Social vulnerability and physical frailty trajectories in later life: a life-course and cross-national analysis of China and the UK

Social vulnerability is increasingly recognized as a key determinant of health in later life, but its dynamic relationship with physical frailty across different societal contexts remains insufficiently understood. Using harmonized longitudinal data from the China Health and Retirement Longitudinal Study (CHARLS) and the English Longitudinal Study of Ageing (ELSA), we examined adults aged ≥ 60 years who were non-frail at baseline. Social vulnerability and physical frailty were measured using deficit accumulation indices. Group-based trajectory modeling was applied to identify longitudinal patterns, while Cox proportional hazards models, logistic regression, and restricted cubic splines were used to assess associations and dose–response relationships. Higher levels of social vulnerability were associated with increased risks of frailty progression in both China (HR = 1.29) and the United Kingdom (HR = 1.35). Distinct trajectory groups revealed that individuals with increasing social vulnerability faced substantially higher risks of transitioning to adverse frailty profiles. A non-linear association with a threshold effect was observed in China, whereas a linear relationship was found in the United Kingdom. Higher social vulnerability was associated with adverse frailty trajectories among older adults in both China and the UK. These findings support a life-course perspective in which social vulnerability shows cumulative and dynamic associations with frailty trajectories, while cross-national differences suggest that welfare contexts may influence the strength and functional form of these associations, with important implications for healthy ageing and health inequalities.