Modelling malaria routine surveillance data to inform seasonal malaria chemoprevention strategy in Moissala, Southern Chad

Background Seasonal malaria chemoprevention (SMC) is a malaria intervention in which antimalarial drugs are administered monthly to children under 5 years of age during the high-transmission season. In the district of Moissala in southern Chad, SMC has been implemented since 2013, with an interruption in 2019, resumption in 2020, and expansion to five rounds of treatment in 2021. Recent World Health Organization (WHO) guidelines allow countries to adapt the timing and number of SMC rounds to local transmission patterns, creating a need to identify optimal strategies for each setting. In this study, we used mathematical modeling for three primary purposes: 1) to estimate the effectiveness of SMC in Moissala from 2018 to 2023, 2) to assess the impact of changes to SMC strategies since 2018, and 3) to determine the optimal SMC strategy in Moissala. Methods and findings We adapted a compartmental, climate-informed malaria transmission model to represent malaria dynamics in the presence of SMC. The model incorporates temperature and rainfall data to capture how climate variability influences malaria transmission over time. It was calibrated to routine surveillance data on malaria cases in children under five years old from 2018 to 2023. Using the calibrated model, we simulated malaria cases under alternative scenarios, including the absence of SMC and variations in the number and timing of SMC rounds. These simulations were then used to estimate the overall effectiveness of SMC, assess the impact of past changes in SMC strategies, and identify the optimal strategy in Moissala. Between 2018 and 2023, SMC reduced malaria cases in children under five by 26% (95% credible interval: 21%, 31%) relative to a scenario without SMC, corresponding to an average of approximately 14400 cases averted each year. The interruption of SMC in 2019 led to an estimated increase of 13600 cases (95% credible interval: 11200, 15800), representing a 31% rise during the high-transmission season. Expanding from four to five SMC rounds in 2021 reduced cases by 7% relative to a four-round schedule, while starting the five-round schedule earlier in June rather than July led to an additional 5% reduction. Overall, the most effective strategy from 2018 to 2023 was a five-round schedule beginning in mid-June. Conclusions Seasonal malaria chemoprevention has substantially reduced malaria incidence among children under five in Moissala. The currently implemented strategy of five rounds of SMC starting in June was estimated to achieve the greatest reduction in cases over the study period. Climate-informed modelling and open-source software can support timely decision-making across settings under changing climate and transmission conditions.