-CoV-2 Antibody Detection

ELISA plates (Thermo Fisher, 442404) were coated with 1 µg/ml of SARS-CoV-2 WA1, variant S2P, or HBsAg (ProspecBio, HBS-875) in PBS, pH 7.4 at 4 °C for 16 h^{7 (link)}. Standard washes and blocking steps were done as described above. The plates were blocked for 2 h. The sera were diluted by 100-fold in 5% skim milk in PBST. A serial 4-fold dilution for weeks 0 and 2 sera or 6-fold dilution for week 6 sera was applied to the 100-fold dilution preparations. The plates were incubated with diluted sera at room temperature for 1 h. The HRP-conjugated anti-mouse secondary antibody (Thermo Fisher, G21040, 1/2000) was used to detect the antibody responses. The endpoint titers were calculated as the dilution that yielded an optical density equivalent to 4×background (secondary antibody alone).