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科学美国人(翻译):COVID-19病毒如何以及为什么传播迅速 2020.03.16

COVID-19: How and Why the Virus Spreads Quickly


This is another in our series of coronavirus episodes of Scientific American’s Science Talk, posted on March 23, 2020. I’m Steve Mirsky.In this two-part episode, our contributing editor W. Wayt Gibbs in Washington state—a state hit early and hard by COVID-19—reports on scientists’ fast-evolving understanding of this new coronavirus and the probable trajectory of this pandemic.Today, in part one, he focuses on why the new coronavirus is spreading so quickly and is so difficult to control.

这是《科学美国人》科学讲座中关于冠状病毒的另一个系列,发表于2020年3月23日。我是史蒂夫·米尔斯基。在这个分为两部分的章节中,我们的特约编辑W. Wayt Gibbs在华盛顿州——一个科学家对这种新型冠状病毒的快速发展的理解和这次大流行的可能轨迹的covid19报告的早期重击的州。今天,在第一部分中,他着重解释了为什么这种新型冠状病毒传播如此迅速,又如此难以控制。

Check back in tomorrow for part two, when Gibbs looks at computer models that are predicting how long we’ll need to shut down large parts of society to prevent hospitals from being overwhelmed. He also looks at how emerging tests for immunity to the virus could pose thorny ethical issues in the months to come.Wayt recorded this episode on March 22nd. The first voice you hear: Governor Jay Inslee of Washington.


JI: “It is not rhetorical or hyperbolic when I say that everyone needs to change their behavior, change the way that we live—temporarily—if we are going to prevent significant loss of life for the people we love in the state of Washington. And when I say everyone, I mean, frankly, everyone. Because we all are potential transmitters of this virus, and we all, to some varying degree, are potential victims of this virus.”


WWG: That was Governor Jay Inslee, pleading with people in my state on March 20th to stay home and stay away from each other. All around the U.S.—and around the world—governors and mayors and prime ministers are urging, begging, in many places even ordering their citizens to shelter in place.


But it’s not easy to resist our hard-wired desires to spend time with our friends, to visit our parents and grandparents, to go to work.So here in Washington, like in lots of other places, compliance has been—kind of spotty. Traffic on some of the major highways here—a good proxy for human intermingling—is down only about 20% or so from normal.


But as scientists learn more about the SARS-CoV-2 virus and the COVID-19 pandemic, it’s becoming apparent that we’re facing a much more serious situation than most of us thought even a couple weeks ago.


Let me give you one example from here in Washington. On March 10th, despite stern official warnings not to gather in groups, 56 people met for an event in Skagit County. All of them were apparently healthy at the time. But ten days later, 43 of those 56 people have either been confirmed to have COVID-19 or are showing symptoms of the disease. Experts suspect that one or more people in the group was a so-called “super-shedder,” someone who has yet to show symptoms but is transmitting lots of infectious virus.


In this two-part episode, we’ll look at several new research studies and new tests announced this week that may help answer four crucial questions.Question 1: Can you catch this disease from someone who isn’t in the same room as you?


Question 2: Can you catch it from somebody who isn’t sick—and is there a way to test for that?

Question 3: What combination of shutdowns and closures will do the most to minimize the number of people who die from the pandemic? And how long will those tough restrictions need to continue?

And question 4: How will each of us know when we’re immune and no longer need to worry about catching COVID-19 or giving it to someone else? And what will we do with that information once we have it?




Let’s start with that first question: do you have to come in contact with someone who has the virus—or with droplets they are spraying from a cough or a sneeze—to catch it?Well, that’s almost certainly the easiest way to get it. So keeping your distance from others greatly reduces your risk of infection.


But in a study out this week in the New England Journal of Medicine, researchers at a Montana biohazard lab run by the National Institutes of Health confirmed what many experts had feared: this coronavirus can also be infectious outside of a living body, in some cases for days.

但在本周《新英格兰医学杂志》(the New England Journal of Medicine)上发表的一项研究中,美国国家卫生研究院(National Institutes of Health)在蒙大拿州的一个生物危害实验室的研究人员证实了许多专家此前的担忧:这种冠状病毒也可能在活体外具有传染性,在某些情况下会持续数日。

The other night I watched the 2011 movie Contagion, which certainly has some eerie parallels to the present. There’s a scene where Kate Winslet’s character, a CDC disease investigator, explains why that kind of durability is so troubling in a virus.[[audio from clip from Contagion]]KW: “So at this point I think we have to believe this is respiratory—maybe fomites, too.


