In line with Gavi's 5.0 vision of "leaving no one behind with immunization," this study investigates the prevalence and determinants of zero-dose (ZD) and under-immunized (UI) children in specific areas of Bangladesh. Utilizing Lot Quality Assurance Sampling (LQAS) from 504 households across hard-to-reach regions and urban slums, the study reveals that 32% of children were either ZD (8%) or UI (26%). Key findings highlight urban slums and wetlands as high-risk areas for ZD/UI, with significant influencing factors including the distance to the nearest EPI center, availability of EPI cards, caregiver age, maternal education and occupation, and household income. The availability of EPI cards emerged as the most critical factor. These insights are vital for policymakers and program managers aiming to enhance immunization coverage and address disparities in Bangladesh.