IntroductionNotably, less than two-thirds of under-5 children received full immunization in 2016 in India. It is critical to understand the inequalities in access to immunization for determining an effective health policy agenda to ensure universal health coverage. Hence, we performed a study to assess the determinants of income-based inequality in the full immunization of children aged 12–23 using Fairlie decomposition analysis. MethodsThis cross-sectional study was a part of a community-based project that aimed to improve maternal and child health in the backward states of India, namely Bihar and Assam. The study was conducted in the rural and urban areas of Munger and Darrang districts of Bihar and Assam, respectively. The degree of income-related inequality in full immunization coverage was obtained through the concentration index. The Fairlie decomposition was employed to quantify the absolute contribution of socio-demographic factors explaining the group differences (higher or lower income) in the probability of having full immunization. ResultsThere were 73 fully and 82 non-fully immunized children. The concentration curve was lying above the line of equality, which implied that full immunization coverage was concentrated towards the lower-income group. Maternal education (7.5%) and place of residence (5.1%) widened the inequality gap, and caste (−13.5%) and age of the child (−2.5%) narrowed down the inequality gap for full immunization among lower and higher-income groups. ConclusionsThe socio-economic inequalities in access to full immunization can be mitigated by multi-sectoral interventions with a focus on children with less-educated mothers and living in urban slums.