“What’s that, fomites?

“It refers to transmission from surfaces. The average person touches their face two or three thousand times a day…three to five times, every waking minute. In between, we’re touching doorknobs, water fountains, elevator buttons, and each other. Those things become fomites.”



The N.I.H. researchers tested whether the SARS-CoV-2 virus can spread via fomites—which, by the way, is how you actually pronounce it. They sprayed infectious droplets onto cardboard, stainless steel, and plastic. Then they checked the spatter as time passed to see how quickly the viral particles dried out and fell apart.


On cardboard, it took a good four hours before the amount of live virus started to fall significantly, and the surface remained quite infectious eight hours later.So when an Amazon box appeared on my front porch a couple days ago, I was glad to see that the delivery man was wearing gloves. But after I opened the box, I still went to the sink and washed my hands.


On stainless steel—think railings, shopping carts, gas pumps, jungle gyms—the coronavirus remained infectious for more than a day.And on plastic—so, our pens, our credit cards, our keyboards and keypads—it stayed viable for two to three days.


Here in the Seattle area, store managers and bus operators have been wiping down surfaces more frequently and making disinfecting wipes available at the door for customers to use. We now know that should be standard practice everywhere.Inside that box I received from Amazon was a set of white cotton gloves that I had ordered for me and my family to wear when we go out.


Tests have shown that a standard face mask really doesn’t protect you from catching this virus because so much of the air you inhale leaks in around the edges rather than going through the filter. Wearing a mask is a nice courtesy, though, if you’re going to be around others who may be vulnerable.


But a pulmonary surgeon advised me to wear cotton gloves, as they do at his hospital, because the natural fibers trap the virus and it dries out and becomes inactive. Also, I find that wearing gloves helps me remember to avoid touching my face.So we can get infected from people we never even see—from the fomites they leave behind.But now let’s turn to that second question: can you catch COVID-19 from someone who hasn’t tested positive, or even from someone who may feel perfectly healthy?


Studies seem to confirm that the answer is yes. In a paper that appeared on March 16th in the journal Science, an international group of researchers analyzed data on the outbreak in China in January. They concluded that only 14% of COVID-19 infections were documented, meaning that 86%—that’s six out of seven cases—never showed up in the statistics as a confirmed case.


Here’s what Jeffrey Shaman of Columbia University, one of the authors of that paper, said about that in a news briefing last week:

JS: “These undocumented infections were … about half as infectious per person as a documented case who has more severe symptoms and maybe shedding more. Because, however, there are many more of these undocumented cases, it’s the undocumented infections that are driving the spread and growth of the outbreak.”

哥伦比亚大学(Columbia University)的杰弗里·萨满(Jeffrey Shaman)是这篇论文的作者之一,他在上周的新闻发布会上是这样说的:


Shaman says these undocumented cases are mostly quite mild.JS: “And so most people may not recognize that they may think they have another cold, or they may not even really recognize that they're ill. If you were to project that number globally, given that we have 150,000 confirmed cases … it says that we're approaching close to a million infections globally…. Generally, you’re looking at about an order of magnitude more cases than have been confirmed.”


VO: Of course, every day the count of confirmed cases rises. But the idea is that if you take the count today and multiply by seven, that’s a more realistic estimate of the number of infections to date.


So who is transmitting the virus? Obviously people who are very sick and have tested positive. Plus maybe seven times more people, many of them young, who have caught it but feel well enough that they wouldn’t think to get tested or to self-quarantine.


And then there’s a third group: the fast-increasing numbers of people who are infected, and who—five or ten days from now—will fall ill and become a confirmed case, but who feel perfectly fine at the moment.In a preprint article published on March 18th, researchers with the World Health Organization studied 94 COVID-19 patients in Guangzhou, China to determine when they became infectious—and when they stopped shedding the virus. The scientists collected throat swabs from the patients as soon as they began showing symptoms and then tested them again every day for the following month.

