Impact of SARS-CoV-2 RNA titer on the level of IgG antibodies in recovered patients with COVID-19 disease

Abstract

Introduction: It is important to better understand the presence of neutralizing anti-SARS-CoV-2 antibodies in the population, as they potentially prevent (re)infection.

Aim: To correlate the Cycle threshold (Ct value) of SARS-CoV-2 and its impact on specific Anti-SARS-CoV-2 IgG titer, to reveal the effect of age and disease severity on antibodies titer.

Methodology: A total of 153 infected participants laboratory-confirmed COVID-19 cases 4-11 months ago, aged 18-85 years old (mean = 43.58, SD ± 15.34) were enrolled in the study. They have not received any COVID-19 vaccine. A questionnaire was prepared including demographic data, age, gender, residence, and symptoms severity they suffered. Five mL of venous blood was taken from each participant to measure SARS-CoV-2 IgG antibodies against the receptor binding domain (RBD) by (VIDAS SARS-COV-2 IgG - Biomerieux kit). Ct values were measured by qRT-PCR (BIO-RAD-CFX96) kit which detected two virus genes, namely (RdRp-N genes).

Results: Lowest Ct values were detected significantly in age group 50-59 and 70-85 respectively. The highest mean of IgG was detected in age groups 70-85 and 50-59, and was found to be significantly correlated with disease severity. There is a direct relationship between Ct values and the titer of specific IgG, as increasing in viral load is associated with a higher level of antibodies. Antibodies were detected several months after infection with the highest mean after 10-11 months.

Conclusions: Specific Anti-viral IgG are significantly associated with increasing age and disease severity, and the direct relation of IgG with viral load. Antibodies are detected several months post-infection but their protective efficacy is controversial.