Despite advancements, Nigeria still faces significant challenges in reaching zero-dose children, particularly in marginalized communities with limited healthcare access, low socioeconomic status, and cultural barriers. This rapid assessment investigates factors hindering routine immunization and aims to improve the national immunization program by focusing on zero-dose children and neglected communities. Conducted in eight LGAs across four states (Bauchi, Borno, Kano, and Sokoto), the study utilized a mixed-methods approach, including evaluations of immunization reporting systems, health facility assessments, household vaccination surveys, and qualitative interviews. Findings highlight issues in data accuracy, facility distribution, staffing, service availability, and cold chain management. High caregiver confidence in vaccines is noted, but practical challenges persist. The study underscores the importance of strengthening data systems, increasing qualified healthcare staff, enhancing community engagement, and addressing affordability and service-related issues to improve immunization coverage and reduce zero-dose children.