SARS-CoV-2 seroassay performance and optimization in a population with high background reactivity in Mali

False positivity may hinder the utility of SARS-CoV-2 serological tests in sub-Saharan Africa. From 312 Malian samples collected prior to 2020, we measured antibodies to the commonly tested SARS-CoV-2 antigens and four other betacoronaviruses by ELISA. In a subset of samples, we assessed antibodies to a panel of P. falciparum antigens by suspension bead array and functional antiviral activity by SARS-CoV-2 pseudovirus neutralization assay. We then evaluated the performance of an ELISA using SARS-CoV-2 spike protein and receptor-binding domain developed in the US using Malian positive and negative control samples. To optimize test performance, we compared single and two-antigen approaches using existing assay cutoffs and population-specific cutoffs. Background reactivity to SARS-CoV-2 antigens was common in pre-pandemic Malian samples. SARS-CoV-2 reactivity varied between communities, increased with age, and correlated negligibly-weakly with other betacoronavirus and P. falciparum antibodies. No pre-pandemic samples demonstrated functional activity. Regardless of the cutoffs applied, test specificity improved using a two-antigen approach. Test performance was optimal using a two-antigen assay with population-specific cutoffs [Sensitivity: 73.9% (51.6-89.8), Specificity: 99.4% (97.7-99.9)]. We have addressed the problem of SARS-CoV-2 seroassay performance in Africa by using a two-antigen assay with cutoffs defined by performance in the target population. Published by Oxford University Press for the Infectious Diseases Society of America 2021. This work is written by (a) US Government employee(s) and is in the public domain in the US.

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