A study published on the pre-print platform ChinaXiv, a Chinese open repository for scientific researchers, indicated that COVID-19 had started spreading in the U.S. around September 2019 "with a high confidence probability."

The study, conducted by researchers from the University of Science and Technology of China and the University of Chinese Academy of Sciences, is based on a data and hybrid model-driven method.

By combining a mathematical model and artificial intelligence technology, qualitative and quantitative analysis of the case data of an infectious disease can reveal its epidemic law and detect its origin and development trend, they said.

"By selecting 12 representative regions in the U.S. for analysis, the dates of the first infected case with 50 percent confidence probability are mostly between August and October 2019, which are earlier than the officially announced date of the first confirmed case in the U.S. on January 20, 2020," the authors wrote.

Using the same methodology, the study concluded that the spread of the COVID-19 in China likely began in late December 2019.

Origin tracing is crucial for infectious disease prevention and control, and a series of previous studies showed that countries including the U.S., Spain, France, Italy and Brazil had been hit by the coronavirus before its outbreak in China, the authors wrote.