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High-resolution sub-district stratification of malaria risk in Zanzibar
Lalji, S. M., Bisanzio, D., Mkali, H. R., Al Mafazy, A.-W., Joseph , J., Nyinondi, S., Abbas, F. B., Ali, M. H., Hassan, W. S., Kitojo, C., Serbantez, N., Reaves, E., McKay, M., Ngondi, J. J. M., Reithinger, R., & Eckert, E. L. (2021). High-resolution sub-district stratification of malaria risk in Zanzibar. American Journal of Tropical Medicine and Hygiene, 105(5 Supplement), 210. https://doi.org/10.4269/ajtmh.abstract2021
To achieve their malaria elimination target, Zanzibar has decided to implement an intervention approach based on risk strata. This study describes a novel approach to create a shehia (sub-district) malaria stratification map for Zanzibar. Case-level malaria data were obtained from the Malaria Case Notification system from January 2017 to December 2019. Data were aggregated and used to estimate the number of cases, incidence, fraction of imported cases, and probability of finding new cases during follow-up investigation of primary index cases for all 388 shehias. For each selected malaria indicator, pre-determined thresholds were assigned to determine risk levels of each shehia. Malaria indicator risk levels were analyzed using a multiple correspondence analysis (MCA) to assign a score for each shehia. The quartiles of the scores were then used to divide 333 shehias in four strata (1st-4th stratum, from high to low burden, with approx. 80 shehia per stratum). The performance of the MCA in creating distinct groups was evaluated comparing the distribution of the indicators among the strata (Wilcoxon signed-rank test). An additional fifth stratum was also created for 55 shehias in Mjini (urban) district due to different approaches for targeted interventions. Two stratification maps were created by varying indicator thresholds, one for the whole archipelago and one for each district. Malaria indicators showed a significant difference among the 1st-4th strata (phighlighting good performance of grouping shehias using MCA. The maps showed that most of the high malaria burden strata aggregated in southern Unguja and low burden strata aggregated in southern Pemba. The resulting stratification will rapidly be implemented in Zanzibar. Intervention packages for shehias in the 2nd, 3rd, and 4th, strata will be based on community-level interventions while the 1st stratum will adopt a shehia-level approach. Specific interventions will be implemented per each urban shehia. This stratified approach will improve intervention effectiveness and programmatic resource allocation to support Zanzibar’s path to elimination.
