Assessing Immunity by Quantitative Measurement of SARS-CoV-2 IgG Antibodies in Fingerstick Samples

COVID-19 has affected billions of people around the world directly or indirectly. The response to the pandemic has focused on preventing the spread of the disease and improving treatment options. Diagnostic technologies have played a key role in this response since the beginning of the pandemic. As vaccines and other treatments have been developed and deployed, interest in understanding and measuring the individual level of immune protection has increased. Historically, use of antibody titers to measure systemic immunity has been constrained by an incomplete understanding of the relationship between antibodies and immunity, the lack of international standards for antibody concentration to enable cross-study comparisons, and insufficient clinical data to allow for the development of robust antibody-immunity models. However, these constraints have recently shifted. With a deeper understanding of antibodies, the promulgation of WHO antibody standards, and the development of immunity models using datasets from multiple COVID-19 vaccine trials, certain types of quantitative antibody tests may now provide a way to monitor individual or community immunity against COVID-19. Specifically, tests that quantitate the concentration of anti-RBD IgG -antibodies that target the receptor binding domain of the S1 spike protein component of the SARS-CoV-2 virus - show promise as a useful and scalable measure of the COVID-19 immunity of both individuals and communities. However, to fulfill this promise, a rapid and easy-to-administer test is needed. To address this important clinical need, Brevitest deployed its point-of-care-capable technology platform that can run a rapid (<15 minute), quantitative antibody test with a sample of 10 l of whole blood from a fingerstick. The test we validated on this platform measures the concentration of anti-RBD IgG in Binding Antibody Units per milliliter (BAU/mL) per WHO Reference Standard NIBSC 20/136. In this paper, we present studies used to characterize the Brevitest anti-RBD IgG assay and evaluate its clinical performance, lower limits of measurement, precision, linearity, interference, and cross-reactivity. The results demonstrate the ability of this assay to measure a patients anti-RBD IgG concentration. This information, together with models developed from recent COVID-19 vaccine clinical trials, can provide a means of assessing the current level of immune protection of an individual or community against COVID-19 infection.