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. 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. 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. 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.

中文翻译：

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量化旅行期间感染的疟疾及其在比奥科岛消除疟疾中的作用


消除疟疾是赤道几内亚比奥科岛的目标。自 2004 年以来实施的强化干预措施降低了患病率，但近年来进展停滞。消除疟疾的一个挑战是居民在前往赤道几内亚大陆旅行期间感染的疟疾。本文量化了岛外对比奥科岛上剩余疟疾流行的影响，并研究了前红细胞疫苗在进一步消除疟疾方面的潜在作用。使用基于 2015 年至 2018 年疟疾指标调查 (MIS) 数据校准的模型来模拟比奥科岛的疟疾传播，包括详细的旅行历史和快速诊断测试 (RDT) 的疟疾阳性率，以及疟疾的地理空间估计流行率。蚊子种群密度进行了调整，以适应当地的传播情况，并以当前控制水平下的输入率和岛内流动性为条件。然后使用模拟来评估两种红细胞前疫苗分配策略的影响：大规模治疗和疫苗接种，以及对离岛旅行者的预防性疫苗接种。最后，通过适合地理空间患病率估计的贝叶斯联合后验概率分布的模拟集合进行敏感性分析。模拟表明，在马拉博这个拥有 80% 人口的城市中，存在一些残余传播，但很大一部分感染是由前往大陆的旅行者在岛外传播的。在马拉博以外，流行主要归因于当地传播。本地传播与输入的不确定性在马拉博境内最低，在境外最高。 使用红细胞前疫苗来保护旅行者比使用疫苗来保护比奥科岛居民免受当地传播有更大的好处。在模拟中，大规模治疗和疫苗接种带来的好处是短暂的，因为随着疫苗功效的减弱，疟疾患病率又回到了目前的水平。对旅行者进行预防性疫苗接种可以更持久地降低患病率。这些预测对于流行率估计的潜在不确定性是稳健的。建模结果表明，从大陆输入的疟疾病例数量是比奥科岛疟疾持续流行的部分驱动因素，而持续的消除工作必须考虑到人类旅行活动。