还有第三类人:被感染的人数迅速增加,从现在起5天或10天内,他们将病倒,成为确诊病例,但目前他们感觉非常好。在3月18日发表的一篇预印本文章中,世界卫生组织(World Health Organization)的研究人员研究了94名covid19名中国广州的患者,以确定他们何时开始感染以及何时停止了病毒的传播。科学家们在病人开始出现症状后立即采集他们的咽喉拭子,然后在接下来的一个月里每天都对他们进行测试。

They saw a lot of variation from patient to patient. But overall, the patients tended to become less infectious as their symptoms progressed. Through some calculations, the scientists concluded that these coronavirus patients shed the most virus, and were probably most infectious to others, up to two days before they started feeling ill. So these are people like Senator Rand Paul of Kentucky, who we learned on March 22nd has tested positive for COVID-19 even though he isn’t yet feeling any symptoms.


These W.H.O. researchers estimate that around half of the people who caught this virus in Guangzhou got it from someone who was still feeling healthy at the time. And other studies from China have documented cases where patients have fully recovered from COVID-19 but continued to test positive for the virus for more than a week after their symptoms disappeared.


This phenomenon of pre-symptomatic and post-symptomatic transmission really complicates efforts to contain this virus, in particular because it will be a long time before we have enough coronavirus tests to check people who aren’t yet showing symptoms.


In a move to help ease shortages of coronavirus tests, the U.S. Food and Drug Administration last week gave emergency authorization to hospitals and labs to use a new automated test for COVID-19 that can detect infections in minutes rather than days. Cepheid, a biotech company in California, says it will start shipping the test kits this week.


The Cepheid test performs essentially the same real-time-PCR process that existing tests do, just much faster, says Cepheid's Chief Medical Officer, David Persing:


DP: “So what we've developed is a reference-lab-quality test that can be run at the point of care in about 45 minutes or less. You take a sample, put it into this new cartridge that we've developed, and about 45 minutes later, you'll have the result. And that test can be run at the hospital as patients are being admitted. So the results are available much more quickly. And that means that those results will play into how those patients are managed: who gets respiratory isolation. Who needs antibiotics who doesn't need antibiotics? Those kinds of decisions can be made in real time.”

Persing says that while the test could be used for screening in the community—and may be sensitive enough to detect pre-symptomatic super-shedders—initially it will be available only for hospitals to use on their patients and healthcare workers.



DP: “We think this will be a very important tool in being able to get rapid, actionable results to let patients know if they're carriers, even if they're asymptomatic, to quarantine. And to know that they're being quarantined for a reason, not because they may have hay fever or some other cause, but really give them a reason for quarantine.”


Faster, easier tests should help us get a better handle on the true extent of this pandemic. But these tests aren’t perfect. A number of studies, including one done by scientists at Wuhan University in February, have found that these RT-PCR tests we’re relying on today aren’t sensitive enough to reliably catch the infection in its early stages. So a negative test result is no guarantee that you’re not carrying the virus.


Nor does it tell you whether or not you are immune to the disease. But new tests for immunity are coming.

These characteristics of the new coronavirus—it’s ability to lie in wait on surfaces and to spread easily among people who feel fine or well enough—that is what make this virus so contagious and hard to stop, as Elizabeth Halloran of the Fred Hutchinson Cancer Research Center explained at news briefing last week:


这些特征的新coronavirus-it谎言的能力之间轻易传染等表面,人感觉好或桥梁是什么让这种病毒会传染的,而且很难停下来,Elizabeth Halloran指出的弗雷德哈钦森癌症研究中心上周在新闻发布会上解释说:

EH: “the basic reproductive number—that is the average number of people that an average person infects at the beginning—is estimated … to be about 2.5. And there are actually estimates that are higher.


“That’s before, of course, all this behavioral reduction and social distancing which would reduce it. But it’s going to be difficult even if it does go down somewhat seasonally in the summer to bring that down necessarily below one.”


And until we can drag that reproductive number from 2.5 or so down to less than one, the pandemic will continue to accelerate.In part two of this episode, we’ll look at how computer models of the pandemic are predicting when we’ll be able to emerge from our state of sequestration and start hanging out with each other again. Also in part 2, I’ll update you on antibody tests for immunity to the virus that are now emerging from research labs. And we’ll look at some tough ethical questions that society may confront as it uses this technology.


Until then, for Scientific American’s Science Talk, I’m Wayt Gibbs.


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