Evaluation of spike protein antigens for SARS-CoV-2 serology

BackgroundSpike protein domains are being used in various serology-based assays to detect prior exposure to SARS-CoV-2 virus. However, there has been limited comparison of human antibody titers against various spike protein antigens among COVID-19 infected patients.MethodsWe compared four spike proteins (RBD, S1, S2 and a stabilized spike trimer (ST)) representing commonly used antigens for their reactivity to human IgG antibodies using indirect ELISA in serum from COVID-19 patients and pre-2020 samples. ST ELISA was also compared against the EUROIMMUN IgG ELISA test. Further, we estimated time appropriate IgG and IgA seropositivity rates in COVID-19 patients using a panel of sera samples collected longitudinally from the day ofonset of symptoms (DOS).ResultsAmong the four spike antigens tested, the ST demonstrated the highest sensitivity (86.2%; 95% CI: 77.8-91.7%), while all four antigens showed high specificity to COVID-19 sera (94.7-96.8%). 13.8% (13/94) of the samples did not show seroconversion in any of the four antigen-based assays. In a double-blinded head-to-head comparison, ST based IgG ELISA displayed a better sensitivity (87.5%, 95%CI: 76.4-93.8%) than the EUROIMMUN IgG ELISA (67.9%, 95% CI: 54.8-78.6%). Further, in ST-based assays, we found 48% and 50% seroconversion in the first six days (from DOS) for IgG and IgA antibodies, respectively, which increased to 84% (IgG) and 85% (IgA) for samples collected ≥22 days DOS.ConclusionsComparison of spike antigens demonstrates that spike trimer protein is a superior option as an ELISA antigen for COVID-19 serology.HighlightsSpike trimer displays the highest antibody titer in SARS-CoV-2 infections among spike protein antigens.Spike trimer IgG ELISA displays a sensitivity of 50% within six days and 86.2% after 14 days from onset of symptoms.IgA and IgG responses to spike trimer antigen were comparable and concomitant in time after infection.16% (IgG) and 15% (IgA) of COVID-19 RT-PCR positive patients did not seroconvert even after 21 days from onset of symptoms.