经济学人下载:缺乏证据不代表没证据(1)

Few stories are as prominent in the study of infectious diseases as that of Mary Mallon,
在传染病研究领域，很少有像玛丽·梅伦的故事那样引人注目。

a cook to wealthy families, and also to a maternity hospital, in New York in the early 1900s.
20世纪初，玛丽·马伦在富裕的家庭和产科医院做过厨师。

As she went from one employer to another, typhoid fever, then deadly in one case in ten, followed in her wake.
她的雇主换了一个有一个，伤寒都紧随其后，10个人中就有一个因其死亡。

Public-health officials eventually joined the dots and identified her as a carrier of Salmonella typhi, the bacterium that causes the disease.
最终公共卫生部进行调查，并确定她是伤寒杆菌（一种会引起疾病的细菌）的携带者。

What was striking about Typhoid Mary, as the newspapers nicknamed her, was that she herself was healthy—
纸媒称她为伤寒玛丽 ，关于她的故事最引人注目的是她本身是健康的——

proof that people could harbour and transmit S. typhi without showing symptoms of the illness it causes.
这证明人们会携带和传染伤寒杆菌，且不出现任何症状。

Such silent transmission, as epidemiologists call the phenomenon, has since been observed in many diseases—among them measles, influenza and HIV/AIDS.
流行病学家称这种现象为寂静传染，很多疾病，如麻疹、流感HIV和AIDS，都出现过这种现象。

A fresh addition to the list is SARS-COV-2, the coronavirus behind the covid-19 pandemic now raging.
最新进入这份名单的是SARS-COV-2，这是冠状病毒是目前covid-19疫情肆意的幕后推手。

Accumulating evidence suggests a substantial chunk of the infections it causes
越来越多的证据表明，它引起的感染中有很大一部分

are transmitted by people whose symptoms have not yet appeared—or even, like Mallon, who never develop symptoms at all.
是由尚未出现症状的人传播的，甚至像马伦一样，根本没有出现症状。

That has implications for the methods countries are employing to curb the pandemic.
这对各国控制全球流行病的方法有影响。

Currently, none of the evidence on asymptomatic transmission is watertight.
目前，关于无症状传染的证据没有一个是无懈可击的。

According to Gerardo Chowell of Georgia State University, in Atlanta,
根据亚特兰大佐治亚州立大学的Gerardo Chowell，

the best way to determine the share of SARS-COV-2 infections that happen in this way
要确定SARS-COV-2以这种方式传染的比列

is to follow up a large number of households in which someone is already infected and then track who subsequently infects whom.
最好的方法是对大量已有感染者的家庭进行跟踪，然后跟踪谁接着感染了谁。

For this to work, everyone involved would have to be tested daily.
这种方法需要每个人每天接受检测。

If this were done, comparing subtle variations from person to person in the virus's genetic material would show who caught it from whom.
做完这一步后，比较病毒基因材料中人与之间的细微差异，就可以发现是谁从谁那里感染了病毒。

Definitive studies of this nature are not yet available, though some are probably in the works, Dr Chowell reckons.
Chowell博士认为，目前还没有关于这一性质的权威研究，尽管一些研究可能正在进行中。

In the meantime, a growing collection of other research is shedding light on the matter. This work comes in three strands.
与此同时，越来越多的其他研究正在阐明这个问题。这项工作分为三个部分。

The first is a set of studies of people in groups for which unusual circumstances have made possible tallying each and every infection.
首先是对人群的一组研究，不同寻常的环境使统计每一个感染患者成为可能。

These studies permit a fairly precise estimate of the share of those infected who have no symptoms.
这些研究允许对没有症状的感染者的比例进行相当精确的估计。

One such group are the passengers and crew of the Diamond Princess, a cruise ship on-board which the infection rate exploded because of a bungled quarantine.
钻石公主号邮轮上的乘客和船员就是这样的一群人。由于疏于检疫，邮轮上的感染率出现了爆炸性增长。

Of 634 people thus infected, 52% had no symptoms at the time of testing, including 18% who never developed symptoms.
634名感染者中，52%的患者在检测时都没任何症状，其中18%从未出现症状。

The residents of Vo, an Italian town in which all 3,300 people were tested twice, is another much-cited example.
另一个被广泛引用的例子是意大利小城Vo的居民，该市所有3300人都接受了两次检测。

Of those in Vo found to be infected, 50-75% had no symptoms at the time of the test.
感染者中，50-75%的人在检测时没有出现症状。