This study examines the impact of expanding a Community Health Worker (CHW) programme on child immunization rates in Nigerian cities over two years. Data were collected through surveys administered by CHWs to parents/guardians of children attending public health centers. A logistic regression model was used to analyse the impact of programme expansion variables on immunization rates. The analysis revealed a significant increase in vaccination coverage from baseline (60%) to end-point (72%), with an odds ratio of 1.59 (95% CI: 1.43-1.78) for children living within CHW service areas compared to those outside. The expanded CHW programme was effective in increasing child immunization rates, particularly among children residing near CHWs' intervention zones. Further expansion of the programme should target urban slums and rural underserved regions to ensure comprehensive coverage. Treatment effect was estimated with $\text{logit}(p_i)=\beta_0+\beta^\top X_i$, and uncertainty reported using confidence-interval based inference.