Acknowledgments
The authors deeply acknowledge the excellent technical assistance of Sina Müller and Carina Bäumler (both Labor Krause und Kollegen MVZ GmbH), as well as of Andrea Hölzgen (Institute for Infection Medicine). Furthermore, we would like to thank all patients for participating in this study.

Supplementary Materials
The following are available online at https://www.mdpi.com/2076-2607/8/10/1572/s1. Table S1: Characteristics of the eight SARS-CoV-2 IgG/total antibody tests. Figure S1: Raw data from the plaque reduction neutralization test used as reference # 2 in the validation part of this study. Figure S2: Raw data from the plaque reduction neutralization test to investigate the kinetics of the virus-neutralizing antibodies. Figure S3: Development of high SARS-CoV-2 IgG avidities in three SARS-CoV-2 patients. Figure S4: Agreement of titers from two independent neutralization tests.
Click here for additional data file.

Author Contributions
Conceptualization, A.K.; methodology, A.S., R.R., O.G., F.N., and A.K.; formal analysis, H.F. and A.K.; investigation, A.S., R.R., O.G., F.N., and A.K.; resources, H.F., T.L., and A.K.; writing—original draft preparation, A.K.; writing—review and editing, H.F. and A.K.; and supervision, H.F., T.L., and A.K. All authors have read and agreed to the published version of the manuscript.

Funding
We acknowledge financial support by DFG within the funding program Open Access Publizieren.

Conflicts of Interest
Mikrogen, Viramed, and Virion-Serion companies kindly provided test kits free of charge, as well as technical support for this study. These companies had no influence on the study design, the collection and testing of the samples, the interpretation of the results, or on the writing of the manuscript or in the decision to publish the results. Some of the data of the assay validation part of this study have already been published as a preprint [33]. All authors declare no conflicts of interest.