Serological tests for detection of anti-severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Abs in blood are expected to identify individuals who have acquired immunity against SARS-CoV-2 and indication of seroprevalence of SARS-CoV-2 infection. Many serological tests have been developed to detect Abs against SARS-CoV-2. However, these tests have considerable variations in their specificity and sensitivity, and whether they can predict levels of neutralizing activity is yet to be determined. This study aimed to investigate the kinetics and neutralizing activity of various Ag-specific Ab isotypes against SARS-CoV-2 in serum of coronavirus disease 2019 (COVID-19) patients confirmed via PCR test. We developed IgG, IgM, and IgA measurement assays for each Ag, including receptor-binding domain (RBD) of spike (S) protein, S1 domain, full-length S protein, S trimer, and nucleocapsid (N) domain, based on ELISA. The assays of the S protein for all isotypes showed high specificity, whereas the assays for all isotypes against N protein showed lower specificity. The sensitivity of all Ag-specific Ab isotypes depended on the timing of the serum collection and all of them, except for IgM against N protein, reached more than 90% at 15-21 d postsymptom onset. The best correlation with virus-neutralizing activity was found for IgG against RBD, and levels of IgG against RBD in sera from four patients with severe COVID-19 increased concordantly with neutralizing activity. Our results provide valuable information regarding the selection of serological test for seroprevalence and vaccine evaluation studies.
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