The novel coronavirus was likely circulating undetected for about two months in Wuhan, China, before the first human cases of COVID-19 were reported late in December 2019, says a study published in the journal Science.
The researchers from the University of California San Diego in the US and colleagues from the University of Arizona and Illumina, Inc. used molecular dating tools and epidemiological simulations and found that mutating virus dies out naturally more than three-quarters of the time without causing an epidemic.
They conducted an in-depth study on 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.
Several efforts have been made to identify when the virus first began spreading among humans, but according to the study authors, the market cluster is unlikely to have initiated the pandemic since the earliest documented COVID-19 cases had no connection to the market.
Molecular clock evolutionary analysis was used to try to home in when the first, or index, case of SARS-CoV-2 occurred. The molecular clock is a term for a technique that uses the mutation rate of genes to deduce when two or more life forms diverged, and in this study, it is when the common ancestor of all variants of SARS-CoV-2 existed. Molecular dating of the most recent common ancestor is often taken to be synonymous with the index case of an emerging disease.
The researchers also used various analytical tools like epidemic simulations based on the virus's known biology, its transmissibility and other factors to model how the SARS-CoV-2 virus may have behaved during the initial outbreak and early days of the pandemic when it was an unknown entity.
While in 29.7 per cent of these simulations, the virus created self-sustaining epidemics, in the remaining 70.3 per cent, the virus-infected comparatively few people before dying out. The average failed epidemic ended merely eight days after the index case.
"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.