哥廷根大学教授柳叶刀论文
https://medicalxpress.com/news/2020-04-covid-average-actual-infections-worldwide.html
结论很可怕：
Insufficient and delayed testing may explain why some European countries, such as Italy and Spain, are experiencing much higher casualty numbers (relative to reported confirmed cases) than Germany, which has detected an estimated 15.6% of infections compared to only 3.5% in Italy or 1.7% in Spain. Detection rates are even lower in the United States (1.6%) and the United Kingdom (1.2%)—two countries that have received widespread criticism from public health experts for their delayed response to the pandemic.
In sharp contrast to this, South Korea appears to have discovered almost half of all its SARS-CoV-2 infections. The authors estimate that on 31 March 2020, Germany had 460,000 infections. Based on the same method, they calculate that the United States had more than 10 million, Spain more than 5 million, Italy around 3 million and the United Kingdom around 2 million infections. On the same day, Johns Hopkins University reported that globally, there were fewer than 900,000 confirmed cases, meaning that the vast majority of infections were undetected.
看了一下研究方法：作者假定武汉是全部病例都被检测到，因此使用武汉的确诊人数分年龄段死亡率作为先验概率。并且从上述数据中，能得到从开始出现症状到死亡的时间，大概是18天。假定出现症状四天后被确诊，那么就可以用3月31日的死亡人数，倒推3月17日的感染人数，再和3月17日实际检测到的感染人数比较，发现实际人数只有理论人数的6%。
用同样的方法，推测出截止到3月31日，美国感染人数可能已达到1000万……
