Cost-effectiveness of outdoor residual spraying, insecticide-treated nets, and their combination for zoonotic malaria control in Sabah, Malaysia: a comparative analysis

BACKGROUND: Following the elimination of human malaria, Malaysia has experienced an increasing burden of zoonotic Plasmodium knowlesi malaria, particularly in Sabah. Evidence on the cost-effectiveness of vector control interventions is needed to inform policy decisions and optimise resource allocation. This study assessed the cost-effectiveness of Outdoor Residual Spraying (ORS), Long-Lasting Insecticidal Nets (LLINs), and their combined use for controlling P. knowlesi malaria in Sabah, Malaysian Borneo. METHODS: A cost-effectiveness analysis was conducted from the healthcare provider perspective over a one-year time horizon in three high-endemic districts: Keningau (LLINs), Ranau (ORS), and Kota Marudu (ORS + LLINs). Capital and recurrent components, were estimated using an ingredient-based costing approach. Effectiveness was measured as the number of P. knowlesi cases averted based on routine surveillance data. Incremental cost-effectiveness ratios (ICERs) were calculated relative to a no-intervention scenario, defined as health promotion activities without vector control. One-way sensitivity analyses assessed uncertainty in key parameters. All costs were reported in Malaysian Ringgit (MYR) and adjusted to 2025 values. RESULTS: Annual provider costs ranged from MYR 3912.92 (no intervention) to MYR 39,764.14 (ORS + LLINs). ORS was associated with the highest number of cases averted (n = 13) and the lowest ICER (MYR 2046 per case averted; approximately USD 470). LLINs and the combined strategy were associated with fewer cases averted and higher ICERs. ORS consistently demonstrated the most favourable cost-effectiveness profile across all sensitivity analyses. CONCLUSION: ORS demonstrated the most favourable cost-effectiveness profile under the observed programme conditions for controlling P. knowlesi malaria in Sabah. A stratified approach prioritising ORS, with targeted use of LLINs among high-risk populations, may represent an economically efficient strategy in resource-limited settings.