How Far Can a Person Spread Coronavirus?
When an infection erupts the way coronavirus has exploded in Wuhan, China, and elsewhere in the world, public-health experts try to gauge the potential for an epidemic—or,worse, a pandemic—by calculating the pathogen’s basic reproduction number.
当一种感染以冠状病毒在中国武汉和世界其他地方爆发的方式爆发时，公共卫生专家试图通过计算病原体的基本繁殖数，来评测流行传染病潜在的可能性，或者更糟糕的是，瘟疫大肆横行的可能性。
The figure, generally written as R0 and pronounced “R naught,” is an estimate of how many healthy people one contagious person will infect. Because viruses spread exponentially, a few cases can quickly blow up to an overwhelming number. An R0 of two suggests a single infection will, on average, become two, then four, then eight.
这个数字通常被计为R0，发音为“R零”，是对一个感染者将感染多少健康人的估计。因为病毒呈指数级传播，少数病例可以迅速感染他者以至于最后爆发成一个压倒性的数字。在两篇论文中，一个R0一次感染平均会变成两个，然后是四个，然后是八个。
Until the infection is contained or runs its course, the doubling will continue. But a pathogen’s basic reproduction number assumes everyone is susceptible to infection. Thanks to vaccines and other conditions, that often isn’t the case, and the effective reproduction number is lower.
直到被感染体得到控制，这一进程仍持续进行，感染的数量将继续增长下去。病原体的基本繁殖数量基于的是假定每个人都容易受到感染。然而多亏疫苗和其他因素的影响，事实并非如此，病菌有效繁殖数已经降低。
When the new strain of coronavirus began to infect humans, there was nothing to slow its progression.
然而，当冠状病毒的新菌株开始感染人类时，没有什么可以减缓它的传播。
Because the disease, which experts have named Covid-19, is infecting humans for the first time, there is no conferred immunity from previous exposure or vaccination. Everyone can potentially catch it, and, according to the MRC Centre for Global Infectious Disease Analysis at Imperial College London, the virus has an R0 of 1.5 to 3.5.
因为这种被专家命名为Covid-19的疾病是第一次感染人类，所以之前的经验或接种还未产生免疫。每个人都可能感染它， 而且，据伦敦帝国学院的MRC全球传染病分析中心的数据分析，该病毒的R0值为1.5至3.5。
“Right now, coronavirus is much more of a concern than SARS ever was,” said Steven Riley, a professor of infectious-disease dynamics at Imperial College, referring to the global outbreak in 2003, when 8,096 people got sick and 774 died. “The main reason is our estimate of the number of people who are currently infectious is higher than the maximum who were ever infectious at one time with SARS.”
帝国理工学院传染病动力学教授史蒂文·莱利在谈到2003年的全球疫情（8,096人确诊，774人死亡）时说：“现在，冠状病毒比当年非典更令人担忧。主要原因是我们对目前传染人数的估计高于曾经感染过SARS的最高感染人数。”
Covid-19 and severe acute respiratory syndrome, or SARS, are both caused by coronaviruses, a family of viruses that causes infections ranging from the common cold to severe respiratory illness.
Covid-19和严重急性呼吸系统综合症（SARS）都是由冠状病毒引起的，冠状病毒是一个可以引起从普通感冒到严重呼吸道疾病的各种感染的病毒种类。
So far, China’s national health commission has reported more than 59,000 cases of Covid-19 and at least 1,300 deaths. In addition, at least 447 cases have been confirmed in 24 other countries, according to the World Health Organization, including 15 infections in the U.S.
当前，中国国家卫生委员会已经报告了59，000多例Covid-19病例，至少有1,300人死亡。此外，至少有447例病例在其他24个国家得到确诊。据世界卫生组织称，这些病例中其中有15例发生在美国。
Because many cases go undetected, the actual total is probably far higher.
因为很多疑似和在潜伏期的病患未确诊，所以实际总数可能要高得多。
Monitoring in China is likely picking up 10% or less of all infections there, according to Neil Ferguson, director of the MRC Centre. In other countries, it’s probably picking up around 25%.
据MRC中心主任尼尔·弗格森说，在中国的监测可能会占到所有感染病例的10%或更少。在其他国家，它可能上升了25%左右。
“Where we’ll look next is Hong Kong and Singapore,”Dr. Riley said. “We may see a growth phase there next,” although, he added, researchers haven’t yet seen exponential growth outside mainland China.
“我们接下来要重视的是香港和新加坡，”莱利博士说，并称，尽管研究人员还没有看到中国大陆以外病例的指数级增长，但“接下来我们可能会看到一个增长阶段，”
To estimate the potential spread, researchers at Imperial College began with two pieces of information: the time when the virus emerged, in December, and the fact that seven travelers who left Wuhan were confirmed to have Covid-19.
为了估计潜在的传播，帝国理工学院的研究人员从两条线索入手：一是病毒出现的时间——12月；二是7名确诊患有Covid-19离开武汉的旅行者 。
To extrapolate how many other people were infected, they observed that 19 million people were in the greater Wuhan area, and on average, 3,301 traveled internationally daily.
为了推断还有多少人被感染，他们发现整个武汉地区共有1900万人，而平均每天有3301人进行国际旅行。
“If you divide 19 million by 3,301, you get the per day probability of international travel,” said Dr. Riley, who was a member of the team that generated the estimate. “That’s approximately a 1 in 5,755 chance of traveling per day.”
“如果你把1900万除以3301，你就得到了每天国际旅行的概率，”得到这一结果的团队成员之一莱利博士说，“每天出游的比例大约是5755分之一。”