Reaching zero-dose children through the Big Catch-Up initiative in the Eastern Mediterranean Region

Routine immunization (RI) is one of the most effective public health interventions for reducing childhood morbidity and mortality. However, recent disruptions to RI have created immunity gaps in the Eastern Mediterranean Region (EMR), increasing the number of zero-dose (ZD) and under-immunized children, particularly in countries affected by conflict, displacement, service disruption, and ongoing vaccine-preventable disease outbreaks. The Big Catch-Up (BCU) initiative aimed to identify and vaccinate missed children, close immunity gaps, and strengthen RI systems. In the six Gavi-supported EMR countries included in this study (Afghanistan, Pakistan, Somalia, Sudan, Syria, and Yemen), implementation targeted children aged 12–59 months. This study assessed progress in reaching ZD children during BCU implementation in these six countries. A descriptive, multi-country programmatic analysis was conducted using quantitative and qualitative data from BCU quarterly reports covering implementation from Q4 2024 to Q1 2026. Additional data sources included World health Organization (WHO and United Nations Children’s Fund (UNICEF) Estimates of National Immunization Coverage (WUENIC), Gavi-approved vaccine dose data, and UNICEF vaccine shipment data. Quantitative analysis summarized administered doses, zero-dose targets, BCU coverage, vaccine consumption, and performance by country and antigen among children aged 12–59 months. Age-disaggregated analyses were conducted for children aged 12–23 months and 24–59 months. Qualitative information from country reports on implementation successes, challenges, and lessons learned was summarized by country and thematic area. During the BCU implementation period, 3.5 million zero-dose children were reached with Penta1 across the six countries, representing 53% of the estimated cumulative population of 6.6 million ZD children. Of the 57 million Gavi-approved vaccine doses, 98% were shipped to countries, and 47.7 million doses were administered, equivalent to 84% of approved doses. Implementation success was supported by targeted identification of areas with high numbers of zero-dose children, integrated outreach, integration with other health services, and recruitment of vaccinators and deployment of community health workers. Key challenges included insecurity, population movement, inaccessible districts, community refusal, and limited community engagement. Substantial progress in reaching missed children was reported during BCU implementation in high-priority EMR countries. Lessons learned highlight the importance of integrating catch-up vaccination with RI, strengthening microplanning, ensuring timely vaccine supply and financing, improving community engagement, and institutionalizing catch-up vaccination.