Data from: Modelling vaccination strategies against Rift Valley fever in livestock in Kenya: model code

The impacts of vaccination on the transmission of Rift Valley fever virus (RVFV) have not been evaluated. We have developed a RVFV transmission model comprising two hosts -- cattle as a separate host and sheep and goats as one combined host (herein after referred to as sheep) -- and two vectors -- Aedes species (spp) and Culex spp -- and used it to predict the impacts of: (1) reactive vaccination implemented at various levels of coverage at pre-determined time points, (2) targeted vaccination involving either of the two host species, and (3) a periodic vaccination implemented biannually or annually before an outbreak. Methodology/Principal Findings: The model comprises coupled vector and host modules where the dynamics of vectors and hosts are described using a system of difference equations. Vector populations are structured into egg, larva, pupa and adult stages and the latter stage is further categorized into three infection categories: susceptible, exposed and infectious mosquitoes. The survival rates of the immature stages (egg, larva and pupa) are dependent on rainfall densities extracted from the Tropical Rainfall Measuring Mission (TRMM) for a Rift Valley fever (RVF) endemic site in Kenya over a period of 1827 days. The host populations are structured into four age classes comprising young, weaners, yearlings and adults and four infection categories including susceptible, exposed, infectious, and immune categories. The model reproduces the 2006/2007 RVF outbreak reported in empirical surveys in the target area and other seasonal transmission events that are perceived to occur during the wet seasons. Mass reactive vaccination strategies greatly reduce the potential for a major outbreak. The results also suggest that the effectiveness of vaccination can be enhanced by increasing the vaccination coverage, targeting vaccination on cattle given that this species plays a major role in the transmission of the virus, and using both periodic and reactive vaccination strategies. Conclusion/Significance: Reactive vaccination can be effective in mitigating the impacts of RVF outbreaks but practically, it is not always possible to have this measure implemented satisfactorily due to the rapid onset and evolution of RVF epidemics. This analysis demonstrates that both periodic and reactive vaccination ought to be used strategically to effectively control the disease.

目前尚无针对疫苗接种对裂谷热病毒（Rift Valley fever virus, RVFV）传播影响的相关评估研究。本研究构建了一款RVFV传播模型，该模型包含两类宿主与两类媒介：宿主分别为单独分类的牛，以及合并为一类的绵羊与山羊（下文统称为绵羊）；媒介则为伊蚊属（Aedes spp.）与库蚊属（Culex spp.）。本研究利用该模型，对三类疫苗接种策略的防控效果展开预测：(1) 在预设时间点实施、覆盖水平各异的应急接种；(2) 针对两类宿主中任意一类的靶向接种；(3) 在疫情暴发前每半年或每年开展的定期接种。
研究方法与主要结果：该模型包含耦合的媒介与宿主模块，媒介与宿主的种群动态通过差分方程组进行描述。媒介种群被划分为卵、幼虫、蛹与成虫四个阶段，其中成虫阶段还可进一步分为三类感染状态：易感蚊、潜伏感染蚊与传染性蚊。未成熟阶段（卵、幼虫与蛹）的存活率，取决于从肯尼亚某裂谷热（Rift Valley fever, RVF）流行区获取的、时长1827天的热带降雨测量任务（Tropical Rainfall Measuring Mission, TRMM）降雨密度数据。宿主种群被划分为四个年龄组，即幼崽、断奶幼畜、周岁畜与成畜，同时分为四类感染状态：易感、潜伏感染、感染与免疫。该模型能够复现目标区域实证调查中记录的2006/2007年RVF暴发疫情，以及其他被认为在湿季发生的季节性传播事件。大规模应急接种策略可大幅降低大规模疫情暴发的风险。研究结果还显示，通过提升疫苗接种覆盖率、针对在病毒传播中发挥主要作用的牛群开展接种，以及联合使用定期与应急接种策略，可进一步提升疫苗接种的防控效果。
结论与意义：应急接种可有效缓解RVF疫情带来的影响，但由于RVF流行病的暴发与演化速度极快，实际中往往难以圆满落实该防控措施。本分析表明，应战略性地联合使用定期与应急接种策略，以实现对该病的有效防控。