Upon receipt of boxes, the sample cards (Whatman® Protein Saver 903) were heat-treated (30 minutes at 56° C); a single blood spot per card was punched (0.25-inch diameter) and transferred to an ELISA plate. Plates were coated with 1 μg/ml of purified RBD diluted in PBS overnight at 4C and blocked with Tris-Buffered Saline with 0.1% Tween 20 (TBST) containing 5% non-fat dry milk. DBS were eluted in 500 μl of TBST overnight at 4C and 50 μl of each sample was added to the ELISA plate preloaded with 50 μl of TBST containing 2% non-fat dry milk. Samples were then assayed for SARS-CoV-2 antibodies according to published protocols.18 (link),19 The RBD protein was produced in-house via transfection of HEK293T cells using polyethylenimine (Plasmid was a generous gift from Pr. F. Kramer mount Sinai School of Medicine). Batches were control for purity by SDS-PAGE followed by Coomassie staining and ELISA using an anti His-tag monoclonal antibody. Optical densities were read at 405 nm, and each 96-well plate contained seven negative controls and one positive control (serum from PCR-confirmed case at 1/100 dilution). Samples were tested against IgG and positive samples were confirmed and tested with anti IgM antibodies. Optical density values were normalized to the mean optical density of negative controls daily.