Private Retail Pharmacy Antimalarial Subsidy Schedules and Effective Treatment Coverage in Nigerian Urban Health Systems: A Stepped-Wedge Cluster Evaluation

Subsidising artemisinin-based combination therapy through private retail pharmacies is a central instrument of Nigerian urban malaria control, yet the design of subsidy schedules is rarely evaluated against the coverage it actually buys. The primary outcome was effective treatment coverage, defined as the proportion of febrile episodes receiving a quality-assured artemisinin-based combination therapy within 48 hours. Clusters were randomised to four sequences and stepped from control to intervention at eight-week intervals over twelve months. Analysis used a generalised linear mixed model with cluster and period random effects, and the intraclass correlation was 0.64. Coverage rose from 21.4% at baseline to 38.7% under the flat-price schedule, 44.2% under the fixed-margin schedule, and 51.9% under the volume-tiered schedule. The volume-tiered schedule outperformed the flat-price schedule by 13.5 percentage points (95% CI 8.2 to 18.8; p < 0.001) and the fixed-margin schedule by 7.7 percentage points (95% CI 3.1 to 12.3; p = 0.001). The fixed-margin schedule outperformed the flat-price schedule by 5.8 percentage points (95% CI 1.4 to 10.2; p = 0.010). The intraclass correlation of 0.64 implies that cluster-level factors dominate individual variation, and that schedule design must be evaluated at the level of the pharmacy cluster rather than the patient. The findings indicate that margin structure, not headline subsidy level, is the operative design lever for effective treatment coverage in urban Nigerian retail pharmacy systems.