Immunization of children against vaccine-preventable diseases is one of the most cost-effective and potentially equitable public health interventions. Nevertheless, approximately 19.9 million of the world's annual birth cohort are either under-immunized or have not been vaccinated at all. Understanding the factors contributing to under-immunization in settinsg such as sub-Saharan Africa which bears a disproportionate burden of vaccine preventable diseases is key to unlocking the full potential that vaccines offer in reducing under-5 morbidity and mortality. The series or articles in this issue of the Journal, mainly through systematic analysis of District Health Surveillance data bases from 35 countries, highlight the challenges faced in improving vaccination coverage rates in sub-Saharan Africa which has stagnated at approximately 72% for completion of the primary series of infant vaccines over the past decade. The reasons for under-immunization of children is sub-Saharan Africa is identified to be multi-factorial and may differ between and within countries. This highlights the need for country-specific, possibly at a district or sub-regional level, interrogation of factors contributing to under-immunization of children, to work toward providing Universal Health Coverage as envisioned in the Sustainable Development Goals.