Quantifying malaria acquired during travel and its role in malaria elimination on Bioko Island

Abstract

Background Malaria elimination is the goal for Bioko Island, Equatorial Guinea. Intensive interventions implemented since 2004 have reduced prevalence, but progress has stalled in recent years. A challenge for elimination has been malaria infections in residents acquired during travel to mainland Equatorial Guinea. The present article quantifies how off-island contributes to remaining malaria prevalence on Bioko Island, and investigates the potential role of a pre-erythrocytic vaccine in making further progress towards elimination. Methods Malaria transmission on Bioko Island was simulated using a model calibrated based on data from the Malaria Indicator Surveys (MIS) from 2015 to 2018, including detailed travel histories and malaria positivity by rapid-diagnostic tests (RDTs), as well as geospatial estimates of malaria prevalence. Mosquito population density was adjusted to fit local transmission, conditional on importation rates under current levels of control and within-island mobility. The simulations were then used to evaluate the impact of two pre-erythrocytic vaccine distribution strategies: mass treat and vaccinate, and prophylactic vaccination for off-island travellers. Lastly, a sensitivity analysis was performed through an ensemble of simulations fit to the Bayesian joint posterior probability distribution of the geospatial prevalence estimates. Results The simulations suggest that in Malabo, an urban city containing 80% of the population, there are some pockets of residual transmission, but a large proportion of infections are acquired off-island by travellers to the mainland. Outside of Malabo, prevalence was mainly attributable to local transmission. The uncertainty in the local transmission vs. importation is lowest within Malabo and highest outside. Using a pre-erythrocytic vaccine to protect travellers would have larger benefits than using the vaccine to protect residents of Bioko Island from local transmission. In simulations, mass treatment and vaccination had short-lived benefits, as malaria prevalence returned to current levels as the vaccine’s efficacy waned. Prophylactic vaccination of travellers resulted in longer-lasting reductions in prevalence. These projections were robust to underlying uncertainty in prevalence estimates. Conclusions The modelled outcomes suggest that the volume of malaria cases imported from the mainland is a partial driver of continued endemic malaria on Bioko Island, and that continued elimination efforts on must account for human travel activity.

Spatiotemporal dynamics of malaria in Zanzibar, 2015–2020

“…Such phenomenon is not unique to Zanzibar, and has been observed in other malaria island elimination settings, such as Bioko island. 52 To ascertain whether cases with a travel history are truly imported, more advanced approaches such as whole-of-genome sequencing 24 would have to be used, which, however, due to infrastructure requirements and costs is not feasible to do routinely on a large scale. Currently, ZAMEP is not yet differentiating autochthonous from travel-associated (or imported) cases, as discussions on what interventions could be effective in reducing imported malaria are still ongoing.…”

Spatial Dynamics of Malaria Transmission

“…In these models, this is determined by a biting distribution matrix ( β ). A similar matrix has appeared in other models for the spatial dynamics of mosquito-borne diseases for which human mobility is based on a concept of “visitation” or time spent – classified as Lagrangian movement [7, 8, 10, 12–15, 17, 18, 45]. Here, β is based on a concept of availability, the weighted, ambient population at risk.…”

“…The fraction of all cases that were imported called the travel fraction can be defined as either: 1) the fraction of incident infections that were imported; or 2) the fraction of prevalent infections that were imported [45, 53]. To compute these travel fractions, we let γ = (1 − υ )𝓍 δ /κ denote the visitor fraction , the fraction of infectious mosquitoes that were infected by visitors.…”

“…Using spatial metrics to identify differences in transmission patterns and the flow of parasites across a landscape can help control programs prioritize drugs, outreach, and medical attention to populations, and vector or larval control to places. Using our differential equation framework to reconstruct the equilibrium analysis presented in [45], we have generated spatial bulk transmission matrices (diag( H ) · ℛ ) among areas for Bioko Island, Equatorial Guinea. In Fig.…”

“…In Supplement 3, we formulate a model using both conventional notation and the modular notation of this framework. In Supplement 1, we have implemented a previously published model of malaria transmission on Bioko Island [45]. In Supplement 4, we extend the discussion of vector dynamics, including a discussion of models with multiple vector species.…”