Determinants of zero-dose status and a framework for institutionalizing the Big Catch-Up in fragile settings: operational evidence from the Central African Republic

Abstract Background Zero-dose children, defined as children who have not received the first dose of pentavalent vaccine (DTP 1), are a key indicator of inequitable access to immunization services. The Central African Republic (CAR) remains among the countries with the highest burden of zero-dose children in the WHO African Region due to chronic fragility, insecurity, population displacement, weak health systems, and limited access to health services. In response, the Big Catch-Up (BCU) initiative was implemented to identify and vaccinate missed children while strengthening routine immunization services. This study aimed to identify determinants of zero-dose status and propose priority actions for institutionalizing the Catch-up approach in CAR routine immunization. Methods We conducted a secondary analysis of operational and program monitoring data collected during implementation of the Big Catch-Up (BCU) initiative in the Central African Republic between 2025 and 2026. The analytical dataset combined information from household visits, outreach activities, routine and mobile vaccination sessions, supportive supervision reports, district review meetings, community health worker and civil society organization reports, program monitoring systems, and the 2023–2024 Expanded Program on Immunization (EPI) Coverage Survey. Determinants of missed vaccination were grouped into four predefined operational domains: geographic and access barriers, health service delivery barriers, community and caregiver-related factors, and immunization information system constraints. Associations with zero-dose status were assessed using modified Poisson regression to estimate adjusted prevalence ratios (aPRs) with 95% confidence intervals, and programmatic lessons were synthesized to inform a framework for institutionalizing the BCU. Results A total of 847 child records was analyzed, of which 364 (43.0%) were classified as zero-dose. The most frequently identified determinants of missed vaccination were lack of caregiver knowledge of the vaccination schedule (291/847; 34.4%) , geographically inaccessible areas (190/847; 22.4%) , caregiver or child absence during vaccination activities (189/847; 22.3%) , distance greater than 5 km from vaccination services (180/847; 21.3%) , cancelled vaccination sessions (149/847; 17.6%) , shortages of trained vaccination personnel (132/847; 15.6%) , and vaccination received but not recorded (124/847; 14.6%) . Information-system constraints represented the largest determinant category (290/847; 34.2%), followed by geographic access barriers (203/847; 24.0%) and service-delivery barriers (110/847; 13.0%). In multivariable analysis, caregiver lack of knowledge of the vaccination schedule (aPR 1.48; 95% CI 1.28–1.71), caregiver or child absence (aPR 1.56; 95% CI 1.30–1.87), geographically inaccessible areas (aPR 1.28; 95% CI 1.10–1.49), vaccination team absence (aPR 1.38; 95% CI 1.12–1.70), shortages of trained personnel (aPR 1.23; 95% CI 1.05–1.44), and vaccination not recorded (aPR 1.27; 95% CI 1.06–1.52) remained independently associated with zero-dose status, whereas availability of a vaccination card was protective (aPR 0.65; 95% CI 0.53–0.78). Conclusions Zero-dose status in CAR is driven primarily by access and health system barriers, compounded by weaknesses in immunization information systems that limit identification and follow-up of vulnerable children. Institutionalizing the BCU as a continuous, data-driven component of routine immunization services, supported by improved community outreach, child tracking mechanisms, and electronic immunization registries, could contribute to sustainable reductions in immunization inequities in fragile and humanitarian settings.