Background: The SARS-CoV-2 Omicron variant (B.1.1.529) has raised concerns the reason behind them being having numerous spike protein mutations and potential impact on disease severity and immune response. This research set out to determine the level of anti-SARS-CoV-2 IgG antibodies targeting the receptor-binding domain (S-RBD) and correlate the antibody levels with COVID-19 infection severity and outcomes. Methods: This prospective cohort study included 61 Sinopharm/Sinovac-vaccinated individuals who came down with SARS-CoV-2 infection. Patients were categorized as moderate, severe, or critically ill. Omicron,B.1.1.529 IgG (anti-S-RBD) antibody concentrations were measured using ELISA at admission and 3 weeks later, correlations with clinical parameters were analyzed. Results: The median antibody concentration at ICU admission for all patients (n=61) was 30.2 ng/ml . The median antibody concentration after 3 weeks for survivor patients (n=43) was 58 ng/ml. A statistically significant negative correlation has been determined between antibody concentration and age. Antibody levels significantly correlated with COVID-19 severity and patient outcomes. The optimal cutoff for predicting mortality was ≤19.49 ng/ml (sensitivity 50%, specificity 72.1%), while the cutoff for critically ill disease was ≤18.43 ng/ml (sensitivity 100%, specificity 87.8%). Conclusion: Serial (S-RBD) IgG antibody levels could function as a crucial prognostic marker for recognizing patients at risk of developing critical illness. The study highlights the future potential utility of antibody measurements in clinical risk stratification and patient management particularly when considered alongside vaccination history and previous infections. Further long term follow up studies with genetic verification are needed to establish causal relationships.