Sensitivity and specificity of SARS-CoV-2 S1 subunit in COVID-19 serology assays

Serological assays such as enzyme-linked immunosorbent assays (ELISA) using SARS-CoV-2 Spike (S) proteins are practical methods to determine the extent of COVID-19 immunity in a population upon SARS-CoV-2 vaccination 1 . However, the robustness of these assays, which depends on the sensitivity and specificity of S proteins in detecting anti-SARS-CoV-2 antibodies, is not well characterized. Here, we report that the S1 subunit of the SARS-CoV-2 S protein has superiority over the receptor-binding domain (RBD) and the native state S trimer in terms of sensitivity and specificity, respectively, in measuring anti-SARS-CoV-2 antibodies from COVID-19 convalescent patients. S1 and S trimer are more sensitive than RBD antigen because they are able to capture non-RBD, as well as RBD-binding COVID-19 antibodies. However, the full-length S trimer, which harbors S2 subunit in addition to S1, cross-reacted with antibodies elicited by circulating coronavirus (CoV), such as HCoV-OC43 and HCoV-HKU1, making it less specific than S1 in detecting COVID-19 antibodies. Our results show that the S1 subunit protein of SARS-CoV-2 is both sensitive and specific in distinguishing COVID-19 antibodies, suggesting its potential implication in large-scale serological testing, especially in determining the efficacy of SARS-CoV-2 vaccine.