【经济学人】缺乏证据不代表没证据（2）
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.
unwittingly adv.不知情地
That is what the second strand of research on the asymptomatic （无症状的）and presymptomatic （症状前感染的）transmission of SARS-COV-2 deals with. It draws on various laboratory studies.
draw on 利用；吸收；鼓励；引起；戴上；临近
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.
nasal and throat 鼻腔和咽喉
swab vt.打扫，擦拭；涂药（于） n.医用海绵，纱布；拖把；无赖
cotton swab 棉签
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（他们体内的病毒数量在这些症状出现之前达到顶峰）, which suggests that it may be easily transmissible at an early stage of infection.
As a persistent cough is a common symptom, it might be expected that those who are symptomatic are more effective in spreading the virus than those who are not. Contrariwise, however, those with symptoms often feel unwell and take to their beds. They are, therefore, coughing mainly onto their sheets and blankets rather than onto strangers in the street.
contrariwise adv.反之
The third strand of research into the question of silent spreading is mathematical modelling. One such study was published in Science on March 31st by Luca Ferretti of Oxford University and his colleagues. It used data on 40 infected people for whom the source of their infection was known with high probability, and the timing of their symptoms and those of the people who infected them was well documented.
mathematical model 数学模型
The researchers estimate that between a third and a half of transmission occurs from people who are without symptoms at that point —— a result which broadly agrees with estimates from similar studies by others. Collectively, all this research may help explain why SARS-COV-2 has spread with such ferocity.
collectively adv.全体的；共同的 ；总的来说
ferocity n.凶猛；残忍；暴行
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（再次感染） at least in the short term also includes these people.
ebb n./vi. 退潮；衰落；减少 ebb tide
ebb and flow 兴衰，起伏；潮的涨落
at a low ebb 处于低谷
flood and ebb 潮汐
pool n.共同物；共用人员；（液体等的）一片、一滩 vt.共同使用；共用
talent pool 人才库
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".
pathogen n.病原体；病菌
herd immunity 群体免疫
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.
silent infections 寂静传染病例
另：我以为这篇文章是让人们注意无症状感染，结果最后还是指向了群体免疫，呵呵...
