The team, including researchers from University of California San Diego in the US, note that their simulations suggest that the mutating virus dies out naturally more than three-quarters of the time without causing an epidemic. "Our study was designed to answer the question of how long could SARS-CoV-2 have circulated in China before it was discovered," said senior study author Joel O Wertheim, an associate professor at UC San Diego School of Medicine. 

"To answer this question, we combined three important pieces of information: a detailed understanding of how SARS-CoV-2 spread in Wuhan before the lockdown, the genetic diversity of the virus in China and reports of the earliest cases of COVID-19 in China," Wertheim said. 

By combining these disparate lines of evidence, the researchers were able to put an upper limit of mid-October 2019 for when SARS-CoV-2 started circulating in Hubei province. Cases of COVID-19 were first reported in late-December 2019 in Wuhan, located in the Hubei province of central China. The researchers used molecular clock evolutionary analyses to try to home in on when the first , or index, case of SARS-CoV-2 occurred. 

"Molecular clock" is a term for a technique that uses the mutation rate of genes to deduce when two or more life forms diverged -- in this case, when the common ancestor of all variants of SARS-CoV-2 existed, estimated in this study to as early as mid-November 2019. Molecular dating of the most recent common ancestor is often taken to be synonymous with the index case of an emerging disease. 

"The index case can conceivably predate the common ancestor -- the actual first case of this outbreak may have occurred days, weeks or even many months before the estimated common ancestor," said study co-author Michael Worobey, a professor at the University of Arizona. "Determining the length of that 'phylogenetic fuse' was at the heart of our investigation," Worobey said. 

Based on this work, the researchers estimate that the median number of persons infected with SARS-CoV-2 in China was less than one until November 4, 2019. Thirteen days later, it was four individuals, and just nine on December 1, 2019, they said. The first hospitalisations in Wuhan with a condition later identified as COVID-19 occurred in mid-December. The researchers used a variety of analytical tools to model how the SARS-CoV-2 virus may have behaved during the initial outbreak and early days of the pandemic when it was largely an unknown entity. 

These tools included epidemic simulations based on the virus's known biology, such as its transmissibility and other factors. In just 29.7 per cent of these simulations was the virus able to create self-sustaining epidemics. In the other 70.3 per cent, the virus infected relatively few persons before dying out. 

The average failed epidemic ended just eight days after the index case. "We saw that over two-thirds of the epidemics we attempted to simulate went extinct. That means that if we could go back in time and repeat 2019 one hundred times, two out of three times, COVID-19 would have fizzled out on its own without igniting a pandemic,” Wertheim noted. "This finding supports the notion that humans are constantly being bombarded with zoonotic pathogens," he added.