Association of Social and Economic Inequality With Coronavirus Disease 2019 Incidence and Mortality Across US Counties

Key Points Question Are racial/ethnic population composition and economic inequality associated with coronavirus disease 2019 (COVID-19) incidence and mortality? Findings This cross-sectional ecological analysis of cumulative COVID-19 incidence and mortality rates for the first 200 days of the pandemic in 3141 US counties confirmed positive associations of incidence and mortality rates with racial/ethnic composition and with income inequality well as a joint association of incidence and mortality with both structural factors. Meaning This study suggests that COVID-19 surveillance systems should take into account county-level income inequality to better understand the social patterning of COVID-19.


Introduction
The disproportionate burden of coronavirus disease 2019 (COVID-19) on minoritized populations in the United States is now well established. 1,2 The latest data indicate that age-adjusted mortality rates among Black and Hispanic populations are 3.6 and 3.2 times higher, respectively, than for the non-Hispanic White population. 3 This difference translates into almost 30 000 excess deaths among Black and Hispanic residents in the United States, 3 a figure expected to increase substantially in the coming months, even under conservative modeling assumptions.
Understanding inequities in the context of COVID-19 requires not just a descriptive analysis of racial/ethnic gaps but also explanatory studies that identify the structural drivers of those gaps. 4 Such work is vitally important for shifting the narrative from one that places explanatory power on race to one that recognizes the centrality of racism in shaping health. 5 Based on previous research in social epidemiology, 6 there is reason to expect that structural drivers shaping health inequities may be reflected in a community's level of income inequality. As an upstream determinant of health, income inequality influences both community-level characteristics (including the quality and accessibility of housing, education, transportation, and other aspects of public infrastructure) and individual-level physiological processes (primarily through the activation of cortisol and other stress responses). 7 However, little is known about the role of income inequality as a driver of racial/ethnic inequities with COVID-19.
Along with income inequality, the structural drivers of health inequities may also be reflected by political attributes of places, signaling the increasing recognition of political determinants of health. 8 A growing literature in the social sciences explores the link between right-wing policies and population health, 9 and COVID-19 studies [10][11][12] have found associations between state-level political characteristics such as a governor's party affiliation, sex, and term limit-as well as between a county's proportion of Republican votes in the 2016 presidential election-and overall incidence and mortality outcomes. Governors facing a binding term limit may exhibit different political behavior from those able to run again. 13 Although researchers have examined the association between COVID-19 outcomes and racial/ ethnic composition in either a simple correlation analysis at the county level or a linear regression analysis at the individual level without focusing on incidence/mortality, 14,15 no study, to our knowledge, has examined the joint associations of income inequality, racial/ethnic composition, and political attributes of places with COVID-19 outcomes. The objective of this investigation is to analyze the US county-level associations of income inequality, racial/ethnic composition, and political attributes with COVID-19 incidence and mortality in the first 200 days of the pandemic, covering the first 2 major peaks in daily case counts in the country. 16

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Association of Social and Economic Inequality With COVID-19 Incidence and Mortality in US Counties

Methods Data
This cross-sectional study used data from the 3142 counties in the 50 states and for Washington, DC.  16 This study represents, to our knowledge, the first attempt to combine these data sources in a single analysis not reported or considered in other places. Because all of the data used in the study are at the county or state level and in the public domain, the study was exempted by the University of Illinois institutional review board. This study followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline.

Measures
The numbers of cumulative confirmed cases and deaths in a county (both per 100 000 population) are the 2 outcome variables. To estimate the association of incidence and mortality with racial/ethnic composition, economic inequality, and political characteristics, we controlled for 6 potential confounding factors. These factors include percentages of population that were male, younger than 20 years, and 70 years or older; population density per square kilometer; number of days since the first confirmed local case; and county population size (used to compute incidence and mortality rates). The analysis included 3 relevant state-level political variables-whether a governor faced a term limit, was Republican, or was male-as well as whether a state implemented Medicaid expansion under the Affordable Care Act (ACA).

Statistical Analysis
Both outcomes are heavily skewed to the right and can be described by a negative binomial distribution. To model such outcomes, multilevel negative binomial regressions with level-2 (counties nested in states) random intercepts and a log link were applied. The log link implied that exponentiated coefficient estimates provide relative risks (or risk ratios [RRs]) of incidence and mortality for ease of interpretation. Two-sided P < .05 indicated statistical significance. Analyses were performed using STATA, version 16.0 (StataCorp LLC).

