**1. Introduction**

In late January 2020, we started to treat real-time polymerase chain reaction (RT-PCR) confirmed COVID-19 patients. We are a medical center that typically cares for patients with moderate to severe diseases. Because of the low prevalence of COVID-19 in Taiwan and local government policy, however, we also admitted COVID-19 patients with mild disease or even those without symptoms for inpatient care. Serial RT-PCR tracking of pharyngeal samples was performed throughout each patient's hospital course. With informed consent from patients or their families, our research was conducted using serum samples remaining

**Citation:** Huang, C.-G.; Dutta, A.; Huang, C.-T.; Chang, P.-Y.; Hsiao, M.-J.; Hsieh, Y.-C.; Lin, S.-M.; Shih, S.-R.; Tsao, K.-C.; Yang, C.-T. Relative COVID-19 Viral Persistence and Antibody Kinetics. *Pathogens* **2021**, *10*, 752. https://doi.org/10.3390/ pathogens10060752

Academic Editors: Philipp A. Ilinykh and Kai Huang

Received: 10 May 2021 Accepted: 10 June 2021 Published: 13 June 2021

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after routine medical tests. We used two ELISA-based kits [1–3] to detect anti-spike protein IgG antibodies, and the results of the two were concordant. The tests were semi-quantitative and measured antibody concentrations relative to a cut-off point value in serial dilutions of serum samples. We cultured virus strains from our patient samples and used one of the strains to quantify the neutralization valence of serum samples in our Biosafety Level-3 laboratory. We also measured cytokines and chemokines in the serial sera samples of four patients using the Bio-Plex Pro Human Cytokine 48-Plex Screening kit from Bio-Rad. By mid-March 2020, we had collected serum samples from 15 consecutive patients. As this was a single-center study, we also collected a complete medical record including detailed travel, occupation, contacts, and cluster history.
