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

Citron, D. T.; Guerra, C. A.; Garcia, G. A.; Wu, S. L.; Battle, K. E.; Gibson, H. S.; Smith, D. L.

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. We quantify how off-island contributes to remaining malaria prevalence on Bioko Island, and investigate the potential role of a pre-erythrocytic vaccine in making further progress towards elimination. Methods: We simulated malaria transmission on Bioko Island using a model calibrated based on data from the Malaria Indicator Surveys (MIS) from 2015-2018, 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. We evaluated the impact of two pre-erythrocytic vaccine distribution strategies: mass treat and vaccinate, and prophylactic vaccination for off-island travelers. We propagated uncertainty through the model 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 prevalence is attributable to malaria importation by travelers. Outside of Malabo, prevalence was mainly attributable to local transmission. We assess the uncertainty in the local transmission vs. importation to be lowest within Malabo and highest outside. Using a pre-erythrocytic vaccine to protect travelers 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 travelers resulted in longer-lasting reductions in prevalence. These projections were robust to underlying uncertainty in prevalence estimates. Conclusions: The modeled 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.