Implementing the Big Catch-Up initiative in Mozambique: a qualitative assessment of implementation barriers, equity enablers, and lessons for routine immunization

Abstract Background The COVID-19 pandemic impacted routine immunization services negatively, increasing the number of zero dose and under immunized children. In 2024, Mozambique implemented the Big Catch-Up (BCU) initiative to reach these missed children. This study examined implementation barriers and enablers and identified lessons for integrating equitable catch-up vaccination into routine immunization services. Methods We conducted a qualitative assessment complemented by a structured desk review. Data collection included 273 semi-structured interviews and 11 focus group discussions involving 55 participants across seven provinces in Mozambique. Participants included national and subnational programme managers and implementers, health workers, technical partners, community health workers, and community actors. Qualitative data were analyzed thematically using a hybrid inductive–deductive approach, with findings triangulated across respondent groups and documentary evidence. Results Four interrelated factors shaped BCU implementation. First, strong district-level coordination and adaptive microplanning enabled teams to translate local information into operational decisions. Second, engagement of community health workers, community leaders, and other trusted actors strengthened community mobilization, acceptance, and identification of missed children, although misinformation, displacement, and sociocultural barriers persisted in some settings. Third, combining routine immunization records with community mapping and local knowledge enabled teams to identify zero-dose and under-immunized children and adapt mobile brigades, concentration points, and house-to-house approaches to local circumstances. Fourth, hybrid paper–digital systems supported continuity of monitoring and local decision-making despite connectivity and infrastructure constraints. Implementation was nevertheless constrained by workforce shortages, insufficient training, logistical and financing challenges, and limitations in supervision and digital infrastructure. Conclusion Mozambique's experience suggests that equitable catch-up vaccination depends not only on identifying missed children but also on adapting implementation to the contexts in which they live. Institutionalizing community-informed microplanning, locally responsive delivery strategies, engagement of trusted community actors, and data-informed decision-making within routine immunization systems may help sustain efforts to reach zero-dose and under-immunized children beyond the BCU.