Nunc 96-well immune ELISA plates were coated with 1 μg/ml of B.1.1.529 (Omicron) Spike RBD protein in carbonate buffer (Sigma Aldrich) for 2 hours at 37°C before washing with PBS (0.05% Tween) (PBST) and blocking at 37°C for 1 hour with PBS containing 1% Bovine Serum Albumin (BSA). Plates were washed in PBST again before application of 50 μl of diluted sera to each well. All serum dilutions were run in duplicate and a four-point dilution series was run for each sample. Following overnight incubation at 4°C, plates were washed with PBST and wells incubated with 1:1000 dilution of
Biotin Mouse Anti-human IgG (BD Pharmingen, 555785) at room temperature for 1 hour. Plates were washed again before application of 1:200 dilution of Streptavidin Horseradish Peroxidase (HRP) (Bio-techne, DY998) for 30 min followed by a final wash and then assay development using 3,3′, 5,5;-tetramethylbenzidine (TMB) substrate (Sigma Aldrich, T0440). Color development was stopped after 5 min by the addition of 0.18M H
2SO
4 and OD450nm values for each well measured using a FLUOstar® Omega Plate Reader. Analysis of ELISA data was performed in
Prism 9.0 for Mac OS (GraphPad). Data for serial dilutions were plotted and area under the curve calculated for each individual serum sample.
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