疾病控制和预防中心主任警告说，多达25％的新冠状病毒感染者可能没有症状，这是一个惊人的数字，使预测大流行的过程和减轻其传播策略的努力变得复杂。
特别是，无症状病例的高水平正在促使CDC考虑扩大其有关谁应该戴口罩的准则。
主任罗伯特·雷德菲尔德（Robert Redfield）博士在周二接受采访时对国家公共广播电台说：“这有助于解释这种病毒在全国范围内扩散的速度。”
该机构一再表示，普通公民除非感到不适，否则就不需要戴口罩。但是，有了关于可能从未感染过疾病或没有生病就已经传播病毒两天的被感染者的新数据，雷德菲尔德说，这种指导已被“严格审查”。
疾病控制与预防中心主任罗伯特·雷德菲尔德（Robert Redfield）博士在2020年3月22日星期日在华盛顿白宫举行的冠状病毒工作队简报会上讲话时听取了唐纳德·特朗普的讲话。（美联社照片/帕特里克·塞曼斯基）更多
研究人员并不确切地知道有多少人没有生病或是否有些只是有症状的而被感染。但是，自从新的冠状病毒在12月浮出水面以来，他们发现了一些不健康的人，他们不知道撒布机，这些轶事似乎很健康。
例如，中国广东一名26岁男子“患者Z”与2月份感染冠状病毒的武汉旅行者密切接触。但是，无论是在接触后的第7天，还是在第10或11天，他都没有任何不妥的迹象。
不过，到了第7天，这种病毒已经在他的鼻子和喉咙里开花了，就像那些生病的人一样大量繁殖。Z患者可能感觉还不错，但他也被感染了。
现在，研究人员说，像Z病患者这样的人不仅仅是轶事。例如，根据一项分析，在钻石公主号游轮上感染该病毒的人中，多达18％从未出现症状。香港的一个研究小组建议，在中国，有20％至40％的传播是在症状出现之前发生的。
秘密传播的高水平可能有助于解释为什么新型冠状病毒是第一种不是引起大流行的流感病毒的病毒。
新病毒的传播和流感一样容易，“什么时候没有人想到除疫苗之外没有任何关于阻止流感传播的想法？” 明尼苏达大学（University of Minnesota）的传染病专家迈克尔·奥斯特霍尔姆（Michael T. Osterholm）博士说。
他和其他专家说，在任何疫苗仍处于早期开发阶段时，减轻大流行的最好方法是远离社会。因为即使人们感觉良好，人们也可能会将病毒传播给其他人，所以仅让身体不适的人呆在家里是不可能的。这就是为什么许多专家违背疾病预防控制中心和世界卫生组织的建议，现在敦促每个人都戴口罩，以防止那些不知道自己感染病毒的人传播病毒。
现在，一些专家说，像流感一样，这种病毒似乎通过大液滴和小于5微米的液滴（称为气溶胶）传播，其中含有被感染的人可能会释放的病毒，尤其是在咳嗽时，甚至在呼气时。他们强调说，两种类型颗粒中的病毒水平都很低，因此被感染者慢跑或走路不会使人们处于危险之中。
香港大学的流行病学家本杰明·考林（Benjamin Cowling）博士说：“如果您与传染病患者有往来接触，那么传播的机会将非常非常低。”
持续接触会增加风险-例如，在面对面的交谈中，或者长时间共享同一空域。考林说，除了在口罩上的混乱立场外，“世卫组织一直在说没有发生气溶胶传播，这也是令人困惑的事情。”
专家们一致认为，没有报告症状的人正在传播感染，即所谓的无症状传播，但他们也注意到该术语有些混乱。
哥伦比亚大学传染病专家杰弗里·沙曼（Jeffrey Shaman）博士说：“没有标准的定义，您可以对自己说，这很荒谬：您有症状还是没有症状。” 他说，但是研究小组的研究表明，有些人从不注意到自己的症状，另一些人则无法将其感染与吸烟者的咳嗽，过敏或其他情况区分开来，还有一些人可能会感到十分痛苦。
关于似乎完全健康但随后生病的人的比例，也存在很大的语义辩论，例如《新英格兰医学杂志》的一篇关于无症状传播者的报告，后来他承认自己感到症状轻微。
萨曼说，最终这些定义并不重要。
他说：“最重要的是，外面有一些人在散播病毒，却不知道自己被感染了。”
定义可能重要的地方在于能够了解大流行的真实范围。
