A study of big data and epidemic models indicates with 50 percent probability that the first COVID-19 infection in the United States may have occurred between August and October 2019, and the earliest possible case was on April 26, 2019, in Rhode Island, according to a preprint of a study published on Wednesday.

The study, conducted by researchers from the University of Science and Technology of China and the University of Chinese Academy of Sciences, is published on the Chinese preprint server ChinaXiv and is still undergoing peer-review.

After analyzing daily epidemic data published by local health authorities from 11 US states and the District of Columbia, the researchers said they are 50 percent confident that the first COVID-19 cases in the US emerged between August and October 2019, considerably earlier than the currently acknowledged date of the first confirmed US case, on Jan 20, 2020.

"The calculations show that the COVID-19 epidemic in the United States has a high probability of beginning to spread around September 2019," according to the paper.

The 11 states are New Jersey, Vermont, Virginia, Michigan, New Hampshire, Louisiana, Connecticut, New York, Pennsylvania, Maryland, Massachusetts, Delaware and Rhode Island.

"A series of previous studies showed that the United States, Spain, France, Italy, Brazil and other countries had been attacked by the coronavirus before its outbreak in China," the researchers wrote.

Last week, a team of Laotian and French researchers published a preprint study saying they had discovered three coronaviruses found in horseshoe bats that live in northern Laotian limestone caves that are the closest known ancestors yet of the COVID-19 virus.

These viruses share a key feature with SARS-CoV-2 in the part of its genome known as the receptor binding domain, a region that allows it to latch onto cells. This research supports the hypothesis that the COVID-19 virus originated from the horseshoe bat species.

Last month, researchers from the University of Milan and the Italian National Institute of Health reported that a different version of the COVID-19 virus may have been circulating in Lombardy, northern Italy, as early as late summer 2019.

The Italian preprint study suggested that a wider geographical area and a broader time span should be considered when investigating the origins of the virus.