Evaluating the use of the first WHO International Standard for anti-SARS-CoV-2 immunoglobulin to harmonise the readouts between commercial SARS-CoV-2 binding antibody tests: the HARMONY multicentre calibration study

Background The COVID-19 pandemic resulted in the development and use of a plethora of SARS-CoV-2 serology assays, each reporting results in its own assay units, confounding attempts to compare results between different studies. In 2021, WHO commissioned the HARMONY study to calibrate some of the most widely used commercial SARS-CoV-2 spike protein-binding antibody assays relative to the first WHO International Standard for anti-SARS-CoV-2 immunoglobulin (National Institute for Biological Standards and Control [NIBSC] code 20/136) and assess the harmonisation of data. Methods Diagnostic test manufacturers participating in HARMONY were provided with three standard preparations: the first WHO International Standard, the NIBSC anti-SARS-CoV-2 Antibody Diagnostic Calibrant (code 20/162), and the US SARS-CoV-2 serology standard (Frederick National Laboratory for Cancer Research). The manufacturers followed a standardised study protocol to titrate their in-house reference (IHR) or calibrant and the three standards, performing each experiment in triplicate. Results were submitted for the centralised statistical analysis of dose–response curve data and calculation of potency estimates relative to the International Standard, expressed in binding antibody units per mL (BAU/mL). Findings 14 manufacturers took part in the study, submitting results from 22 different test and target combinations. Each manufacturer’s IHR material was successfully calibrated against the first WHO International Standard and assigned a unitage in BAU/mL relative to the International Standard. Quantification of IgG levels in BAU/mL for each secondary standard showed harmonisation of results for assays measuring binding to the same antigenic target (receptor binding domain [RBD], spike subunit 1 [S1], or spike). For the US serology standard, good agreement was observed for the three participating S1 IgG assays (geometric mean 739 BAU/mL [95% CI 617–885]; geometric coefficient of variation [GCV] 18%) and the six RBD IgG assays (701 BAU/mL [640–768]; GCV 12%). Similar concordance was observed for the NIBSC anti-SARS-CoV-2 Antibody Diagnostic Calibrant 20/162, with BAU/mL estimates for S1 IgG (2356 BAU/mL [2118–2620]; GCV 12%, n=3) and RBD IgG (2096 BAU/mL [1856–2368]; GCV 17%, n=6). Interpretation These findings provide proof-of-concept evidence that for the biologically important SARS-CoV-2 spike protein, levels of anti-RBD, S1, or spike IgG binding antibodies can successfully be compared across a range of well validated commercial assays with different designs and formats through previous calibration and reporting in standardised units. The BAU/mL values assigned to the two secondary standards enable their use as alternative calibration reagents. Funding WHO and the Swiss Agency for Development and Cooperation.