A paper in Malaria World this week attracted my attention for additional reasons. The research is organised by University of Notre Dame, where I earned my PhD. It was carried out in Kenya which I will visit next week. I am looking forward to leaving winter behind for a few weeks! The study, by Ochomo et al published in The Lancet, used a double blinded placebo methodology to find that that spatial repellents significantly reduced the hazard rate of malaria infection by ~33%.
The research is sponsored by household chemical consumer good company SC Johnson who are promoting combating malaria with a pyrethroid insecticide, transfluthrin, impregnated mesh product called Mosquito Shield and a similar to product called Guardian (example in picture). The product is intended to repel rather than kill mosquitos.
The study did not contain all the methods and referenced 2022 paper by the same team published in Trials. This paper had more information but not a description of the chemical composition of the placebo (the article stated the trial design included a placebo product of matched design to the Mosquito Shield™ but with inert ingredients only). I am uncertain if this is important and have asked the corresponding author for more information.
The study itself was a cluster-randomised, controlled trial in Busia County, Kenya to quantify the efficacy of the spatial repellent against human malaria infection following mass distribution of insecticide treated nets. Investigators, staff, and study participants were masked to cluster allocation. Infection incidence was measured by microscopy in children aged 6 months to younger than 10 years during a 4-month baseline (March–July 2021) and 24-month follow-up period with intervention (October, 2021–October, 2023). From 58 clusters (29 intervention, 29 placebo), a total of 1526 and 1546 participants from two consecutive, 12-month cohorts were assessed for first-time malaria infection (primary endpoint) by survival analysis at interim and end-of-trial timepoints, respectively.
The two groups were similar, although the placebo group, had slightly more houses with mud walls and on average had fewer windows. This might indicate that cluster locations for the placebo were poorer and poverty is linked to higher incidence of malaria (discussed in a previous post). A weakness of the authors’ hypothesis that the incidence of malaria was reduced by the use of the transfluthrin impregnated mosquito repellent, is that the study did not show that the active repellent caused a statistically significant decrease in Anopheles mosquito numbers.
So is the product effective and if so why? The research used the usual 95% statistical boundary which leaves a 5% possibility the result is not correct. And there is the usual problem with most efficacy studies that the researchers are paid by the beneficiaries of a positive result. If as I suspect, mosquitos, are not responsible for spreading malaria the result can only have been positive because of a structural bias of some sort in the study, or a random false positive result.