Controlling Pandemic Flu: The Value of International Air Travel Restrictions

BackgroundPlanning for a possible influenza pandemic is an extremely high priority, as social and economic effects of an unmitigated pandemic would be devastating. Mathematical models can be used to explore different scenarios and provide insight into potential costs, benefits, and effectiveness of prevention and control strategies under consideration.
Methods and FindingsA stochastic, equation-based epidemic model is used to study global transmission of pandemic flu, including the effects of travel restrictions and vaccination. Economic costs of intervention are also considered. The distribution of First Passage Times (FPT) to the United States and the numbers of infected persons in metropolitan areas worldwide are studied assuming various times and locations of the initial outbreak. International air travel restrictions alone provide a small delay in FPT to the U.S. When other containment measures are applied at the source in conjunction with travel restrictions, delays could be much longer. If in addition, control measures are instituted worldwide, there is a significant reduction in cases worldwide and specifically in the U.S. However, if travel restrictions are not combined with other measures, local epidemic severity may increase, because restriction-induced delays can push local outbreaks into high epidemic season. The per annum cost to the U.S. economy of international and major domestic air passenger travel restrictions is minimal: on the order of 0.8% of Gross National Product.
ConclusionsInternational air travel restrictions may provide a small but important delay in the spread of a pandemic, especially if other disease control measures are implemented during the afforded time. However, if other measures are not instituted, delays may worsen regional epidemics by pushing the outbreak into high epidemic season. This important interaction between policy and seasonality is only evident with a global-scale model. Since the benefit of travel restrictions can be substantial while their costs are minimal, dismissal of travel restrictions as an aid in dealing with a global pandemic seems premature.

背景 针对潜在流感大流行的防控规划属于极高优先级事项，因未受干预的大流行所引发的社会与经济后果将极具破坏性。数学模型可用于探索各类防控场景，为待评估的预防与控制策略的潜在成本、收益及实施效果提供决策依据。
方法与结果 本研究采用基于随机方程的流行病模型，探究流感大流行的全球传播动态，涵盖旅行限制与疫苗接种的影响，并同时考量干预措施的经济成本。研究假设初始暴发存在不同的时间与地点，以此分析病毒首次到达美国的时间（First Passage Times, FPT）分布，以及全球各大都市区的感染人数规模。仅实施国际航空旅行限制仅能小幅延迟病毒首次到达美国的时间；若在源头管控的同时结合旅行限制，可实现更长的延迟时长。若进一步在全球范围内推行防控措施，则全球（尤其是美国）的确诊病例数将显著下降。但若未将旅行限制与其他防控措施联用，反而可能加剧局部疫情的严重程度：因限制引发的传播延迟可能将本地暴发推至高流行季。针对美国经济而言，国际及国内主要航线航空客运旅行限制的年度成本极低，约占国民生产总值（Gross National Product, GNP）的0.8%。
结论 国际航空旅行限制或可小幅但显著延缓大流行的传播，尤其是在该限制争取到的窗口期内同步落实其他疾病防控措施的情况下。但若未配套其他防控措施，传播延迟反而可能将疫情推至高流行季，加剧区域疫情的严重程度。政策与季节流行性之间的这一重要交互效应，仅能通过全球尺度的模型得以显现。鉴于旅行限制的收益可观且成本极低，直接否定其作为全球大流行防控辅助手段的价值显然为时过早。