The risk of COVID-19 transmission in heterogeneous age groups and effective vaccination strategy in Korea: a mathematical modeling study.

Objectives COVID-19 Vaccines have been developed and introduced in several countries since the pandemic COVID-19 occurred in 2020. In Korea, vaccination began on February 26, 2021, and the object is forming a herd immunity in 2021. In this study, the possibility and conditions of herd immunity (keeping reproductive number consistently below 1) are analyzed using a mathematical modeling, considering both vaccination and social-distancing-based scenarios. Methods The total population was divided into five age groups (0-17, 18-29, 30-59, 60-74, and 75 years old or older). Maximum likelihood estimation was used to estimate the transmission rate between each age group, and the estimated transmission rate matrix was applied into the mathematical model including vaccination. The model was simulated to reflect the actual epidemic situation until May 25, 2021. Since then, simulations are extended until December 31, 2021, with various strategies for vaccination and social distancing. Results Simulation considering vaccinating 70% of the population showed that reproductive number becomes consistently below 1 in August if the third-wave-like situation does not reoccur during the eased distancing phase. However, it was also observed that in the scenario considering the priority of the elderly, reproductive number becomes consistently below 1 in the end of 2021 if the third-wave-like situation reoccurs. Conclusion Model simulations showed both stable and aggravated situations. The results of best and worst scenarios highlight the need for adequate social distancing controls along with sufficient vaccine supply, in order to simultaneously and effectively achieve the mortality minimization and keeping reproductive number consistently below 1 in 2021.