Impact of armed conflict on measles surveillance and zero-dose vaccination status in Tigray, Ethiopia, 2018 - 2024

BackgroundArmed conflict disrupts health systems, undermining routine immunization and disease surveillance. These disruptions can delay outbreak detection and allow population susceptibility to accumulate unnoticed. This study assessed the impact of the 2020 - 2022 conflict in Tigray, Ethiopia, on measles epidemiology, focusing on surveillance reporting, age distribution of cases, and vaccination status. MethodsWe conducted a retrospective longitudinal analysis of national case-based measles surveillance data from Ethiopia (2018-2024; n = 69,866). The study period was classified into pre-conflict (2018 - 2019), conflict peak (2020 - 2022), and post-conflict recovery (2023 - 2024) phases. Two-way analysis of variance examined regional differences in age at infection across phases. Multivariable logistic regression estimated adjusted odds ratios (aORs) for confirmed measles cases being unvaccinated (zero-dose), using the pre-conflict period as the reference and adjusting for age and sex. Surveillance quality was assessed using demographic data completeness. ResultsDuring the conflict peak, reported measles cases from Tigray declined to 0.01% of nationally reported cases, consistent with near-total surveillance collapse. After hostilities ended, a marked pediatric shift emerged, with the median age of infection in Tigray declining from 24.0 years during the conflict to 5.0 years in the post-conflict period (p < 0.0001), a pattern not observed in other regions. Compared with the pre-conflict baseline, the odds that a confirmed measles case was zero-dose were substantially higher during the conflict peak (aOR: 71.43; 95% CI: 14.1 - 1000.0) and remained elevated during post-conflict recovery (aOR: 2.49; 95% CI: 1.17 - 5.52). During the conflict peak, 50% of confirmed cases in Tigray lacked sex-disaggregated data. ConclusionThe conflict in Tigray severely disrupted immunization services and surveillance, delaying detection of a large susceptible pediatric cohort. These findings underscore the need for age-targeted catch-up vaccination and resilient surveillance systems during post-conflict recovery. Key questionsO_ST_ABSWhat is already known?C_ST_ABSO_LIArmed conflict disrupts routine immunization and increases the risk of measles outbreaks in low- and middle-income countries. C_LIO_LIConflict also weakens disease surveillance systems, which can delay outbreak detection and obscure the true burden of vaccine-preventable diseases. C_LIO_LIMeasles is highly sensitive to disruptions in vaccination coverage and is often among the first diseases to resurge following health system breakdown. C_LI What are the new findings?O_LIDuring the 2020-2022 conflict in Tigray, measles surveillance reporting declined to near zero, despite rising national measles incidence. C_LIO_LIAfter the cessation of hostilities, measles cases in Tigray shifted sharply toward young children, revealing a large cohort of unvaccinated children that had accumulated during the conflict period. C_LIO_LICompared with the pre-conflict period, confirmed measles cases in the post-conflict period had substantially higher odds of being zero-dose, indicating prolonged interruption of routine immunization services. C_LI What do the new findings imply?O_LISurveillance data from conflict-affected settings may substantially underestimate disease burden during periods of active conflict, leading to delayed recognition of outbreaks. C_LIO_LIPost-conflict recovery strategies should prioritize age-stratified catch-up vaccination campaigns targeting children born during conflict periods, rather than relying solely on routine immunization services. C_LIO_LIStrengthening surveillance resilience in fragile and conflict-affected settings is essential to prevent delayed detection of measles and other vaccine-preventable disease outbreaks. C_LI