考林的团队分析了大流行期间各个阶段来自中国的数据。世卫组织对中国的访问得出的结论是，大多数被该病毒感染的人都有明显的症状。但他的分析表明，在流行的最初几周里，中国为确诊的感染病例设定了高门槛，即需要呼吸道症状，发烧和进行肺部X光检查。
他们的定义排除了轻度和无症状的病例，因此，该团队大大低估了那里爆发的规模和性质。
考林说：“我们在中国估计有20％至40％的传播事件是在症状出现之前发生的。”
对钻石公主号游轮上数百名被隔离的人的单独分析证明了这一规模。该船于2月5日停靠在日本后，研究人员对所有乘客进行了测试，并在两周内多次审查了对该病毒呈阳性反应的乘客。他们发现18％的受感染乘客在整个过程中都没有症状。
佐治亚州立大学的流行病学家Gerardo Chowell博士说：“ COVID-19的无症状比例相当惊人。”
Chowell指出，船上的乘客往往年龄较大，因此更有可能出现症状。他估计，普通人群中大约40％的人可能被感染而没有迹象表明。
也有许多暗示（微妙的和没有暗示），表明该病毒可以通过气溶胶传播。西雅图合唱团的60名成员于3月10日聚集在一起参加练习，时间超过2 1/2小时。他们没有一个人感到不适，他们彼此之间没有任何接触。但是到了这个周末，数十名成员病倒了，其中两名已经死亡。
他们的经验表明，通过气溶胶的空气传播比世卫组织和疾病预防控制中心所强调的大液滴传播得更远。一项研究显示，这种病毒最有可能通过咳嗽或打喷嚏而被排出，最远可达8米（约26英尺）。但是对流感和其他呼吸道病毒（包括其他冠状病毒）的研究表明，人们可以通过呼吸或说话（或者大概是通过唱歌）释放含有该病毒的气溶胶。
乔威尔说：“我认为越来越多的证据表明该病毒不仅通过飞沫传播，还通过气溶胶传播。” “至少鼓励在包括超级市场在内的封闭空间中使用口罩是很有意义的。”
现在有几项研究表明，感染新冠状病毒的人在开始出现症状之前约一到三天具有最强的传染性。对于引起严重急性呼吸系统综合症和中东呼吸系统综合症的冠状病毒，这种症状前传播并不正确。
卡尔博士说：“这是SARS非常幸运的地方，它的确直到人们出现症状后才传播，这使得更容易发现并通过积极的公共卫生措施将其关闭。”西雅图大学华盛顿大学新兴传染病专家伯格斯特伦。
有了新的冠状病毒，好看的人会传播这种病毒，通常症状很严重，病死率很高。他说：“整体来看，采取标准的公共卫生措施非常困难。”
疾病预防控制中心在周二进行的另一项分析提供了新的证据，表明在美国，患有严重冠状病毒感染的人中有很大一部分患有基础疾病。该机构调查了7,162例病例，仅占美国122,000例病例的一小部分，但调查结果却显示出鲜明的形象。在该亚组的457名重症监护病房中，32％患有糖尿病，29％患有心脏病，21％患有肺病。总体而言，入ICU的COVID-19患者中有78％至少患有一种疾病。该研究没有研究死亡。
快速检测感染可能有助于发现被感染但感觉正常的人，尤其是医护人员。口罩可能会有所帮助。但是，专家们一直将社会隔离作为长期阻止传播链的唯一最佳工具-不一定是封锁，而是取消大规模活动，在可能的情况下在家工作并关闭学校。
伯格斯特姆说：“我们不能假设我们任何人都不是潜在的媒介。” “这就是为什么即使我感觉很好，感觉很好并且没有接触过任何有任何症状的人，这就是为什么我今天出去走走是不负责任的。”
本新闻最早发布于《纽约新闻》
由网友昼夜颠翻译转载
新闻原文：
Infected but Feeling Fine: The Unwitting Coronavirus Spreaders
As many as 25% of people infected with the new coronavirus may not show symptoms, the director of the Centers for Disease Control and Prevention warns — a startlingly high number that complicates efforts to predict the pandemic’s course and strategies to mitigate its spread.
