Brief summary: Knowledge concerning SARS-CoV-2-neutralizing antibodies (NAbs) is 14 indispensable for COVID-19 convalescent plasma selection, evaluation of vaccines, and immunity 15 certificates. Our in-cell ELISA (icELISA) test allows rapid (<48h) and high-throughput detection 16 and quantification of SARS-CoV-2-specific NAbs and antiviral activity of drug candidates. 17 18 # Correspondence to 19 Mirko Trilling; Mirko.Trilling@uk-essen.de; ORCID: M Trilling Abstract 28The coronavirus disease 2019 caused by the severe acute respiratory syndrome 29 coronavirus 2 (SARS-CoV-2) is currently the most pressing medical and socioeconomic challenge. 30 Constituting important correlates of protection, determination of virus-neutralizing antibodies 31 (NAbs) is indispensable for convalescent plasma selection, vaccine candidate evaluation, and 32 immunity certificates. In contrast to standard serology ELISAs, plaque reduction neutralization 33 tests (PRNTs) are laborious, time-consuming, expensive, and restricted to specialized laboratories. 34To replace microscopic counting-based SARS-CoV-2 PRNTs by a novel assay exempt from 35 genetically modified viruses, which are inapplicable in most diagnostics departments, we 36 established a simple, rapid, and automated SARS-CoV-2 neutralization assay employing an in-cell 37 ELISA (icELISA) approach. 38 After optimization of various parameters such as virus-specific antibodies, cell lines, virus doses, 39 and duration of infection, SARS-CoV-2-infected cells became amenable as direct antigen source 40 for quantitative icELISA. Using commercially available nucleocapsid protein-specific antibodies, 41 viral infection could easily be quantified in human and highly permissive Vero E6 cells by 42 icELISA. Antiviral agents such as human sera containing NAbs or antiviral interferons dose-43 dependently reduced the SARS-CoV-2-specific signal. Applying increased infectious doses, the 44 icNT was superior to PRNT in discriminating convalescent sera with high from those with 45 intermediate neutralizing capacities.46The SARS-CoV-2 icELISA test allows rapid (<48h in total, read-out in seconds) and automated 47 quantification of virus infection in cell culture to evaluate the efficacy of NAbs as well as antiviral 48 drugs, using reagents and equipment present in most routine diagnostics departments. We propose 49 the icELISA and the icNT for COVID-19 research and diagnostics. 50 65 the outbreak a global pandemic. Since its beginning, the centre of the pandemic shifted from China, 66 via Europe and Northern Americas to Central and Southern Americas. This dynamic nature of the 67 pandemic poses an inherent danger of repetitive local and temporal reintroduction circles. Thus, 68 even countries which coped relatively well with the first wave must prepare in terms of diagnostics 69 capacities for potential future re-emergences.70Most SARS-CoV-2 infections lead to mild or moderate illnesses. However, a considerable fraction 71 of cases proceeds to severe pneumonia or life-threatening acute respir...