UI-FHS aimed to strengthen the routine immunization system in Ethiopia by implementing the Reaching Every District using Quality Improvement Methods (RED-QI) approach. This document briefly summarizes the methodology and results from combined surveys conducted at baseline and after RED-QI implementation that sought to understand why disease outbreaks were occurring in areas with high reported coverage. It presents how three data sources (1) administrative reports; (2) facility registers and home-based vaccination cards; and (3) parental recall were triangulated to estimate vaccination coverage.