Why, how and in what contexts interventions on social determinants of health work on socio-health inequities? The case of returnees from Spain and the United Kingdom in the post-Brexit scenario.

Return migration, the second most common migration path, is shaped by social determinants of health (SDH). However, little is known about how interventions targeting these determinants work or for whom, especially in the dynamic and complex post-Brexit context. This study aimed to identify SDH-focused interventions to understand for whom, under what circumstances and why they may impact on socio-health inequities in returnees from Spain and the United Kingdom (UK) and to explore how Brexit may have affected the regulation and provision of health and social services for these population. Using a realist synthesis approach following RAMESES guidelines, it retrieved and analyzed academic and grey literature documents alongside expert consultation and co-production workshops. Six context-mechanism-outcome (CMO) configurations were developed: three for Spanish returnees ("Disaffection" "Lost and forsaken" and "Empowerment for action") and three for British returnees ("In no one's land" "Face to face and at home" and "The best of both worlds"). These configurations reveal challenges such as disaffection, administrative barriers, and loss of rights. Spanish returnees struggled with professional recognition and weakened welfare systems, while British returnees faced uncertainty and rigid administrative requirements, impacting their quality of life. While programs targeting human capital can empower returnees, their exclusivity may exacerbate inequities. Findings highlight the need for tailored interventions addressing SDH to reduce inequities and provide actionable insights for inclusive policy design supporting returnees' well-being and fostering their sense of belonging.