Mapping Strategies and Interventions to Reach Zero-Dose and Under-Immunized Children: A Scoping Review

Background: Zero-dose (ZD) children, classified as those who have not received their first dose of diphtheria–tetanus–pertussis vaccine, remain at high risk of vaccine-preventable diseases globally. Despite progress, an estimated 14.3 million children were ZD in 2024. We systematically mapped global evidence on interventions to reach ZD and under-immunized children to inform policy and planning towards vaccine equity. Methods: We conducted a scoping review of peer-reviewed studies published between 2015 and 2025 reporting interventions to reach ZD or under-immunized children under five years. We searched PubMed, Scopus, Web of Science, and EBSCOhost databases. Two reviewers independently extracted data using standardized forms. We analyzed interventions thematically and by World Bank income classification. Results: Of the 37 included studies, 24% (n = 9) originated from low-income countries (LICs), 65% (n = 24) from lower-middle-income countries, 5% (n = 2) from upper-middle-income countries, and 5% (n = 2) from high-income countries. Mobile outreach programmes were the most prevalent intervention at 46% (17/37), followed by engagement with religious and community leaders at 41% (15/37) and community healthcare worker involvement at 27% (10/37). We identified eight thematic intervention areas: community engagement, community and health workforce, health systems strengthening, outreach and service delivery, technology-driven approaches, monetary incentives, integrated human–animal campaigns, and leveraging military and security forces. Intervention patterns varied by income level, with LICs emphasizing community engagement and mobile outreach, while higher-income countries utilized technology and incentive-based approaches. Five studies specifically reported on ZD children, with most reporting improvements in first dose of DTP/Penta vaccine uptake following intervention implementation. Conclusions: Multi-faceted, context-adapted interventions show promise for reaching ZD children. However, clearer definitional standards are needed to distinguish ZD from under-immunized populations. Strategies identified in this review can guide policymakers in designing and prioritizing further evaluation of interventions to achieve equity in immunization coverage globally.