In particular, the high level of symptom-free cases is leading the CDC to consider broadening its guidelines on who should wear masks.
“This helps explain how rapidly this virus continues to spread across the country,” the director, Dr. Robert Redfield, told National Public Radio in an interview broadcast Tuesday.
The agency has repeatedly said that ordinary citizens do not need to wear masks unless they are feeling sick. But with the new data on people who may be infected without ever feeling sick, or who are transmitting the virus for a couple of days before feeling ill, Redfield said that such guidance was “being critically rereviewed.”
Dr. Robert Redfield, director of the Centers for Disease Control and Prevention, listens as President Donald Trump speaks during a coronavirus task force briefing at the White House, Sunday, March 22, 2020, in Washington. (AP Photo/Patrick Semansky)More
Researchers do not know precisely how many people are infected without feeling ill or if some of them are simply presymptomatic. But since the new coronavirus surfaced in December, they have spotted unsettling anecdotes of apparently healthy people who were unwitting spreaders.
“Patient Z,” for example, a 26-year-old man in Guangdong, China, was a close contact of a Wuhan traveler infected with the coronavirus in February. But he felt no signs of anything amiss, not on Day 7 after the contact nor on Day 10 or 11.
Already by Day 7, though, the virus had bloomed in his nose and throat, just as copiously as in those who did become ill. Patient Z might have felt fine, but he was infected just the same.
Researchers now say that people like Patient Z are not merely anecdotes. For example, as many as 18% of people infected with the virus on the Diamond Princess cruise ship never developed symptoms, according to one analysis. A team in Hong Kong suggests that from 20% to 40% of transmissions in China occurred before symptoms appeared.
The high level of covert spread may help explain why the novel coronavirus is the first virus that is not an influenza virus to set off a pandemic.
The new virus spreads about as easily as flu, “and when’s the last time anyone thought anything about stopping influenza transmission, short of the vaccine?” said Dr. Michael T. Osterholm, an infectious disease expert at the University of Minnesota.
With any vaccine still in early development, the best way to mitigate the pandemic is social distancing, he and other experts said. Because people may be passing the virus on to others even when they feel fine, asking only unwell people to stay home is unlikely to be enough. This is why many experts, going against recommendations by the CDC and the World Health Organization, are now urging everyone to wear masks — to prevent those who are unaware they have the virus from spreading it.
Like influenza, some experts now say, this virus appears to spread both through large droplets and droplets smaller than 5 micrometers — termed aerosols — containing the virus that infected people might release especially while coughing but also while merely exhaling. They emphasized that the level of virus in both types of particles is low, so simply jogging or walking by an infected person does not put people at risk.
“If you have a passing contact with an infectious person, you would have a very, very low chance of transmission occurring,” said Dr. Benjamin Cowling, an epidemiologist at the University of Hong Kong.
The risk goes up with sustained contact — during face-to-face conversation, for example, or by sharing the same air space for a prolonged time. In addition to its confusing stance on masks, “the WHO has been saying aerosol transmission doesn’t occur, which is also perplexing,” Cowling said, adding, “I think both are actually wrong.”
Experts agreed that infections were being passed along by people who do not report symptoms — what they call asymptomatic transmissions — but they also noted some confusion around the term.
“There’s no standard definition for it, and you could say to yourself, well, that’s kind of ridiculous: You either have symptoms or you don’t,” said Dr. Jeffrey Shaman, an infectious diseases expert at Columbia University. But studies by his team have shown, he said, that some people never notice their symptoms, others are unable to distinguish the infection from their smoker’s cough or allergies or other conditions, and still others may feel every pain acutely.
