Quantification of IgG specific antibodies in recovered COVID -19 patients and its association with disease severity

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Background: Serological tests for anti-SARS-CoV-2 antibodies can provide valuable information in epidemiologic and public health research. However, the data is limited on serological response post COVID-19 infection. Objective: To investigate the use of ARCHITECT SARS CoV-2 IgG II Quantitative assay to quantify the IgG antibodies in recovered COVID-19 infections. Methods: Abbott SARS-CoV-2 IgG assays were performed using the ARCHITECT Quantitative chemiluminescent microparticle immunoassay to detect IgG against SARS-CoV-2 S1 receptor binding domain (RBD) in serum and plasma. Patients having IgG concentrations above ≥50 AU/mL were defined as positive. We analyzed 333 COVID-19 patients who were tested negative by RT-PCR not more than three months after testing positive. Results: The mean age of the population was 34.3 years and 95% were male. About 93% of the patients had mild disease and rest had moderate disease. The mean IgG levels in the entire cohort was 1860.27 AU/mL, the lowest and highest value detected by ARCHITECT SARS CoV-2 IgG II Quantitative Assay (Abbott Laboratories, USA) were 39.8 AU/mL and 43657.4 AU/mL, respectively. While patients with moderate COVID-19 disease had numerically higher IgG levels than patients with mild disease, the difference was not statistically significant (1787.7 AU/mL vs. 2865.39 AU/mL, p=0.21). In 97% of the patients, IgG antibodies could be detected up to 95 days after testing negative on RT-PCR. Conclusion: Patients with moderate COVID-19 disease may develop higher IgG antibodies than in patients with mild disease. Antibodies can be detected in majority of recovered individuals for more than 90 days which may be valuable in developing future vaccination strategy.