How many incoming travelers (I₀ at time 0, equivalent to the ‘founders’ in evolutionary genetics) infected with SARS-CoV-2 who visit or return to a region could have started the epidemic of that region? I₀ would be informative about the initiation and progression of epidemics. To obtain I₀, we analyze the genetic divergence among viral populations of different regions. By applying the ‘individual-output’ model of genetic drift to the SARS-CoV-2 diversities, we obtain I₀ < 10, which could have been achieved by one infected traveler in a long-distance flight. The conclusion is robust regardless of the source population, the continuation of inputs (I_t for t > 0) or the fitness of the variants. With such a tiny trickle of human movement igniting many outbreaks, the crucial stage of repressing an epidemic in any region should, therefore, be the very first sign of local contagion when positive cases first become identifiable. The implications of the highly ‘portable’ epidemics, including their early evolution prior to any outbreak, are explored in the companion study (Ruan et al., personal communication).

中文翻译：

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关于 COVID-19 爆发的创始人效应：有多少受感染的旅行者可能是这一切的始作俑者？

有多少感染 SARS-CoV-2 的入境旅客（时间 0 时I为₀，相当于进化遗传学中的“奠基人”）访问或返回某个地区可能引发了该地区的流行病？I ₀将提供有关流行病的发生和发展的信息。为了获得I ₀，我们分析了不同地区病毒种群之间的遗传差异。通过将遗传漂变的“个体输出”模型应用于 SARS-CoV-2 多样性，我们得到I ₀ < 10，这可以由一名受感染的旅行者在长途飞行中实现。无论源群体、输入的连续性（当t > 0 时I _t  ）或变体的适应度如何，结论都是稳健的。因此，在任何地区抑制流行病的关键阶段，当首次发现阳性病例时，应该是当地传染的第一个迹象。同伴研究探讨了高度“可移植”流行病的影响，包括它们在任何爆发之前的早期演变（Ruan等人，个人通讯）。