Infectious Disease as a Mechanism Linking Health and Income Inequality
Authors: Bhattacharya Joydeep , Bhattacharya Jay , Kim Min 
Published: October 2021

Abstract
BackgroundWithin-country inequality has been rising worldwide rapidly since the 70s. An extensive literature has examined the effect of inequality on health, finding health outcomes to be worse in more unequal countries. Among the measures of health used are life expectancy, mental illness, obesity, infant mortality, teenage births, homicides, imprisonment, etc., or some weighted index of such measures. While these measures of health are informative, they are indirect. Secondly, many studies fail to establish an independent effect of inequality on health. Finally, as noted, if the individual-level relationship between health and income is non-linear, cross-section studies may detect a spurious association between inequality and health due to an aggregation problem.This paper studies the relationship between the incidence of infectious disease, a direct and vital measure of health, and ambient income inequality. Our hypothesis is, the more income-unequal a society, the higher the chance a random mixing of people from different income strata brings the infected and uninfected closer, thereby raising disease spread. This implies two countries with similar per capita incomes but with varying levels of income inequality can exhibit very different trajectories of disease spread. We investigate this causal pathway by examining whether countries with elevated levels of income inequality have higher rates of Tuberculosis (TB) incidence per capita. The choice of TB is appropriate because it is an enduring, serious threat to global public health, the leading cause of death from infectious diseases worldwide. Moreover, it is well known that the four stages of TB pathogenesis (exposure to infection, progression to disease, late or inappropriate diagnosis and treatment, and treatment adherence) vary across rich and poor individuals.MethodsWe used publicly available panel data for one hundred and thirty-three countries between 1997 and 2013. The data include TB incidence and prevalence per 100,000 people. It also contains data on income inequality (Gini coefficient) both across countries and across time within countries. Our multivariate regression model controlled, among other variables, for economic output per capita, HIV prevalence, public health expenditures, population density, and poverty, and also incorporated a country-level fixed effect and time-fixed effects. A novel correction for “aggregation bias” using data on diabetes (a non-communicable disease) is also applied.FindingsOverall, elevated levels of income inequality were positively associated and causally connected to tuberculosis prevalence. All else same, countries with income-Gini coefficients a mere 10% apart would likely see a 5% difference in tuberculosis prevalence.InterpretationLike any air-borne infectious disease, TB is akin to a pollutant that spoils air quality and makes it unhealthy for all who breathe it. Our findings suggest a significant cause of this externality is ambient income inequality. In effect, TB is a negative externality whose reach amplified by income inequality. Around the world, the emergence of COVID-19 has renewed focus on the importance of reducing income differences. We join in that chorus by arguing that policy action aimed at reducing income inequities could directly contribute to a reduced TB burden by reducing the chance of infection spread via contact between the poor and the rich.