Abstract
<title>Abstract</title> <p>Background Countries facing the same public health crisis may diverge not in clinical symptoms but in the social dimensions of well-being, yet single-country studies cannot capture such contrasts. South Korea and Japan, which are demographically similar yet institutionally distinct, offer an informative comparative case. This study compares the longitudinal determinants of depression, loneliness, and life satisfaction in the two countries across the transition from the pandemic to the endemic phase of COVID-19. Methods In a two-wave panel design, 1,614 adults (900 Korean, 714 Japanese; aged 20–69) recruited through census-benchmarked online quota sampling completed measures of depression (PHQ-9), loneliness (UCLA-3), and life satisfaction (SWLS) at both waves in 2023. The same respondents were assessed before (March–April) and after (October–November) the WHO declaration ending the public health emergency, and generalized estimating equations were estimated separately for each country and supplemented by a pooled model with country as a predictor. Results The two societies diverged in social rather than clinical dimensions of well-being: Japanese participants reported higher loneliness (B = .768, 95% CI [.695, .842]) and lower life satisfaction (B = − .595, 95% CI [− .724, − .467]) than Korean participants, whereas depression levels were comparable. The pattern of determinants also differed: relational stressors, including family and workplace conflict, were associated with depression and loneliness in Korea, whereas socioeconomic position and age were the more consistent correlates in Japan. Changes from the first to the second wave were small in both countries. Conclusions Neighboring societies facing a common public health crisis diverged primarily in loneliness and life satisfaction rather than in depression, with distinct national patterns of risk. Population mental health planning should treat social connectedness as a distinct policy target and tailor support to the dominant sources of risk in each national context.</p>