To explore the relationship between social isolation and frailty trajectories, and to provide evidence for targeted interventions and healthy aging.

Based on four waves (2011-2018) of the China Health and Retirement Longitudinal Study, 7 805 adults aged 45 years and above were included. Latent Class Mixed Models (LCMM) wereapplied to identify longitudinal trajectories of social isolation and the frailty index.Multinomial logistic regression was used to analyze factors associated with each trajectory, and conditional and joint probabilities were calculated to evaluate the correspondence between social isolation and frailty trajectories.

Four trajectories of social isolation were identified: Low-isolation stable (36.0%), Low-isolation rapid increase (2.7%), Moderate-isolation stable(52.7%), andHigh-isolation slow increase(8.7%)—along with three frailty trajectories: Robust stable(27.0%), Prefrail stable (54.2%), and Frailty worsening (18.8%).Joint probabilities further indicated that the combination of Moderate-isolation stable and Prefrail-stable had the highest overall probability (π_jk=0.29).

Levels and temporal patterns of social isolation are strongly associated with frailty development. Middle-aged and older adults with higher or increasing isolation levels, as well as those at the prefrail stage, should be prioritized in community-based interventions aimed at strengthening social support, maintaining physical function, and improving psychological well-being to delay frailty progression and promote healthy aging.