Modeled Impact of Seasonal Malaria Chemoprevention on District-Level Suspected and Confirmed Malaria Cases in Chad Based on Routine Clinical Data (2013-2018).

Sulfadoxine-pyrimethamine plus amodiaquine is delivered to children aged 3-59 months as seasonal malaria chemoprevention (SMC) in areas where transmission is highly seasonal such as Chad and other Sahelian countries. Although clinical trials show a 75% reduction in malaria cases, evidence of SMC's impact at scale remains limited. Using data from the Chadian National Health Management Information System, we analyzed associations between SMC implementation during July-October and monthly district-level malaria incidence (suspected and confirmed outpatient cases) among children aged 0-59 months at health facilities in 23 health districts with SMC implementation during 2013-2018. Generalized additive models were fitted with separate cyclic cubic spline terms for each district to adjust for seasonality in cases. SMC implementation in Chad was associated, compared with no implementation, with lower monthly counts of both suspected (rate ratio [RR]: 0.82, 95% CI: 0.72-0.94. P = 0.006) and confirmed malaria cases (RR: 0.81, 95% CI: 0.71-0.93, P = 0.003), representing around 20% reduction in malaria incidence. Sensitivity analyses showed effect sizes of up to 28% after modifying model assumptions. Caution should be exercised in interpreting our findings, which may not be comparable with other studies, and may over- or underestimate impact of SMC; not all malaria cases present at health facilities, not all suspected cases are tested, and not all facilities report cases consistently. This study's approach presents a solution for employing readily available routine data to evaluate the impact of health interventions at scale without extensive covariate data. Further efforts are needed to improve the quality of routine data in Chad and elsewhere.