Main Findings
This study analyzed the association between COVID-19 incidence and mortality rates of 3141 US counties and the following: the percentage of Black and Hispanic populations; the Gini index for income inequality; a governor's sex, political affiliation, and term limit; percentage of population voting Republican in 2016 in a county; a state's health care system type; and a set of control variables.
Within the 3141 analyzed counties, the mean Black population was 9.365% (range, 0-86.593%); the mean Hispanic population was 9.754% (range, 0.648%-96.353%); the mean Gini ratio was 44.538  characteristics-Republican governor, male governor, or governor facing a term limit-were associated with COVID-19 incidence or mortality in the multilevel analysis. However, an association was discovered for ACA Medicaid expansion: a county in a state with ACA Medicaid expansion had a mean 32% lower incidence rate compared with counties in states that did not participate in ACA expansion (models 1 and 2). Participation in ACA expansion was not associated with mortality rates (models 3 and 4).

Interactions Between Racial/Ethnic Composition and Income Inequality
An interaction was found between racial/ethnic composition and income inequality.  Hispanic composition), indicating that the interaction between Black composition and income inequality was not as strong as that between Hispanic composition and income equality. Finally, income inequality was most strongly associated with higher COVID-19 incidence in counties with relatively low proportions of Black or Hispanic residents, indicated by the greater separation of the 2 curves toward the lower end of Figure 1   The Gini index ranges from 0, indicating perfect equality, to 100, indicating perfect inequality. In a similar presentation, Figure 2 plots estimated COVID-19 mortality rates by racial/ethnic composition and income inequality with their interaction by controlling other covariates. The graph presents similar findings to those in Figure 1, especially in terms of the separation between the 2 mortality curves at the low and high levels of inequality were not narrower than those for the higher inequality level. Finally, when income inequality was high, the association between estimated mortality rate and Black composition was still positive, but it was almost absent between estimated mortality rate and Hispanic composition.

Discussion
Many studies have concluded that COVID-19 has revealed the fault lines of inequality in the United States. Already, published research has shown a disproportionate burden of COVID-19 on minoritized populations. 1,2 This study expands that picture by illustrating how county-level income inequality matters, in itself and through its interaction with racial/ethnic composition to systematically disadvantage Black and Hispanic communities. For the nation to gain control of COVID-19 incidence and mortality, it is essential that public health and government officials know which places are most affected. This analysis found racial/ethnic composition, while important, does not reveal the full The Gini index ranges from 0, indicating perfect equality, to 100, indicating perfect inequality. complexity of the story. Income inequality-a measure not typically included in public health countylevel surveillance-also needs to be considered as a driver of the disproportionate burden borne by minoritized communities across the United States.

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Past studies that did not rely on multilevel analysis showed the association between state-level political factors and COVID-19 outcomes. [10][11][12] In contrast to their findings, the present study did not substantiate such political influence once states' specificities were modeled with a multilevel data analysis framework. This analytic framework, however, did not erase all state-level influences.
Counties in states that participated in ACA Medicaid expansion were found to have a sizable reduction in incidence rates (models 1 and 2). This study used county-level population data of the first 200 days of the pandemic in the United States to add to the growing literature documenting the harmful effects of racialized economic inequality. 6 It must be emphasized that these processes are social rather than biological or genetic. 24 The analyses point to the fundamental cause of racialized economic inequality, not race, as a risk factor producing health inequities. It also points to the complex association between racial/ethnic composition and COVID-19 incidence and mortality, conditional on income inequality. Across all counties, a higher Black or Hispanic population composition was always associated with a higher incidence and a higher mortality. However, at lower levels of inequality, the association between racial/ethnic composition and COVID-19 outcomes was greater than when inequality levels were In most scenarios in this analysis, the association among racial/ethnic composition, income inequality, and COVID-19 outcomes was positive. The only exception was the association between mortality and racial/ethnic composition when income inequality was low. This is because very few counties had low inequality and a sizable Black and Hispanic composition, yielding a greater uncertainty for estimation. The differential association between incidence and mortality and racial/ ethnic composition between low and high economic inequality levels suggests that when one structural factor is at work, there is less room for the other structural factor to come into play. This association also supports the notion put forth by earlier studies for the use of composite variables that measure both income inequality and racial/ethnic composition simultaneously, such as the index of concentration at the extremes, as a critical tool for equity-focused public health surveillance. 29,30 The overall positive associations found in this study suggest that with health equity as a stated goal of the United States, real progress on this front will likely only come with a dedicated commitment to dismantling structural racism and economic inequality, particularly racialized economic inequality.

Conclusions
This cross-sectional study reports a county-level ecological analysis of cumulative COVID-19 infection and mortality rates for the first 200 days of the pandemic in the United States since January 22, 2020, by examining the association between infection incidence and mortality on the one hand and the structural factors of racial/ethnic composition and income inequality on the other, with control for other important covariates. This analysis confirms the association between racial/ethnic composition and COVID-19 incidence and mortality. A higher level of Black or Hispanic composition in a county is associated with a higher COVID-19 incidence and mortality; a higher level of economic inequality is also associated with a higher level of incidence and mortality. The study also substantiated that the association between racial/ethnic composition and incidence/mortality is stronger when income inequality is relatively low, especially for Hispanic populations. More generally, the study suggests that high levels of income inequality may harm population health irrespective of racial/ethnic composition.