Analysis of the Total Immunoglobulin G (IgG) and Its Subclasses Over Time in Coronavirus Disease 2019-Recovered Patients and Its Association With Disease Severity: A Single-Center Prospective Cohort Study.

Purpose: In order to address concerns regarding the diminishing levels of antibodies over time and the variations in response between mild and severe cases, efforts are being made to determine how long immunoglobulin G (IgG) antibodies persist in patients with coronavirus disease 2019 (COVID-19).

Methods: The present study was conducted in a longitudinal setting over a period of 6 months in 37 unvaccinated COVID-19-recovered patients. The spike protein and receptor-binding domain (RBD)-specific serum levels of IgG and IgG subclasses were measured at 3 time points (within ≤1 month, 1-3 months, and 3-6 months) of recovery from COVID-19.

Findings: Our study found a significant (P < 0.05) reduction in the levels of antispike and anti-RBD antibodies within 3 to 6 months after recovery from COVID-19 infections. The group of patients who developed severe illness had higher levels of antispike and anti-RBD IgG compared with the group of patients who recovered from mild disease. There was a statistically significant difference in the contribution of IgG1 and IgG3 over time in COVID-19-recovered patients, indicating a potential alteration in the distribution of IgG subclasses. Serum levels of IgG1 were found to be 1.5 folds higher within 1 to 3 months of recovery from severe acute respiratory syndrome coronavirus 2 infections in severe cases than mild cases.

Implications: This study found that severe COVID-19 cases in unvaccinated patients had higher antibody titers and a greater likelihood of antispike antibodies persisting after infection. The levels of IgG1 and IgG3 increased significantly with the severity of COVID-19, indicating a heightened immune response in more severe cases. The estimation of serum levels of IgG subclass may determine the vaccination strategy and the process of treatment.