According to the official report, the first case of COVID-19 and the first death in the United States occurred on January 20 and February 29, 2020, respectively. On April 21, California reported that the first death in the state occurred on February 6, implying that community spreading of COVID-19 might have started earlier than previously thought. Exactly what is time zero, i.e., when did COVID-19 emerge and begin to spread in the U.S. and other countries? We develop a comprehensive predictive modeling framework to address this question. Using available data of confirmed infections to obtain the optimal values of the key parameters, we validate the model and demonstrate its predictive power. We then carry out an inverse inference analysis to determine time zero for 10 representative states in the U.S., plus New York City, United Kingdom, Italy, and Spain. The main finding is that, in both the U.S. and Europe, COVID-19 started around the new year day.

中文翻译：

---

COVID-19开始的最佳推断

根据官方报告，美国首例COVID-19病例和首例死亡分别于2020年1月20日和2月29日发生。4月21日，加利福尼亚州报告说，该州的首例死亡发生在2月6日，这意味着社区传播COVID-19的时间可能早于先前的想象。到底什么时候是零，即COVID-19何时出现并开始在美国和其他国家/地区传播？我们开发了一个全面的预测建模框架来解决这个问题。使用已确认感染的可用数据来获取关键参数的最佳值，我们验证模型并证明其预测能力。然后，我们进行逆推断分析，以确定美国10个代表州以及纽约市，英国，意大利和西班牙的零时区。主要发现是，在美国和欧洲，COVID-19都是在新年前后开始的。