The COVID-19 pandemic severely disrupted routine immunization globally and across Cameroon, causing a surge in “zero- dose” (ZD) children and exacerbating “missed opportunities for vaccination” (MOV). This necessitated accelerated recovery initiatives like the Big Catch-Up (BCU). This study explores BCU’s operationalization, implementation, and sustainability in Cameroon, assessing how vertical funding interacted with national health systems to reach ZD and under-immunized children. Focusing on four districts (urban, conflict-affected, isolated island, and remote rural), we triangulated data from 34 document reviews (e.g. national policies, operational plans, budget frameworks, and Expanded Program on Immunization reports), 37 key informant interviews across all health system levels, and 11 focus group discussions with 86 community stakeholders, including caregivers. Thematic analysis reveals that while BCU remained a campaign-style intervention with parallel reporting structures, its funding acted as a critical accelerator to operationalize Cameroon’s preexisting but under-implemented 2021 National Policy and Guidelines on Catch-up Immunization. However, centralized financial control created top-down pressures that were mitigated through bottom-up co-creation during local micro-planning. Operationally, reducing MOV required a shift from passive screening to active “Catch and Vaccinate” models, necessitating context-specific delivery adaptations in conflict zones and island populations. Despite these operational successes, stakeholders expressed deep concern regarding the long-term institutionalization due to an impending fiscal cliff. The system’s heavy reliance on external donor financing threatens the sustainability of community health workers’ incentives essential for demand generation. Furthermore, while the parallel IASO digital platform enabled real-time geolocation of missed children, it created double-reporting burdens, highlighting the structural challenge of synchronizing campaign tools with the national routine health information system. Ultimately, while recovery funding can successfully catalyze national policies and drive immediate equity gains, sustained resilience requires an urgent transition to domestic financing, the embedding of adaptive service delivery models into routine primary care, and the harmonization of digital tracking systems.

Catch-up reports and briefs

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• The Big Catch-up

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