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Absence of evidence is not evidence of absence

沒有患病的證據不代表沒有患病



How important is silent transmission in the covid-19 pandemic?

在新冠肺炎大流行病中,隱性感染情況如何?

Few stories are as prominent in the study of infectious diseases as that of Mary Mallon, a cook to wealthy families, and also to a maternity hospital, in New York in the early 1900s. As she went from one employer to another, typhoid fever, then deadly in one case in ten, followed in her wake. Public-health o?cials eventually joined the dots and identified her as a carrier of Salmonella typhi, the bacterium that causes the disease. What was striking about Typhoid Mary, as the newspapers nicknamed her, was that she herself was healthy—proof that people could harbour and transmit S. typhi without showing symptoms of the illness it causes.

瑪麗·梅倫于20世紀早期在幾家富裕人家和一所婦產醫院當廚師,在傳染病研究中,很少有什么故事能像她的故事一樣有名。她從一個雇主處去往另一個雇主處,身上攜帶的傷寒病便留存下來,那時十個傷寒病患者中會有一人死亡。公共衛生官員最終通過各種細節確定她攜帶了引起傷寒病的傷寒桿菌。當時的報紙稱她為傷寒瑪麗,在傷寒瑪麗的故事中最不同尋常之處就在于她本人是健康的,這證明了人們可以在沒有任何癥狀的情況下攜帶并傳染傷寒桿菌。



Definitive studies of this nature are not yet available, though some are probably in the works, Dr Chowell reckons. In the meantime, a growing collection of other research is shedding light on the matter. This work comes in three strands.

考維爾博士認為,這方面的明確研究還沒有什么消息,不過可能有人正在進行研究。與此同時,越來越多的其他研究闡明了這一問題。他們的研究工作分為了三種類型。

The first is a set of studies of people in groups for which unusual circumstances have made possible tallying each and every infection. These studies permit a fairly precise estimate of the share of those infected who have no symptoms. 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. Of 634 people thus infected, 52% had no symptoms at the time of testing, including 18% who never developed symptoms. The residents of Vo, an Italian town in which all 3,300 people were tested twice, is another much-cited example. Of those in Vo found to be infected, 50-75% had no symptoms at the time of the test. A smaller but similarly useful cohort was several planeloads of Japanese evacuated from Wuhan, the Chinese city where the epidemic began. Among the 12 people in this group found to be infected, five have never developed symptoms.

第一種是針對群體的研究,不尋常的情況使得每一次感染都可以計算出來。這些研究可以對無癥狀感染者的數量進行相當精確的估計。一個例子是“鉆石公主號”的乘客和船員。這是一艘游輪,由于隔離不力,感染率激增。在被感染的634人中,有52%的人在檢測時沒有癥狀,包括18%從未出現過癥狀的人。另一個被廣泛引用的例子是意大利小鎮沃·歐加諾的居民,全鎮3300人都接受了兩次檢測。在沃·歐加諾小鎮發現被感染的人中,有50-75%的人在檢測時沒有癥狀。一個相對較小但同樣有用的例子是從流行病開始的中國城市武漢撤離的幾架日本飛機。 在這組被發現感染的12個人中,有5個人從未出現癥狀。

The rest is silence

隱性傳染是關鍵

All this suggests that the number of infected people unwittingly infecting others could be quite large. What is unclear is how infectious these people actually are. That is what the second strand of research on the asymptomatic and presymptomatic transmission of SARS-COV-2 deals with. It draws on various laboratory studies. In several of these the amount of the virus in nasal and throat swabs taken from infected people who were presenting no symptoms at the time was similar to the amount found in those who had symptoms. 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, which suggests that it may be easily transmissible at an early stage of infection.

這些例子都表明,在不知不覺中傳染他人的感染者人數可能很多。目前尚不清楚這些人實際上有多大的傳染力。這就是第二種研究,其研究內容關于新冠病毒的無癥狀和癥狀前傳播。它參考了各種實驗室研究。在其中一些研究里,從當時沒有癥狀的感染者身上提取的鼻部和咽拭子中的病毒數量與有癥狀感染者身上發現的數量相當。實際上,對于那些之后出現癥狀的感染者來說,他們身上的病毒數量在癥狀開始發作時達到峰值,這表明在感染的早期階段,病毒很容易傳播。



Collectively, all this research may help explain why SARS-COV-2 has spread with such ferocity. But the study, in particular, of those who are infected but never present symptoms is also crucial to understanding how that spread may ebb—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. Pandemics end when the pathogen causing them runs out of individuals to infect. Some of those susceptible will have died. Enough of the rest would then be immune for the population to have developed “herd immunity”. 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.

總的來說,所有這些研究都可能有助于解釋為什么新冠病毒的傳播程度如此嚴重。但是,對那些從未出現癥狀的感染者進行的研究對于理解病毒傳播如何消退也至關重要,因為被傳染進而至少在短期內產生免疫的感染者也包括這些人群。當導致大流行病的病原體無人可感染時,大流行病便結束了。到那時,其中一些易感者已病亡。這樣,其余的人就可以充分免疫,從而形成“群體免疫”。就當前的新冠病毒大流行而言,隱性傳染越多,這種群體免疫就會越快實現。