Abstract
Background
Zanzibar has achieved and sustained very low malaria prevalence through high coverage of indoor residual spraying (IRS) and insecticide-treated nets (ITNs). However, residual transmission persists, particularly on Pemba Island. Increasing evidence suggests that the predominance of exophagic vectors and outdoor human activity patterns may reduce the effectiveness of indoor-focused interventions. This study aimed to quantify malaria transmission risk arising from the interaction between human behavior and mosquito biting activity indoors and outdoors on Pemba Island.Methods
From February 2024 to January 2025, adult malaria vectors were collected using human landing catches (HLCs) indoors and outdoors at sentinel sites across four districts of Pemba Island. Concurrent human behavior observations (HBOs) were conducted hourly to record location, activity, age, sex, and ITNs use within households. Human biting rates (HBRs) and behavior-adjusted biting rates were calculated to estimate human behavior-adjusted exposure and identify protection gaps by age and sex.Results
A total of 981 Anopheles mosquitoes were collected, of which 98% of An. gambiae s.l. were An. arabiensis. Mosquito collection was significantly higher outdoor than indoor (Relative rate = 4.37, 95% CI 3.61-5.29, p Conclusions
Residual malaria transmission on Pemba Island is driven primarily by outdoor exposure linked to human behaviour patterns and exophagic An. arabiensis. While ITNs continue to protect young children indoors, they do not address most exposure among older children and adults. Malaria elimination efforts in Zanzibar will require complementary interventions that target outdoor transmission and behavioral risk, alongside existing indoor-focused strategies.