Area Deprivation and Health Outcomes in Preschool Children in Japan: A Nationwide Cohort Study

Background: Despite Japan’s universal health insurance system, health disparities have increased since the 1990s. However, the impact of area deprivation on various aspects of child health remains understudied. Methods: This population-based cohort study followed 38,554 children born in Japan (May 10–24, 2010) from birth to age 5.5 years. Using an outcome-wide approach, Bayesian three-level logistic regression models (individuals in municipalities within eight major regions) assessed associations between municipality-level Area Deprivation Index (ADI) at birth and multiple preschool health outcomes (hospitalizations for all causes; respiratory infections; gastrointestinal diseases; Kawasaki disease; medical visits for asthma, allergic rhinitis, atopic dermatitis, food allergy, injury, intussusception; prevalence of overweight/ obesity), adjusting for individual-level factors. Results: Higher ADI was associated with increased risk of all-cause hospitalization (adjusted odds ratio [aOR] per 1-standard-deviation increase in ADI, 1.04; 95% credible interval [CrI], 1.01–1.07), respiratory infections (aOR, 1.08; 95% CrI, 1.04–1.13), gastrointestinal diseases (aOR, 1.11; 95% CrI, 1.03–1.20), asthma (aOR, 1.10; 95% CrI, 1.01–1.19). Overweight/obesity at age 5.5 years also increased with higher ADI (aOR, 1.11; 95% CrI, 1.06–1.16). Higher ADI was inversely associated with Kawasaki disease (aOR, 0.86; 95% CrI, 0.77–0.96), though not robust in sensitivity analysis. Geographic clustering was observed for all outcomes, particularly at municipality level. Conclusion: We found persistent municipal-level health inequalities across various childhood health outcomes in Japan, despite its universal health insurance system. These findings suggest that policymakers should address health inequalities through comprehensive strategies targeting broader social determinants beyond health care access.