COVID-19 transmission dynamics in South Korea prior to vaccine distribution

In early 2020, South Korea experienced a large coronavirus disease 2019 (COVID-19) outbreak. However, despite its proximity to China, where the virus had emerged, and the high population density of the Seoul metropolitan area, a major international hub, South Korea effectively contained the spread of COVID-19 using non-pharmaceutical interventions until vaccine distribution in 2021. Here, we built a metapopulation model with a susceptible-exposed-infectious-quarantined-recovered (SEIQR) structure and combined it with the ensemble adjustment Kalman filter to infer the transmission dynamics of COVID-19 in South Korea from February 2020 until vaccine deployment. Over the study period, the fraction of documented infections (ascertainment rate) was found to increase from 0.50 (95% credible interval (CI): 0.26--0.77) to 0.62 (95% CI: 0.39--0.86). The cumulative number of total infections, including both documented cases and undocumented infections, was less than 1% of the South Korean population at the end of the simulation period, indicating that the majority of people had yet to be infected when vaccine administration began. These findings enhance understanding of the COVID-19 outbreak in South Korea and highlight the importance of preparedness and response in managing global pandemics.