Surrogate SARS-CoV-2 virus neutralisation test

The surrogate virus neutralisation test (sVNT) (GenScript cPass™ SARS-CoV-2 Neutralization Antibody Detection Kit, Genscript, The Netherlands) was performed according to the manufacturer's instructions. Both laboratories used the same lot number (20E012157). Briefly, serum samples as well as positive and negative assay controls were diluted 1:10 in sample dilution buffer and mixed with an equal volume of HRP-conjugated RBD. Controls were tested in duplicates and samples in singular. After a 30 min incubation at 37°C, 100 µl of this mixture was transferred to a 96-well plate coated with recombinant ACE2. After incubation at 37°C for 15 min, the supernatant was removed and the plate was washed 4x using the provided wash buffer. 100 µl tetramethylbenzidine substrate was added and incubated for 15 min at room temperature before the reaction was stopped by addition of 50 µl stop solution. Plates were read at 450 nm immediately afterwards. Percentage reduction (%reduction) for each sample was calculated by using the following formula: