Background: SARS-CoV-2 genome encodes different kinds of structural and nonstructural proteins. The spike structural protein is the prime target for vaccine manufacturing, drug development, and diagnostic assays.

Methods: In this cross-sectional study, we assessed the developed Receptor Binding Domain (RBD) protein in 520 serum samples. The samples were stratified according to vaccine status into two groups: Group 1 consisted of 274 pre-vaccinated samples collected from April to December 2020 (109 were asymptomatic contacted with COVID-19-patients; 100 patients had positive RT-PCR test for SARS-CoV-2 infection, and 65 were convalescent-COVID-19 cases), and group 2 consisted of 246 post-vaccinated participants with or without past-infection recruited from students, coworkers, and staff members from November 2021 to March 2022. Indirect Enzyme-Linked Immunosorbent Assay (ELISA) was performed to evaluate the seroprevalence of the SARS-CoV-2 immunoglobulin G (IgG) across both groups.

Results: The sensitivity and specificity of the RBD-developed protein were 88.4% (95% CI: 84% - 92%) and 100% (95% CI: 88%- 100%), respectively in the pre-vaccinated group. 213 out of 274 of the pre-vaccinated samples were positive. Meanwhile, the RBD-developed protein provided 100% RBD IgG seropositivity in the recovered COVID-19 cases. Moreover, the results were seropositive in all post-vaccinated candidates (n=246) and the results were reliable in 70 negative pre-pandemic sera specimens. Conclusion: The developed S-RBD protein has the potential to be a successful screening method in COVID-19-vaccinated or infected individuals.