The MSD pseudoneutralization/ACE2 inhibition assay measures the ability of participant plasma to inhibit ACE2 binding to spike protein. Plasma was thawed and ACE2 blocking was measured using the ACE2 MSD V-PLEX SARS-CoV-2 ACE2 kits according to the manufacturer’s protocol at a dilution of 1:100. Plates come pre-coated with spike proteins corresponding to variants of interest. They were washed and incubated with plasma for one hour, human ACE2 protein conjugated with a SULFO-TAG (light-emitting label) added for another hour, washed, read buffer added, and read with a MESO QuickPlex SQ 120 instrument. If the plasma fully bound the coated spike protein and blocked binding of the added ACE2, then no light is emitted during the read phase of the assay, corresponding to 100% ACE2 inhibition. If there was no binding of spike by participant plasma, then the added ACE2 fully binds the coated spike protein and illuminates during reading, corresponding to 0% inhibition. An 8-point calibration curve was included in each plate. The last point only contained assay diluent. Results were reported as percent ACE2 inhibition based on the equation provided by the manufacturer ((1 – Average sample ECL/Average ECL signal of blank well) x100).