Whole ectodomain of SARS-CoV-2 S protein with His Tag (Sino Biological Inc., Beijing, China; catalogue number: 40589-B08V1) was diluted with coating buffer (0.1 M NaHCO3, 34 mM Na2CO3) and a total of 20 ng of protein was loaded into individual wells of a 96 well plate (Nunc, Roskilde, Denmark) and allowed to coat overnight at 4 °C. Plates were then washed four times with 0.05% Tween 20 in PBS (PBST) and blocked with 5% bovine serum albumin (BSA)/PBST for 30 min before murine antibodies serially diluted with blocking buffer were added to desired wells for 1 hour. Plate were washed four times with PBST before incubation for 1 hour with HRP-conjugated goat anti-mouse IgG (Thermo Fisher Scientific) secondary antibodies diluted in blocking buffer, and washed four times with PBST. Visualisation of bound secondary antibodies was done by the addition of 3,3',5,5'-tetramethylbenzidine (TMB) substrate (Thermo Fisher Scientific) for 5 min in the absence of light and the reaction was stopped with 2 M sulphuric acid. Optical density at 450 nm (OD450nm) was determined by a Tecan (Männedorf, Switzerland) Infinite M1000 reader and normalised OD450nm was obtained by subtracting background absorbances determined in BSA coated wells.