The role of community engagement (CE) in improving demand for immunization merits investigation. The International Rescue Committee developed a CE strategy to implement a vaccine defaulter-tracing tool and a color-coded health calendar aimed at increasing uptake of immunization services in north-west Ethiopia ('The Fifth Child Project'). We report findings from a formative evaluation of this project. In May/June 2016 we conducted 18 participant observations of project activities, 46 semi-structured interviews and 6 focus groups with caregivers, health workers, community members/leaders. Audio-recordings and fieldnotes were transcribed, anonymized, translated and analyzed thematically using inductive and deductive coding. Additional data was collected in November 2016 to verify findings. The project was suitably integrated within the health extension program and established a practical system for defaulter-tracing. The calendar facilitated personalized interactions between health workers and caregivers and was a catalyst for health discussions within homes. At the community level, a regulation exercise of sanctions was observed, which served as a deterrent against vaccine default. Pre-existing community accountability mechanisms supported the CE, although varying levels of engagement between leaders and health workers were observed. The benefits of shared responsibility for immunization were evident; however, more transparency was required about community self-regulatory measures to ensure health-related discussions remain positive.