Abstract
In Ghana, indoor residual spraying (IRS) has been a key intervention for malaria control since 2008. After seven years of IRS with an organophosphate insecticide, which substantially reduced malaria incidence, IRS programs transitioned to neonicotinoid-based products between 2018 and 2019 as part of an insecticide rotation plan. This change was largely informed by entomological data from a limited number of pilot districts, with little evidence of epidemiological impact. We assessed the effect of this transition using monthly, district-level malaria incidence from 2015 to 2022 in 22 IRS districts across four northern regions. Incidence was calculated from routinely reported confirmed malaria cases in the Ghana Health Service District Health Information Management System, adjusted for testing rates, with population denominators from census-based estimates. An interrupted time series model with generalized estimating equations was fit to 2015-2019 data, when organophosphates were in use, to generate counterfactual trends through 2022, adjusting for rainfall, temperature, vegetation index, seasonality, and initiation of seasonal malaria chemoprevention. Observed incidence exceeded counterfactual estimates in the first transmission season after the switch and remained higher in subsequent campaigns. Across all districts, malaria incidence was 26% higher under neonicotinoid IRS than expected under continued organophosphate use (IRR = 1.26, 95% CI 1.07-2.08). Regional differences were observed, with significant increases in the Northern (IRR = 1.76, 95% CI 1.04-2.74), North East (IRR = 1.83, 95% CI 1.08-3.40), and Upper East (IRR = 4.26, 95% CI 2.17-8.50) regions, but not in the Upper West (IRR = 1.25, 95% CI 0.90-1.70). These findings suggest that the transition to neonicotinoids may have reduced IRS effectiveness in northern Ghana. Future pilots of new IRS products should incorporate both entomological and epidemiological outcomes to guide region-specific selection before national scale-up.