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A Surrogate Virus Neutralization Test to Quantify Antibody-Mediated Inhibition of SARS-Cov-2 in Finger Stick Dried Blood Spot Samples (WP-21-11)

Amelia Sancilio, Richard D’Aquila, Elizabeth McNally, Matt Velez, Michael Ison, Alexis Demonbreun, and Thomas McDade

Background The spike protein of SARS-CoV-2 engages the human angiotensin-converting enzyme 2 (ACE2) receptor to enter host cells, and neutralizing antibodies are effective at blocking this interaction to prevent infection. Widespread application of this important marker of protective immunity is limited by logistical and technical challenges associated with live virus methods and venous blood collection. To address this gap, the researchers validated an immunoassay-based method for quantifying neutralization of the spike-ACE2 interaction in a single drop of capillary whole blood, collected on filter paper as a dried blood spot (DBS) sample.

Methods Samples are eluted overnight and incubated in the presence of spike antigen and ACE2 in a 96-well solid phase plate. Competitive immunoassay with electrochemiluminescent label is used to quantify neutralizing activity. The following measures of assay performance were evaluated: dilution series of confirmed positive and negative samples, agreement with results from matched DBS-serum samples, analysis of results from DBS samples with known COVID-19 status, and precision (intra-assay percent coefficient of variation; %CV) and reliability (inter-assay; %CV).

Results Dilution series produced the expected pattern of dose-response. Agreement between results from serum and DBS samples was high, with concordance correlation = 0.991. Analysis of three control samples across the measurement range indicated acceptable levels of precision and reliability. Median % neutralization was 46.9 for PCR confirmed convalescent COVID-19 samples and 0.1 for negative samples.

Conclusions Large-scale testing is important for quantifying neutralizing antibodies that can provide protection against COVID-19 in order to estimate the level of immunity in the general population. DBS provides a minimally-invasive, low cost alternative to venous blood collection, and this scalable immunoassay-based method for quantifying neutralization of the spike-ACE2 interaction can be used as a surrogate for virus-based assays to expand testing across a wide range of settings and populations.

Amelia Sancilio, Postdoctoral Fellow, Institute for Policy Research, Northwestern University

Richard D’Aquila, Howard Taylor Ricketts, MD, Professor of Medicine, Northwestern University

Elizabeth McNally, Elizabeth J. Ward Professor of Genetic Medicine, Northwestern University

Matt Velez, Center for Genetic Medicine, Northwestern University

Michael Ison, Professor of Medicine and Surgery, Northwestern University

Alexis Demonbreun, Assistant Professor of Pharmacology, Northwestern University

Thomas McDade, Carlos Montezuma Professor of Anthropology and IPR Fellow, Northwestern University

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