There is also a largely semantic debate about the proportion of people who appear to be perfectly fine but then become ill — as in the report in The New England Journal of Medicine of an apparently asymptomatic spreader who later acknowledged having felt mild symptoms.
Ultimately, Shaman said, these definitions are unimportant.
“The bottom line is that there are people out there shedding the virus who don’t know that they’re infected,” he said.
Where the definitions may matter is in being able to understand the true scope of the pandemic.
Cowling’s team has analyzed data from China at various stages in the pandemic. The WHO’s mission to China concluded that most people who were infected with the virus had significant symptoms. But in the early weeks of the epidemic, his analysis shows, China set a high bar for what constituted a confirmed case of infection — requiring respiratory symptoms, fever and a chest X-ray for pneumonia.
Their definition left out mild and asymptomatic cases and, as a result, the team vastly underestimated the scale and nature of the outbreak there.
“We’ve estimated in China that between 20% and 40% of transmission events occurred before symptoms appeared,” Cowling said.
A separate analysis of the hundreds of people cloistered aboard the Diamond Princess cruise ship bears out this scale. Once the ship docked in Japan on Feb. 5, researchers tested all of the passengers and reviewed those who tested positive for the virus on multiple occasions over a two-week period. They found that 18% of the infected passengers remained symptom-free throughout.
“The substantial asymptomatic proportion for COVID-19 is quite alarming,” said Dr. Gerardo Chowell, an epidemiologist at Georgia State University who worked on the analysis.
Chowell noted that the passengers on the ship tended to be older and therefore more likely to develop symptoms. He estimated that about 40% in the general population might be able to be infected without showing signs of it.
There have also been many hints, subtle and not, that the virus can be transmitted via aerosols. Sixty members of a choir in Seattle gathered March 10 for a practice session for more than 2 1/2 hours. None of them felt ill, and they made no contact with one another. But by this weekend, dozens of the members had fallen ill, and two had died.
Their experience points toward airborne transmission via aerosols, which can travel farther than the large droplets the WHO and the CDC have emphasized. The virus is still most likely to be expelled with a cough or a sneeze, as far as 8 meters (about 26 feet), according to one study. But studies on influenza and other respiratory viruses, including other coronaviruses, have shown that people can release aerosols containing the virus simply by breathing or talking — or, presumably, by singing.
“I think increasing evidence suggests the virus is spread not just through droplets but through aerosols,” Chowell said. “It would make a lot of sense to encourage at the very least face mask use in enclosed spaces including supermarkets.”
Several studies have shown now that people infected with the new coronavirus are most contagious about one to three days before they begin to show symptoms. This presymptomatic transmission was not true of the coronaviruses that caused severe acute respiratory syndrome and Middle East respiratory syndrome.
“This is where we got very lucky with SARS, was that it really didn’t transmit until after people were showing symptoms, and that made it much easier to detect it and shut it down with aggressive public health measures,” said Dr. Carl Bergstrom, an expert in emerging infectious diseases at the University of Washington in Seattle.
With the new coronavirus, there is transmission by healthy-seeming people, and often severe symptoms and a high fatality rate. “That whole combination makes it very, very tough to fight using standard public health measures,” he said.
A separate analysis from the CDC on Tuesday offered new evidence that a significant portion of people with severe coronavirus infections in the United States have underlying medical conditions. The agency looked at 7,162 cases, a small subset of the 122,000 cases in the U.S., but the findings provided a stark portrait. Of 457 people in that subset who were admitted to intensive care units, 32% suffered from diabetes, 29% had heart disease and 21% had lung disease. Overall, 78% of people with COVID-19 admitted to the ICU had at least one preexisting condition. The study did not look at deaths.
Rapid tests for infection might help detect people, especially health care workers, who are infected yet feel normal. Masks may help. But experts kept returning to social distancing as the single best tool for stopping the chain of transmission in the long term — not lockdowns, necessarily, but canceling mass events, working from home when possible and closing schools.
“We can’t assume that any of us are not potential vectors at any time,” Bergstrom said. “This is why even though I’m feeling great, and have felt great and haven’t been exposed to anybody with any symptoms of anything, that’s why it would be irresponsible of me to go out and about today.”