The study was approved by the local ethics committee (BC-07829) of the Ghent University Hospital, a 1062-beds tertiary care teaching hospital in Belgium. From March 12th to 31st 2020, 23 consecutive sera from 7 different patients, admitted to the hospital with a confirmed diagnosis of COVID-19 based on positive RT-PCR [6], were collected on day 2 to 18 after symptom onset. The study group consisted of 2 females and 5 males. The median age was 58 years (range 39–81 years). The collected sera were divided into 2 groups: day 2–7 after symptom onset (7 samples) and day 8–18 after symptom onset (16 samples) in order to evaluate clinical sensitivity.