Associations between income inequality at municipality level and health depend on context - a multilevel analysis on myocardial infarction in Sweden.

This study investigates whether a) income inequality in Swedish municipalities increases the risk of myocardial infarction (AMI); b) the association between income inequality and AMI is mediated by level of residential segregation, measured as homogeneity in parishes (as a proxy for neighbourhoods) within municipalities; and c) there is an interaction between parish homogeneity and individual level social position. The study population consisted of all individuals aged 40-64 years in 1990 who lived in municipalities with >50,000 inhabitants (n = 1,284,955). Data on socioeconomic, demographic information and diagnosis data on AMI were obtained by linkage between authority-administered registers and the National Patient Register. All individuals were followed from 1991 onwards until the first relevant discharge, death or end of observation period (1998). We used a multilevel Poisson model where individuals were nested within 729 parishes which in turn were nested in 41 municipalities. We found that the risk for AMI was lower in the municipalities with higher degree of income inequality. Segregation of households in the highest income quintile diluted, but did not eliminate, the association between income inequality and risk of AMI - the degree of parish affluence seemed to be more important as a mediator than other parish characteristics, even when individual level characteristics were added to the model. Interaction analyses showed that the divide between manual workers and non-manual employees became more apparent in parishes with a higher degree of parish affluence. This was more apparent in municipalities with higher income inequality and was due to a decreasing risk among high level non-manual employees and an unchanged risk among manual workers. The results give some support to the idea that income inequality might serve as a proxy for social stratification even in a comparatively egalitarian context.