Background After relaxing social distancing measures, South Korea experienced a resurgent second epidemic wave of coronavirus disease 2019 (COVID-19). In this study, we aimed to identify the transmission dynamics of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections and assess the impact of COVID-19 case finding and contact tracing in each epidemic wave. Methods We collected data on COVID-19 cases published by local public health authorities in South Korea and divided the study into two epidemic periods (19 January–19 April 2020 for the first epidemic wave and 20 April–11 August 2020 for the second epidemic wave). To identify changes in the transmissibility of SARS-CoV-2, the daily effective reproductive number (Rt) was estimated using the illness onset of the cases. Furthermore, to identify the characteristics of each epidemic wave, frequencies of cluster types were measured, and age-specific transmission probability matrices and serial intervals were estimated. The proportion of asymptomatic cases and cases with unknown sources of infection were also estimated to assess the changes of infections identified as cases in each wave. Results In early May 2020, within 2-weeks of a relaxation in strict social distancing measures, Rt increased rapidly from 0.2 to 1.8 within a week and was around 1 until early July 2020. In both epidemic waves, the most frequent cluster types were religious-related activities and transmissions among the same age were more common. Furthermore, children were rarely infectors or infectees, and the mean serial intervals were similar (~ 3 days) in both waves. The proportion of asymptomatic cases at presentation increased from 22% (in the first wave) to 27% (in the second wave), while the cases with unknown sources of infection were similar in both waves (22 and 24%, respectively). Conclusions Our study shows that relaxing social distancing measures was associated with increased SARS-CoV-2 transmission despite rigorous case findings in South Korea. Along with social distancing measures, the enhanced contact tracing including asymptomatic cases could be an efficient approach to control further epidemic waves.

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Background Since the supply of coronavirus disease 2019 (COVID-19) vaccines will be limited worldwide, it is essential to prioritize vaccination based on scientific evidence. Although several frameworks and studies on vaccine distribution have been published, no published systematic review has evaluated the prioritization of the COVID-19 vaccine. Materials and Methods We searched 4 different databases, PubMed, SCOPUS, Web of Science, and EMBASE for articles published between January 2019 and December 31, 2020. Studies were included if they contained the primary search terms “vaccine”, “COVID-19”, and “prioritization”. In addition, we manually included reports from national and international websites. Results Thirteen studies met the inclusion criteria. In these studies, older adults were the most frequently mentioned group, and healthcare workers (HCWs) were mentioned as the 1st priority group. HCWs and patients with comorbidities were the 2nd and 3rd most frequently mentioned groups in the reviewed papers. Reducing severe COVID-19 was the most frequently mentioned goal. Conclusion Since vaccination programs have been initiated in several countries, scientific evidence on vaccination prioritization is needed to increase our knowledge of general vaccine prioritization and improve vaccine acceptance. Our results showed that HCWs and older adults were the most frequently valued in studies.

In order to identify the temporal change in the possible risk of superspreading events (SSE), we estimated the overdispersion parameter in two different periods of COVID-19 pandemic. We identified the possible risk of SSE was reduced 90% during the second epidemic period in South Korea.