A theoretical exploration of the origin and early evolution of a pandemic

大流行起源及早期演化的理论探索

A virus that can cause a global pandemic must be highly adaptive to human conditions.Such adaptation is not likely to have emerged suddenly but,instead,may have evolved step by step with each step favored by natural selection.It is thus necessary to develop a theory about the origin in order to guide the search.Here,we propose such a model whereby evolution occurs in both the virus and the hosts(where the evolution is somatic;i.e.,in the immune system).The hosts comprise three groups–the wild animal hosts,the nearby human population,and farther-away human populations.The theory suggests that the conditions under which the pandemic has initially evolved are:(i)an abundance of wild animals in the place of origin(PL_(0));(ii)a nearby human population of low density;(iii)frequent and long-term animal-human contacts to permit step-by-step evolution;and(iv)a level of herd immunity in the animal and human hosts.In this model,the evolving virus may have regularly spread out of PL_(0) although such invasions often fail,leaving sporadic cases of early infections.The place of the first epidemic(PL_(1)),where humans are immunologically naïve to the virus,is likely a distance away from PL_(0).Finally,this current model is only a first attempt and more theoretical models can be expected to guide the search for the origin of SARS-CoV-2.

能引起全球疫情的病毒,从生物进化的观点来看不会是一步到位,而是一个逐步进化的过程.因此我们需要一个理论架构,以指导关于病毒起源的研究.在此,我们提出了病毒和宿主共进化的适应性模型.在该模型中,宿主分三类:野生动物宿主、附近的人口和较远的人口.基于数学理论,我们推衍病毒起源地(PL_(0))具备如下特性:(1)野生动物丰富度大;(2)邻近人口密度低;(3)野生动物与人类长期接触;(4)起源地的动物与人类均有一定的群体免疫.该模型表明病毒在早期可能会多次从PL_(0)入侵到其它地方,但这种入侵通常会以失败告终,从而导致零星的早期感染病例.与起源地不同,第一个疫情区(PL_(1)),应该是群体免疫较弱,人口较密的地方.溯源研究,必须先建立理论架构.本研究提出一个非常好的理论模型,可以指导对SARS-CoV-2起源的探索.