Social Determinants and Outbreak Dynamics of the 2025 Measles Epidemic in Mexico: A Nationwide Analysis of Linked Surveillance Data.

Measles resurgence threatens elimination achievements in the Americas. We conducted a nationwide analysis of Mexico's 2025-2026 measles outbreak, integrating individual-level surveillance data from the Special Surveillance System for Febrile Exanthematous Diseases with municipal-level social determinants from eight national databases, complemented by molecular surveillance data. We analyzed 6892 confirmed cases using spatial autocorrelation (Moran's I and LISA), effective reproduction number estimation, logistic regression models for municipal case presence, and multivariable logistic regression for risk factors for complications. Cases concentrated in Chihuahua (65.2%), with 47 LISA hot-spot municipalities containing 64.4% of cases. Molecular surveillance confirmed two independent introductions: D8/MVs/Ontario.CAN/47.24 (98.1%), linked to the North American outbreak, and B3 (1.9%) in Oaxaca. Transmission followed a three-stage pattern: introduction through seasonal agricultural worker networks, amplification in undervaccinated communities, and diffusion to marginalized indigenous populations. A dual-model analysis revealed that school non-attendance among children aged 6-14 years may have mediated the effect of very high marginalization on municipal case presence (OR 1.26; p < 0.001), identifying a potentially actionable vaccination pathway. Vaccine effectiveness was 98.1%, confirming susceptible accumulation rather than vaccine failure. Wave-stratified analysis showed late outbreak phase as an independent risk factor for complications (aOR 1.68, 95% CI: 1.42-2.00), converging with an age of <1 year (aOR 3.36), indigenous status (aOR 1.89), and unvaccinated status (aOR 1.96) in the most marginalized communities. Indigenous individuals comprised 29.1% of cases but 76% of the 25 deaths. This outbreak demonstrates that national vaccination thresholds are insufficient when municipal pockets of susceptibility remain systematically underserved.