a Schematic of the spike and RBD constructs used to generate recombinant proteins. Abbreviations are 3 C, rhinovirus 3 C protease cleavage site; Strep2x2 dual Strep2 epitope tag, T7 bacteriophage T7 fibritin trimerization domain, SBP streptavidin-binding peptide. b SDS-PAGE Coomassie Blue staining of purified (1) McLellan/VRC spike, n = 27, (2) Mt Sinai spike, n = 6, (3) Mt Sinai RBD, n = 5, and (4) Ragon RBD, n = 13, proteins. c Analytical size-exclusion chromatography of purified McLellan/VRC (n = 27) and Mt Sinai spike (n = 6) proteins. Peak elution volumes of sizing standards are noted (670 kDa—thyroglobulin, 158 kDa—gamma-globulin, 44 kDa—ovalbumin, 17 kDa—myoglobin). Inset: transmission electron microscopy of McLellan/VRC and Mt Sinai spike trimers. Ladder unit = kDa. d Left: full spike ectodomain at three different concentrations of protein coating density for both McLellan/VRC (NIAID Vaccine Research Center, blue) and Sinai (Mount Sinai, orange) constructs. Right: RBD constructs at three different concentrations of protein coating for both Ragon (Ragon Institute, blue) and Sinai (Mount Sinai, orange) constructs. Anti-spike or anti-RBD monoclonal recombinant antibody spiked into negative serum at 1:100, 1:1000, and 1:10000 dilution. Data are means ± SD, n = 4 independent study replicates on two independent protein preparations. Source data are provided as a Source Data file.