Abstract
<title>Abstract</title> <p>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 program. Methods This study used a sequential explanatory mixed-methods design in five purposively selected sites across the two states between July and August 2025. We obtained BCU call-in data (Rounds 1–3) and conducted 67 key informant interviews and 18 focus groups with purposively sampled program managers, partners, RI providers, and community members, including caregivers of children under five. Quantitative data were analysed descriptively and qualitative data thematically, with triangulation across datasets. Results Across the three rounds, BCU achieved 70% coverage (Penta2) in Bauchi and 75% (Penta3) in Rivers, with outreach reaching the most children in both states. Bauchi vaccinated more children aged 24–59 months, whereas Rivers vaccinated more children aged 12–23 months. Effective BCU planning relied on collaborative, multi-stakeholder engagement involving government, partners, and communities and leveraging existing immunisation governance structures and staff; detailed microplanning with geospatial mapping; and expanded vaccine supply. Effective BCU implementation combined multi-strategy service delivery adapted to local realities, community mobilisation through community leaders and groups (e.g., mama-to-mama), 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 reaching zero-dose children is feasible with adequate resources, coordination, and community engagement. Sustaining these gains requires formal policy positioning catch-up as a continuous RI component, supported by domestic financing, health system investment, operational flexibility, and formalised roles for community volunteers, rather than a periodic response to zero-dose children.</p>