Association of armed conflict and global measles cases: A structural equation modeling analysis of 193 countries from 2000 to 2023

BACKGROUND: Global armed conflict and population displacement are increasing, yet their association with population health remains poorly understood. We developed and tested four theoretical models linking armed conflict, population displacement, and socioeconomic development to measles burden across 193 countries from 2000 to 2023. METHODS AND FINDINGS: We analyzed longitudinal country-level data comprising 4,632 country-year observations, combining fixed-effects panel regression and structural equation modeling (SEM). Observed variables included battle-related deaths (BRDs) and forcibly displaced population sizes, while socioeconomic development was modeled as a latent variable incorporating gross domestic product (GDP) per capita, life expectancy, and mean years of schooling. Outcomes were total measles cases and incidence per million population. All four constructed models demonstrated excellent fit (Comparative Fit Index [CFI] 0.991-0.996; Tucker-Lewis Index [TLI] 0.976-0.989; Root Mean Square Error of Approximation [RMSEA] 0.046-0.062). Higher contemporaneous BRDs were associated with higher measles cases (β = 0.17; 95% Confidence Interval [CI] [0.14, 0.20]; p < 0.001), adjusting for population displacement and economic development. When prior-year BRDs were included as an observed variable, the direct effect of contemporaneous BRDs was no longer significant (β = 0.05; 95% CI [-0.01, 0.11]; p = 0.091), while the effect of prior-year BRDs was significantly associated with measles cases (β = 0.14; 95% CI [0.08, 0.20]; p < 0.001) but not incidence after accounting for displacement (β = 0.04; 95% CI [-0.02, 0.11]; p = 0.164). Each standard deviation (SD) increase in a country's standardized log-transformed BRDs was associated with an approximately 0.20 SD increase in measles cases, equivalent to 2,500 additional reported cases for every 3,700 BRDs. In all models, BRDs had a slight negative association with socioeconomic development (β = -0.10; 95% CI [-0.13, -0.07]; p < 0.001), and each SD increase in displaced population corresponded with a 0.20 SD decline in socioeconomic development (95% CI [-0.23, -0.17]; p < 0.001), which was the primary pathway by which displacement was associated with measles. Socioeconomic development, in turn, had a significant direct association with both measles cases and incidence, with each SD increase in socioeconomic development corresponding to a 0.32 to 0.34 and 0.34 to 0.36 SD reduction in cases and incidence, respectively (p < 0.001). Key limitations include reliance on national-level annual aggregates, possible under-reporting of both battle-related deaths and measles cases, and the possibility of unmeasured time-varying confounders that preclude causal interpretation. CONCLUSIONS: Armed conflict is associated with an increased measles burden, both directly and indirectly through associations with lower socioeconomic development and greater population displacement. These findings suggest that mitigating infectious disease risks in volatile settings requires a dual strategy: preserving the structural foundations of health and education while systematically integrating displaced populations into routine immunization programs. Future research using subnational and higher-frequency data is needed to clarify the precise mechanisms and timing of these associations across other vaccine-preventable diseases.