Risk Factors and Vaccination Dose Associated with COVID-19 Mortality: A Population-Based Study in Gyeongsangbuk-do, South Korea

Vaccination remains an effective intervention for reducing coronavirus disease 2019-related deaths, while the population-level evidence regarding the association between vaccination dose and mortality remains limited. This study aimed to investigate the association between vaccination status, independent risk factors, and COVID-19 mortality, using population-based surveillance data from Gyeongsangbuk-do, South Korea. A population-based retrospective cohort study was conducted using 698,537 confirmed cases from Gyeongsangbuk-do, between January 2021 and June 2022, including 1008 deaths. Univariable and multivariable logistic regression analyses were performed to identify factors associated with mortality. Additional analyses were conducted among adults aged ≥ 65 years. The overall case fatality rate was 0.14%, increasing to 0.88% among older adults. Older age (≥65 years; OR = 87.262, 95% CI: 67.265–114.472) and underlying disease (OR = 20.394, 95% CI: 17.136–24.270) were strongly associated with mortality. After adjustment for sex, age, and underlying disease, unvaccinated individuals had substantially higher odds of death than vaccinated individuals (aOR = 7.897, 95% CI: 6.903–9.034). Booster vaccination (≥3 doses) was associated with markedly reduced mortality in both the overall population (aOR = 0.094, 95% CI: 0.081–0.109) and older adults (aOR = 0.119, 95% CI: 0.102–0.139). Advanced age, underlying diseases, and lack of vaccination were significant factors associated with COVID-19 mortality during the mass vaccination period. These findings support continued efforts to improve booster vaccination coverage among older adults and individuals with underlying diseases.