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. 2021 Feb 17;106:363–369. doi: 10.1016/j.ijid.2021.02.058

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© 2021 Published by Elsevier Ltd on behalf of International Society for Infectious Diseases.

Since January 2020 Elsevier has created a COVID-19 resource centre with free information in English and Mandarin on the novel coronavirus COVID-19. The COVID-19 resource centre is hosted on Elsevier Connect, the company's public news and information website. Elsevier hereby grants permission to make all its COVID-19-related research that is available on the COVID-19 resource centre - including this research content - immediately available in PubMed Central and other publicly funded repositories, such as the WHO COVID database with rights for unrestricted research re-use and analyses in any form or by any means with acknowledgement of the original source. These permissions are granted for free by Elsevier for as long as the COVID-19 resource centre remains active.

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Types of COVID-19 clusters categorized by K-means clustering from January to September 2020 in the Seoul metropolitan area (see Appendix Figure 2) in phase 1 (January 23 to August 11, 2020) and phase 2 (August 11 to September 24, 2020). Cluster duration refers to the time between the first and the last case reported within a cluster. Cluster size refers to the number of cases per cluster. *Number of cases are in log scale.