经济学人下载:缺乏证据不代表没证据(2)

All this suggests that the number of infected people unwittingly infecting others could be quite large.
所有这些都表明，被感染者的人数可能相当庞大。

What is unclear is how infectious these people actually are.
现在还不清楚这些人到底有多大的传染性。

That is what the second strand of research on the asymptomatic and presymptomatic transmission of SARS-COV-2 deals with.
这是关于SARS-COV-2无症状和症状前传染的第二部分研究。

It draws on various laboratory studies.
它利用了各种实验室研究。

In several of these the amount of the virus in nasal and throat swabs taken from infected people
在其中几个研究中，从无症状感染者鼻腔和咽喉拭子中采集的病毒量

who were presenting no symptoms at the time was similar to the amount found in those who had symptoms.
和那些有症状的人的病毒量相似。

Indeed, for those who do go on to develop symptoms, the amount of virus they have in them peaks close to the onset of those symptoms,
确实，对于那些继续出现症状的人来说，他们体内的病毒数量在这些症状出现之前达到顶峰，

which suggests that it may be easily transmissible at an early stage of infection.
这表明在感染的早期阶段，它可能极易传播。

As a persistent cough is a common symptom, it might be expected that those who are symptomatic are more effective in spreading the virus than those who are not.
持续的咳嗽是一种常见的症状，因此有症状的人比没有症状的人更能有效地传播病毒。

Contrariwise, however, those with symptoms often feel unwell and take to their beds.
但相反，那些有症状的病人通常会因为不舒服而卧床休息。

They are, therefore, coughing mainly onto their sheets and blankets rather than onto strangers in the street.
因此，他们主要是对着床单和毯子咳嗽，而不是对着街上的陌生人。

The third strand of research into the question of silent spreading is mathematical modelling.
关于寂静传染问题研究的第三部分是数学建模。

One such study was published in Science on March 31st by Luca Ferretti of Oxford University and his colleagues.
牛津大学的Luca Ferretti和他的同事在3月31日的《科学》杂志上发表了一项这样的研究。

It used data on 40 infected people for whom the source of their infection was known with high probability,
该研究40名感染患者的数据，这些患者的感染源基本已知，

and the timing of their symptoms and those of the people who infected them was well documented.
而且他们出现症状的时间和感染他们的人都有详细的记录。

The researchers estimate that between a third and a half of transmission occurs from people who are without symptoms at that point—
研究人员估计，三分之一到二分之一的传染发生在当时没有症状的人身上——

a result which broadly agrees with estimates from similar studies by others.
这一结果与其他类似研究得出的估计大致一致。

Collectively, all this research may help explain why SARS-COV-2 has spread with such ferocity.
总的来说，所有这些研究可能有助于解释为什么SARS-COV-2传播地如此凶猛。

But the study, in particular, of those who are infected but never present symptoms is also crucial to understanding how that spread may ebb—
但这项研究，特别是对那些无症状感染者的研究，对理解这种传播将如何减弱也至关重要——

for the pool of those who have been infected and are, therefore, immune to reinfection at least in the short term also includes these people.
因为这些患者也属于因被感染而对再次感染有短期免疫的人群。

Pandemics end when the pathogen causing them runs out of individuals to infect. Some of those susceptible will have died.
当引起流行病的病原体没有足够的个体可以感染时，流行病就结束了。其中一些易感者将会死亡。

Enough of the rest would then be immune for the population to have developed "herd immunity".
剩下的人就获得免疫力，从而形成“群体免疫”。

In the case of the current pandemic of SARS-COV-2, the more silent infections there have been, the faster this herd immunity will arrive.
在当前SARS-COV-2大流行的情况下，寂静传染病例越多，这种群体免疫到来得越快。