Reevaluation of Seroprevalence using a Semi-quantitative Anti-spike IgG in Health Care workers at an Academic Medical Center in Boston, Massachusetts

Abstract

AbstractBackgroundAccurate measurement of antibodies is a necessary tool for assessing exposure to SARS-CoV-2 and facilitating understanding of the role of antibodies in immunity. Most assays are qualitative in nature and employ a threshold to determine presence of antibodies. Semi-quantitative assays are now available. Here we evaluate the semi-quantitative SARS-CoV-2 IgG II (anti-spike (S)) assay. We aim to reassess the seroprevalence using anti-S assay and subsequently compare it to the previously measured IgG (anti-nucleoprotein (N)) in health care workers at an academic medical center in Boston.Methods1743 serum samples from HCWs at Boston Medical Center were analyzed for SARS-CoV-2 anti-S IgG and IgM using the Abbott SARS-CoV-2 IgG II and□Abbott AdviseDx□SARS-CoV-2 IgM assay, respectively. Precision, linearity, positive and negative concordance with prior RT-PCR test were evaluated for anti-S IgG. Seroprevalence and its association with demographics variables was also assessed.ResultsLinearity and precision results were clinically acceptable. The positive and negative concordance for anti-S IgG with RT-PCR was 88.2% (95% CI: 79.4% - 94.2%) and 97.43% (95% CI: 95.2% - 98.8%), respectively. Overall, 126 (7.2%) of 1,743 participants were positive by anti-S IgG. Among the 1302 participants with no prior RT-PCR, 40 (3.1%) were positive for anti-S IgG antibody. The original agreement in this population with the qualitative, anti-N IgG assay was 70.6%. Upon optimizing the threshold from 1.4 to 0.49 S/CO of the anti-N IgG assay, the positive agreement of the assay increases to 84.7%.ConclusionThe anti-S IgG assay demonstrated reproducible and reliable measurements. This study highlights the presence of asymptomatic transmission among individuals with no prior history of positive RT-PCR. It also highlights the need for optimizing thresholds of the qualitative SARS-CoV-2 IgG assay for better agreement between assays by the same vendor.