Impact of social isolation on mental health changes by socio-economic status: A moderated mediation analysis among non-migrant, migrant, and refugee subpopulations in Germany, 2016-2020.

Background: Populations experiencing precarity face heightened mental health inequities, especially during crises. In this regard, it is established that socio-economic status (SES) and social isolation are critical factors influencing mental health outcomes, which interact syndemically. Understanding their interrelated mechanisms is crucial for developing effective public health strategies to support populations experiencing precarity in future crises. Methods: Data from the German Socio-Economic Panel (2016-2020) were analysed in a longitudinal design. A moderated mediation analysis examined the effect of SES on mental health (PHQ4) prior to and during the pandemic, with social isolation change as mediator and migration status as moderator, distinguishing non-migrant (n = 3841), migrant (n = 766), and refugee (n = 331) subpopulations. The model was adjusted using month of interview, baseline PHQ-4 scores, age, gender, and baseline social isolation. Results: High SES was linked to smaller mental health declines compared to low SES (β = -0.635, 95 % CI: -0.822, -0.447). This effect was partially mediated by change in social isolation (-0.049, 95 % CI: -0.082, -0.015). The mediation effect was weaker in non-migrant (-0.030, 95 % CI: -0.066, 0.005) and non-significant in migrant subpopulations (-0.062, 95 % CI: -0.157, 0.034). For refugees, SES effects were non-significant, with baseline and changes in social isolation playing a more critical role. Findings: SES and social isolation critically influenced mental health changes, with substantial variability across subpopulations. Targeted interventions addressing SES inequities and fostering social connections are essential for mitigating mental health disparities, particularly among low SES groups, migrants, and refugees.