The COVID-19 pandemic and the fast global spread of the disease resulted in unprecedented decline in world trade and travel. A critical priority is, therefore, to quickly develop serological diagnostic capacity and identify individuals with past exposure to SARS-CoV-2. In this study serum samples obtained from 309 persons infected by SARS-CoV-2 and 324 of healthy, uninfected individuals as well as serum from 7 COVID-19 patients with 4–7 samples each ranging between 1–92 days post first positive PCR were tested by an “in house” ELISA which detects IgM, IgA and IgG antibodies against the receptor binding domain (RBD) of SARS-CoV-2. Sensitivity of 47%, 80% and 88% and specificity of 100%, 98% and 98% in detection of IgM, IgA and IgG antibodies, respectively, were observed. IgG antibody levels against the RBD were demonstrated to be up regulated between 1–7 days after COVID-19 detection, earlier than both IgM and IgA antibodies. Study of the antibody kinetics of seven COVID 19 patients revealed that while IgG levels are high and maintained for at least 3 months, IgM and IgA levels decline after a 35–50 days following infection. Altogether, these results highlight the usefulness of the RBD based ELISA, which is both easy and cheap to prepare, to identify COVID-19 patients even at the acute phase. Most importantly our results demonstrate that measuring IgG levels alone is both sufficient and necessary to diagnose past exposure to SARS-CoV-2.

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