This mixed-methods study assesses immunization service delivery, coverage, and barriers among Forcibly Displaced Myanmar Nationals (FDMN) and adjacent host communities in Bangladesh. Combining social mapping, household surveys, and qualitative research, the study examines zero-dose (ZD) and under-immunized children, as well as factors influencing vaccine uptake in humanitarian settings. Findings show higher under-immunization among FDMN populations compared to host communities and highlight key barriers including limited awareness, mobility, service access constraints, and social dynamics. The study identifies actionable strategies—such as strengthening community engagement, coordination, and service delivery systems—to improve immunization uptake and better reach missed populations.