Globally, the COVID-19 pandemic has had extreme consequences for the healthcare system and has led to calls for diagnostic tools to monitor and understand the transmission, pathogenesis, and epidemiology, as well as to evaluate future vaccination strategies. In this study, we have developed novel, to our knowledge, flexible ELISA-based assays for specific detection of human SARS-CoV-2 Abs against the receptor-binding domain, including an Ag sandwich ELISA relevant for large population screening and three isotype-specific assays for in-depth diagnostics. Their performance was evaluated in a cohort of 350 convalescent participants with previous COVID-19 infection, ranging from asymptomatic to critical cases. We mapped the Ab responses to different areas on protein N and S and showed that the IgM, A, and G Ab responses against receptor-binding domain are significantly correlated to the disease severity. These assays and the data generated from them are highly relevant for diagnostics and prognostics and contribute to the understanding of long-term COVID-19 immunity.