Background: Zanzibar has made significant progress in malaria control reaching a population prevalence of around 1% with programme-led interventions. Nevertheless, malaria infections persist in people with no recent travel history implying locally acquired infections. Understanding risk factors of local transmission is crucial to refine elimination strategies.
Methods: From May 2017 to October 2018, a rolling cross-sectional survey was conducted in five Zanzibar districts, linked to routine malaria surveillance procedures. The study involved testing all individuals living in households of clinical malaria cases (index cases) routinely detected at a health facility, and a sample of neighbouring households using malaria rapid diagnostic tests and qPCR. Information on socio-demographic and household characteristics, recent travel and bed net use were collected during interviews.
Results: Among 17,891 surveyed individuals, 15,151 (85%) had not travelled outside Zanzibar in the last sixty days. Of these, 7286 were tested for malaria by qPCR and 2.6% [95% confidence interval (CI) 2-3%] tested positive. Individuals living in neighbouring or nearby index households had significantly lower odds of malaria infection (aOR = 0.1, 95% CI 0-0.1, p < 0.001) compared to those residing in the index households. Individuals aged 5-15 and 16-25 years had higher odds of malaria infection compared to older age groups (aOR = 3.5, 95% CI 2.2-5.7 and aOR = 1.9, 95% CI 1.2-3.0, respectively). Locally acquired infections were more likely in people living in houses with windows open (aOR = 2.1, 95% CI 1.4-3.3), but significantly lower among members of households with bed nets (aOR = 0.5, 95% CI 0.3-0.9), higher in those going to sleep early and in people living in areas with a moderate local index of wetness (aOR = 3.4, 95% CI 2.0-5.8).
Conclusion: Combatting locally acquired malaria in Zanzibar requires interventions and strategies that promote uptake of existing interventions such as bed nets, housing improvements, and the identification and targeting of individuals at higher risk of infection.
Keywords: Elimination; Malaria; Residual transmission; Tanzania; Zanzibar.
© 2025. The Author(s).