Areas with higher levels of unequal distribution of income have up to a 30% increased likelihood of rising coronavirus caseloads and an up to a 50% raised risk for more deaths from the disease, the data showed.
This was particularly true during the summer of 2020, when case counts in the United States reached an early peak of up to 75,000 per day, based on Centers for Disease Control and Prevention figures.
"We found that county-level income inequality in the U.S. was associated with higher COVID-19 cases and deaths last spring and summer," study co-author Michelle Odden told UPI in an email.
"Income inequality can exacerbate socioeconomic differences, and worsen the effects of low income on health," said Odden, an associate professor of epidemiology at Stanford University in Palo Alto, Calif.
Prior research has found that low-income communities and communities of color nationally have been at increased risk for coronavirus infection and death since the start of the pandemic.
The new study is based on an analysis of county-level income and COVID-19 case and death data for all of the more than 3,200 counties in the United States and Puerto Rico.
Income inequality was measured using the Gini coefficient, a commonly used measure for income distribution developed by Italian researcher Corrado Gini in 1912, with 0 being "perfect equality" and 1 being maximum inequality.
The Gini coefficient, per the World Bank, takes into account the number of people living in households with incomes below the "perfect equality" threshold for a given country or region.
Across the United States, the average Gini coefficient was between 0.4 and 0.5, Odden and her colleagues said.
For each 0.05-point increase in Gini coefficient, risk for higher COVID-19 cases increased by 18% in March and April 2020, by 23% in May and June 2020 and by 28% in July and August 2020, the data showed.
Similarly, for each 0.05-point increase in Gini coefficient, risk for higher deaths from the disease increased by 25% in March and April, by 20% in May and June and by 46% in July and August, the researchers said.
"Income inequality may put lower income individuals at greater risk because they are less able to engage in risk mitigation -- for example, lower income individuals are often employed in public-facing jobs and cannot work from home," Odden said.
In addition, "lower income people may be forced to stay in the workforce, even if it is risky, just to survive in a place that also has high wealth," she said.