Must Whatever Goes up Come Down? Mortality Gradients in the Emergence of COVID-19

According to Fundamental Cause Theory, social inequalities in mortality persist even as disease burdens shift. Emerging infectious diseases however, present a particular challenge for studying these inequalities, as protective interventions are introduced while epidemics unfold, making it difficult to disentangle the effects of pre-existing social disadvantage from unequal access to interventions. To address this challenge, I develop a theory-guided mechanistic modeling framework that embeds pathways to mortality inequalities within a geospatial epidemic model. I apply this framework to the COVID-19 pandemic in Brazil using a susceptible-exposed-infectious-recovered-deceased model. The model incorporates pre-existing social inequalities through disparities in household transmission and infection fatality rates. I simulate non-pharmaceutical interventions (NPIs) and vaccination under nine counterfactual distribution scenarios ranging from no interventions to observed real-world allocation and alternative more equitable strategies. In the absence of interventions, mortality becomes increasingly concentrated in the most vulnerable municipalities as the epidemic spreads. NPIs adopted following observed, socially stratified patterns accelerate rather than create this gradient, suggesting that pre-existing structural inequalities were the primary drivers of Brazils COVID-19 mortality gradient, while unequal intervention uptake reinforced these dynamics. However, prioritizing the most vulnerable municipalities for vaccination reverses the gradient, an effect further compounded by equal adoption of NPIs, neutralizing the impact of these pre-existing conditions. These findings can help guide more equitable policy responses to future emerging infectious disease crises. This study is, to our knowledge, the first mechanistic model to operationalize Bambras pathways framework of inequalities in emerging infectious diseases. HighlightsO_LIFundamental cause theory applied to mechanistic epidemiological modeling C_LIO_LIPre-existing social inequities can drive epidemic mortality disparities C_LIO_LIIntervention distribution can reinforce or reverse mortality inequalities C_LIO_LINational geospatial epidemic model illustrates COVID-19 inequality dynamics in Brazil C_LIO_LIFirst model to operationalize Bambras inequality pathways framework C_LI