Quantitative measurement of IgG to SARS‐CoV‐2 antigens using monoclonal antibody‐based enzyme‐linked immunosorbent assays

OBJECTIVE: Standardised quantitative analysis of the humoral immune response to SARS‐CoV‐2 antigens may be useful for estimating the extent and duration of immunity. The aim was to develop enzyme‐linked immunosorbent assays (ELISAs) for the quantification of human IgG antibodies against SARS‐CoV‐2 antigens. METHODS: Enzyme‐linked immunosorbent assays were developed based on monoclonal antibodies against human IgG and recombinant SARS‐CoV‐2 antigens (Spike‐S1 and Nucleocapsid). The WHO 67/086 immunoglobulin and WHO 20/136 SARS‐CoV‐2 references were used for standardisation. Sera of a study group of COVID‐19‐positive subjects (n = 144), pre‐pandemic controls (n = 135) and individuals vaccinated with BioNTech–Pfizer BNT162b2 vaccine (n = 48) were analysed. The study group sera were also tested using EuroImmun SARS‐CoV‐2‐ELISAs and a quantitative S1‐specific fluorescence enzyme immunoassay (FEIA) from Thermo Fisher. RESULTS: The ELISA results were repeatable and traceable to international units because of their parallelism to both WHO references. In the study group, median anti‐S1‐IgG concentrations were 102 BAU mL(−1), compared to 100 and 1457 BAU mL(−1) in the vaccination group after first and second vaccination, respectively. The ELISAs achieved an area under the curve (AUC) of 0.965 (S1) and 0.955 (Nucleocapsid) in receiver operating characteristic (ROC) analysis, and a specificity of 1 (S1) and 0.963 (Nucleocapsid) and sensitivity of 0.903 (S1) and 0.833 (Nucleocapsid) at the maximum Youden index. In comparison, the commercial assays (S1‐FEIA, S1 and Nucleocapsid ELISA EuroImmun) achieved sensitivities of 0.764, 0.875 and 0.882 in the study group, respectively. CONCLUSIONS: The quantitative ELISAs to measure IgG binding to SARS‐CoV‐2 antigens have good analytical and clinical performance characteristics and units traceable to international standards.