The rACE2 displacement assay was performed as previously described (33 (link), 34 ), except assay plates were Maxisorp (ThermoFisher Scientific). Briefly, the 384-well plates were coated with RBD (supplied by NRC), blocked with BSA in PBS-T, incubated with 1:10 or 1:40 sera dilutions, then with biotinylated ACE2 (supplied by Jim Rini, University of Toronto), and treated with Streptavidin-Peroxidase Polymer Ultrasensitive (Sigma-Aldrich). Incubation with ELISA Pico Chemiluminescent Substrate and reading on the EnVision were performed as in direct detection. All values were normalized to sample-free blanks on the same plate. The resulting relative ratios (RR) were converted to IU/mL using the WHO International Standard 20/136 as a calibrant (33 (link)) and the following formula: log2(IU/mL at sample dilution d)=RR/(−0.2308)+5.7898+log2(d). Neutralization threshold was determined for the 1:10 dilution as 2×MAD (median absolute deviation) from the mean of blanks.