Abstract

Childhood immunization is one of the most effective public health measures, yet inequities remain in marginalized populations across Asia, where parental adherence is essential to sustaining the Expanded Program on Immunization. This narrative review examines determinants of adherence among under-five children in disadvantaged communities. Following PRISMA guidelines, searches of PubMed, Scopus, and Google Scholar identified studies published between 2015 and 2025, with earlier key works included as relevant. Twenty-one studies from South, Southeast, and East Asia were analyzed. Five domains were associated with adherence: socioeconomic and access factors, where maternal education, household income, and possession of immunization cards were positive predictors, while remote residence was a barrier; trust, cultural beliefs, and social norms, with misinformation and vaccine controversies reducing uptake, and provider trust and supportive norms improving it; migration and mobility, as migrant, stateless, and left-behind children had lower coverage due to weak registration and disrupted caregiving; household and caregiver dynamics, where decision-making by family or community members shaped uptake, while large family size and maternal employment limited adherence; and health system capacity, with inadequate infrastructure and follow-up hindering coverage and integration with maternal–child health services facilitating it. Addressing these intersecting barriers through equity-focused strategies is critical to achieving universal immunization coverage.