Reaching zero-dose children: lessons learned from the planning, implementation, and monitoring of Nigeria’s Big Catch-up initiative

Background The COVID-19 pandemic disrupted routine immunisation (RI) services globally, leaving millions of zero-dose children, with Nigeria bearing the highest burden. In response, Nigeria conducted the Big Catch-Up (BCU) RI intensification between December 2024 and April 2025 to reach and vaccinate children aged 12–59 months who had missed vaccinations. This case study documented lessons learned from planning, implementing, and monitoring BCU activities in Bauchi and Rivers States to inform policy decisions on integrating catch-up vaccination into Nigeria’s immunisation programme. Methods This study used a sequential explanatory mixed-methods design at five purposively selected sites across the two states between July and August 2025. We obtained BCU monitoring data (Rounds 1–3) and conducted 67 key informant interviews and 18 focus groups with purposively sampled programme managers, partners, RI providers, and community members. We analysed quantitative data descriptively and qualitative data thematically, triangulating findings across datasets. Results Across three rounds, BCU reached 75% of the target cohort with Penta3 (Rivers) and 70% with Penta2 (Bauchi), with outreach reaching the most children in both states. Bauchi reached more children aged 24–59 months, whereas Rivers reached more children aged 12–23 months. Effective BCU planning relied on collaborative, multi-stakeholder engagement involving government, partners, and communities; leveraging existing immunisation governance structures and staff; detailed microplanning with geospatial mapping; and facility-tailored vaccine forecasting and allocation. Effective BCU implementation combined multi-strategy service delivery adapted to local realities, community mobilisation through community leaders and groups, and tailored communication to reach zero-dose children and address socio-cultural barriers. Effective BCU monitoring required age-segregated reporting tools, real-time digital data, daily review meetings, supportive supervision, and ward-level data validation, which enabled feedback, rapid problem-solving, and quality control. Collectively, the mechanism that worked leveraged routine systems rather than a parallel structure but depended in part on unpaid volunteers. Conclusions BCU demonstrated that identifying and vaccinating zero-dose children with catch-up doses is feasible with adequate resources, coordination, and community engagement. Sustaining these gains requires formal policy that positions catch-up as a continuous RI component, supported by domestic financing, systems integration, operational flexibility, and formalised roles for community volunteers, rather than a periodic response to zero-dose children.