Northern-Southern Inequalities in COVID-19 Vaccination Uptake Among Women of Reproductive Age in Nigeria: A Multivariate Decomposition Analysis.

Background: The vaccination of women against COVID-19 remains a priority in the post-pandemic era. Regional inequalities in COVID-19 vaccination uptake among Nigerian women are a key factor of interest in public health research. The objective of this study was to examine the northern-southern inequality in COVID-19 vaccination among Nigerian women of reproductive age. Methods: We analysed a nationally representative sample of 36,555 women aged 15-49 years from the 2024 Nigeria Demographic and Health Survey. Nigeria's six geopolitical zones were grouped into a northern region (North Central, North East, North West) and a southern region (South East, South, South West). Multivariate decomposition analysis for non-linear models was used to partition the northern-southern difference in COVID-19 vaccination into an endowment (compositional) component and a coefficient (unexplained) component. Results: COVID-19 vaccination was reported by 29.0% (95% CI: 27.2-33.0) of southern women and 24.1% (95% CI: 21.9-26.4) of northern women (p<0.05). Higher percentages of vaccination were observed among women aged 45-49 years and among those with higher education, health insurance coverage, employment, classification as empowered, the lowest level of socioeconomic disadvantage, media exposure, residence in the richest households, residence in communities with high willingness for vaccine uptake, and urban residence. In the decomposition, the endowment (compositional) component accounted for -34.5% and the coefficient (unexplained) component for 134.5% of the northern-southern difference. Importantly, the negative compositional contribution indicates that differences in the measured characteristics of women between regions did not explain the northern disadvantage. Rather, the gap was predominantly associated with regional differences in the effects of these characteristics. Thus, under a counterfactual scenario in which northern women had the same observed characteristics as southern women, the vaccination gap would not necessarily narrow and could instead widen in favour of the south, suggesting an important role for contextual factors not captured in the model. Educational attainment, religion, employment, media exposure, community-level vaccine willingness, community-level illiteracy, community-level nativity, and community-level urbanicity were the categories with the largest contributions. Conclusion: COVID-19 vaccination uptake among women of reproductive age was lower in northern than southern Nigeria. The decomposition indicates that the regional gap was driven predominantly by the coefficient component (134.5%) rather than the compositional component (-34.5%), suggesting that differences in how individual, socioeconomic, and community characteristics operate across regions are more important than differences in population composition alone. Therefore, equalizing measured characteristics between regions may not eliminate the disparity; region-specific interventions should address the contextual, sociocultural, behavioural, and health-system factors shaping vaccination decisions.