Serial evaluation of antibody titres in patients recovered from COVID-19 and their correlation with disease severity

On March 11, 2020, the World Health Organization (WHO) declared COVID-19 a pandemic. According to the findings of various studies conducted around the world, the serological response varies greatly among different populations, with the determinants of variable response still unknown, including the role of disease severity, which is thought to have a definite correlation. The purpose of this study was to assess serial SARS-CoV-2 IgG antibody response in COVID-19 patients and correlate it with disease severity. It was a longitudinal observational study in which 45 patients (age >18 yrs), were enrolled who had recovered from COVID-19 and were reporting to the post-COVID Care OPD Clinic. Patients who had been on long-term immunosuppressive therapy prior to SARS-CoV-2 infection were not eligible. All patients had not been immunized against SARS-CoV-2 and had no history of contact with recent COVID-19 cases. The patients underwent serial blood tests to determine serum IgG titers specific for SARS-CoV-2 at 30, 60, and 90 days after being diagnosed with COVID-19. Chemiluminescence was used to perform a semi-quantitative evaluation of the SARS-CoV-2 IgG antibody. At 30 days after confirmed SARS-CoV-2 infection, 98.78% had detectable serum IgG levels, and sero-reversion (loss of previously detectable antibodies) occurred in 2.5% at 60 days and 90 days. Serum IgG was found to peak at 30 days out of the three time points of measurement (30, 60, and 90 days from diagnosis). Serum IgG levels at 90 days were significantly lower than those at 30 days (p<0.0001) and 60 days (p=0.002). The current study's findings shed light on the presence and persistence of serum SARS-CoV-2-specific IgG antibodies following a natural infection. The findings point to a long-lasting immune response with increasing severity of initial COVID-19 disease.