In Kenya's informal settlements, immunization coverage remains suboptimal despite government efforts. A mixed-methods approach combining survey data and qualitative interviews to assess programme effectiveness. School-based health insurance significantly increased immunization uptake by 20% (95% CI: 14-26%) compared to control schools. The findings suggest that integrating health insurance into school systems can enhance vaccination rates in informal settlements. Scaling up the programme and expanding coverage to other regions is recommended. immunization, health insurance, Kenya, informal settlements Treatment effect was estimated with $\text{logit}(p_i)=\beta_0+\beta^\top X_i$, and uncertainty reported using confidence-interval based inference.