Chinese researchers have found that the COVID-19 pandemic might have started to spread in the United States around September 2019, months earlier than their first reported case. The Chinese Academy of Sciences study set up an optimized model utilizing the epidemic transmission model and big-data analysis method. The result shows that for 12 U.S. states with a probability of 50%, the possible dates of their first infection fall mostly between August and October 2019, with the earliest in late April.