The Intersection of Health Inequalities and COVID-19: Evidence from a Nationwide Cohort Study in South Korea

Background: Since the onset of COVID-19, a concept of syndemics has increasingly attracted attention in public health and social sciences, highlighting how synergistic interactions among the virus and pre-existing structural, social, and health conditions contribute to higher morbidity and mortality in certain communities and populations (Bambra et al., 2020; Mendenhall et al., 2022). A syndemic pandemic characterizes the dynamics in which inequalities in COVID-19 are interconnected with existing disparities in chronic diseases and the social determinants of health (Bambra et al., 2020; McGowan & Bambra, 2022; Mendenhall et al., 2022). Alongside genetic factors, the surrounding social, economic, and geographical conditions are crucial when studying health outcomes, as they are influenced by both genetic and environmental factors and their interactions. In particular, socioeconomic status plays an important role, as it significantly influences access to resources including healthcare, which in turn shapes health behaviors. However, there is limited empirical evidence on how the health impacts of the COVID-19 pandemic intersect with the pre-existing disparities in financial resources over time since the onset of the pandemic. In this paper, we fill the gap in the literature by exploring whether negative health outcomes tend to concentrate in specific populations, particularly among low-income groups and identifying the health disparities resulting from the COVID-19 pandemic.

Methods: We examine the associations between income disparities and health outcomes among confirmed COVID-19 cases in South Korea from 2020 to 2023. Using national administrative health data provided by the Korean National Health Insurance Service (KNHIS), we employ a logistic regression model for over 10 million randomly sampled cases from a total of more than 34 million registered positive cases in the country.

Results: The descriptive statistics for the full sample of over 10 million confirmed COVID-19 cases in Korea, spanning from October 2020 to June 2023. Of the sample, approximately 4% were hospitalized, 0.13% developed severe symptoms, and the fatality rate was 1.27%. Our findings reveal that individuals from higher-income backgrounds are less likely to experience negative health outcomes compared to the lowest income group, including hospitalization (OR=0.450, p-value=0.000), severe illness (OR=0.701, p-value=0.000), and fatalities (OR=0.397, p-value=0.000). Furthermore, we conducted subsample analyses based on various demographic characteristics—such as sex, age, and employment status—to account for potential heterogeneity in the associations between income and health. These analyses consistently showed that individuals from higher-income backgrounds face a lower risk of adverse health outcomes. For example, the OR of 0.514 for hospitalization among cases aged 65 and older (p-value=0.001) indicates that elderly cases from the highest income background are half as likely to be hospitalized compared to those from the lowest income group. The ORs of 0.696 (p-value=0.001) for severe cases and 0.451 (p-value=0.001) for case fatality reinforce this pattern.

Discussion and Conclusion: These results indicate that disadvantaged individuals are disproportionately affected by severe health consequences, deepening health inequities. This paper emphasizes the need for targeted public policies to address these disparities during health crises.