DPT3 Vaccination Coverage, Systemic Capacity, and DPT-Related Disease Inequality in the Western Pacific

Background Routine childhood immunization has reduced vaccine-preventable disease burden, yet inequalities in DPT-related outcomes persist in the WHO Western Pacific Region. This study examined whether DPT3 vaccination coverage was associated with DPT-related disease burden and inequality, and whether this association varied with systemic capacity, defined as the governance, health service coverage, and logistics environment in which immunization programs operate. Methods We constructed a 2000–2023 country–year panel for 32 countries and areas using Global Burden of Disease 2023 estimates, WHO–UNICEF DPT3 coverage data, and World Bank indicators. A composite DiseaseIndex for under-five DPT-related incidence, mortality, and DALYs was derived using principal component analysis; the first component explained 74.5% of total variance. Cross-country inequality was measured using Theil and Gini indices. We estimated mean and distributional associations using fixed-effects and distributional regression models, and used counterfactual simulations and decomposition analyses to explore model-based inequality patterns. A restricted mechanism analysis of 17 countries and areas examined modification by systemic capacity. Results Higher DPT3 coverage was associated with lower average DiseaseIndex and lower cross-country inequality. In two-way fixed-effects distributional models, DPT3 coverage was negatively associated with RIF-Theil inequality (β = –6.130, 95% CI: –8.260 to –4.000) and RIF-Gini inequality (β = –2.941, 95% CI: –3.841 to –2.041). Exploratory counterfactual simulations suggested that model-implied inequality patterns varied across coverage-improvement scenarios, with convergence-oriented changes showing more consistent reductions than uniform or threshold-based increases. Decomposition analyses showed that observable characteristics explained 13.3% of the Theil gap and 15.9% of the Gini gap between low- and high-coverage settings, leaving a large unexplained component. In the 17-country mechanism sample, the association between DPT3 coverage and lower inequality was stronger in lower-SystemIndex settings, possibly reflecting larger marginal gains where baseline disease burden or coverage gaps were greater. Conclusions Higher DPT3 coverage was associated with lower DPT-related disease burden and inequality, but coverage alone did not fully account for persistent disparities. These ecological observational findings suggest that systemic capacity may condition how vaccination coverage is translated into equitable health gains, and should be interpreted as associative rather than causal.