Fig. 2

SARS-CoV-2 N-protein ELISA. In total 113 sera samples were collected from persons infected with SARS-CoV-2 and grouped based on the time period from symptom onset (blue) and for samples with unknown symptom onset by the date of a positive PCR test (black) until the blood sample was taken: 0–6 days (group 1, n = 23), 7–13 days (group 2, n = 22), and 14–55 days (group 3, n = 68, median: 33 d) as well as 1500 serum samples collected before 2015. Results were normalized by defining the positive control as 100% and negative control as 0% for each plate. The cut-off value was 30% and the grey zone below the cut-off ranging from 20 to 30% was considered as negative