a–c Spike (McLellan/VRC) and RBD (Ragon) antigens tested against 100 archival controls collected from 2014 to 2018 (blue) compared with 14 SARS-CoV-2 PCR+ controls (red) for a IgG, b IgM, and c IgA. d Microsampler controls of PCR+ samples against healthy controls. e–g Seropositivity thresholding for e IgG, f IgM), and g IgA calculated from average signal intensity of archival controls. n = 100 archival controls (black), n = 14 nasal swab SARS-CoV-2 PCR+ patients (red). Manual ELISA, titer = 1:400 (serum) or 1:10 (microsampler eluate). Source data are provided as a Source Data file.