The effectiveness of long-lasting spatial repellent emanators against malaria in humanitarian crisis settings in northern Nigeria: a two-arm pragmatic, open-label, controlled trial

Background: In 2025, the UN estimated that 305 million people would need humanitarian aid, and by 2030, two-thirds of the poorest populations will reside in areas of fragility, conflict, and violence. When individuals are forcibly displaced into malaria endemic areas, the risk of severe disease and mortality can be high. Novel vector control tools suited to the emergency context, for which conventional interventions suffer from biological and operational limitations, are urgently needed to interrupt vector-borne disease transmission among the world's most vulnerable. This study aimed to evaluate the effectiveness of spatial repellent emanators against malaria during a protracted humanitarian crisis. 

Methods: We conducted a 6-month, two-arm pragmatic, open-label, controlled trial in 24 camps for people who are internally displaced in Maiduguri, Nigeria. Children aged 6–10 years were enrolled into a monthly cohort to estimate the primary trial epidemiological outcome of intervention impact on malaria infection incidence. The study team allocated 12 camps, which were spatially clustered, to receive the intervention and 12 other camps were allocated to the control group and received no intervention. Spatial repellent emanators, designed to remain efficacious for trial duration, were installed by the study team, or the householder under supervision, at the label coverage, in 50 eligible households per camp allocated to the intervention group. Monthly entomological monitoring measured changes in vector density (primary entomological outcome) and blood feeding (secondary entomological outcome). Intervention acceptability and feasibility was assessed using focus group discussions (tertiary trial outcomes). Patients were analysed according to treatment received and analyses were conducted using all non-missing observations, allowing for inclusion of participants with partial follow-up. This trial was registered with ClinicalTrials.gov (NCT06179732) and is completed. 

Findings: Between June 20 and July 3, 2023, 1655 participants were recruited, and 842 participants were assigned to the intervention group, and 813 patients were assigned to the control group. 770 patients were male and 885 were female. Post-intervention, after adjusting for baseline malaria prevalence, spatial repellent emanators significantly reduced malaria infection incidence over 6 months of follow-up (incidence rate ratio 0·713, 95% CI 0·584–0·869; p=0·001; n=1655). The estimated protective efficacy of spatial repellent emanators against first-time malaria infection was 22·5% (hazard ratio 0·775, 95% CI 0·637–0·943; p=0·011). Spatial repellent emanators also significantly reduced pyrethroid-resistant female Anopheles gambiae sensu lato density (incidence rate ratio 0·0231, 95% CI 0·00661–0·0807; p<0·0001) and blood feeding (0·0195, 0·00590–0·0646; p<0·0001). Camp residents displayed high levels of intervention acceptability, driven by perceived entomological impact, including willingness to pay. 

Interpretation: These findings contribute to expanding the toolbox of efficacious vector control products for the humanitarian emergency context, strengthening the growing evidence for the public health value of spatial repellent emanators, and providing the first epidemiological efficacy data for spatial repellent emanators against malaria in west Africa. 

Funding: US Department of the Army, US Army Contracting Command, Aberdeen Proving Ground, Edgewood Contracting Division.