a–d, Reactivity to iRBD (a), mRBD (b), iSpike (c) and mSpike (d). Red, green and black data points/lines show the results for sera from SARS-CoV-2-infected individuals, a convalescent serum sample post-NL63 infection and other negative control samples, respectively. e–h, Data from the same experiment as in a–d, respectively, but plotted as AUCs to obtain a better quantitative impression (control samples: n = 50 for iRDB, iSpike and mSpike; n = 59 for mRBD; convalescent samples: n = 4 for iRBD and iSpike; n = 16 for mRBD and mSpike). Statistical analyses were performed using an unpaired two-tailed Student’s t-test in GraphPad Prism. Horizontal lines represent mean values. i,j, Reactivity of the 50 negative control samples from a–h against spike protein from human coronaviruses 229E (i) and NL63 (j). k, Correlation between ELISA titers and microneutralization titers (n = 12; the three samples from negative control sera overlap and are displayed as a single point). Statistical analysis was performed using Pearson’s rank test in GraphPad Prism. The experiments were performed once. IC50, half-maximum inhibitory concentration.