Equity in out-of-pocket health expenditure: Evidence from a health insurance program reform in Mexico.

Background: The fragmentation of health systems in low- and middle-income countries (LMICs) deepens health inequities and shifts the economic burden of health care to families via out-of-pocket spending (OOPHE). This problem has been addressed by introducing public health insurance programs for poor people; however, there is a lack of knowledge about how equitable these programs are. We aimed to analyse the long-term effects of the Seguro Popular (SP) voluntary health insurance program, recently phased out and replaced by the Health Institute for Welfare (Instituto de Salud para el Bienestar (INSABI)), on OOPHE equity in the poor Mexican population. Methods: We conducted a pooled cross-sectional analysis using eleven waves of the National Household Income and Expenditure Survey (2002-2020). We identified the effect of SP by selecting households without social security (with SP or without health insurance (n = 169 766)) and matched them by propensity score to reduce bias in the decision to enrol in SP. We estimated horizontal and vertical equity metrics and assessed their evolution across subpopulations. Results: The program's entry years (2003-2010) show a positive redistributive effect associated with a focalised stage of the program, while oversaturation could have diluted these effects during 2010-2014, with adverse results in terms of vertical equity and re-ranking among insured families. SP is more horizontally inequitable than for those uninsured. Within SP, the redistributive effect could improve up to 13% if all families with similar expenditures were spending equal OOPHE and horizontal equity was eliminated. Regarding vertical equity, SP outperforms the insured population with middle-range coverage some years after the implementation, but this progress disappears. Conclusions: To achieve universal health coverage, health authorities need to create and execute financial protection mechanisms that effectively address structural inequalities. This involves implementing a more comprehensive risk-pooling mechanism that makes social insurance sustainable in the long-run by increasing the social-economic influx of resources. It is essential to monitor oversaturation and financial sustainability to achieve optimal results. The replacement of the SP with INSABI highlights the complexity of maintaining a social insurance program where the ideology of different governments can influence the program structure, regulation, financing, and even its existence.