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

Highlights

• Spike 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.

Abstract

Background

Spike 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 antibody titers against various spike protein antigens among COVID-19 infected patients.

Methods

We 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 of onset of symptoms (DOS).

Results

Among 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 from DOS.

Conclusions

Comparison of spike antigens demonstrates that spike trimer protein is a superior option as an ELISA antigen for COVID-19 serology.