A new collaborative study suggests SARS-CoV-2 may have been circulating in China as early as October 2019 – way before the first human cases with COVID-19 had been reported in Wuhan.
"A lot has been learned over the last year about this pandemic, but one of the most important questions of all has remained unanswered: When exactly did the outbreak begin?" said co-corresponding author Michael Worobey, professor and head of the University of Arizona Department of Ecology and Evolutionary Biology in a statement.
"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," stated senior author Joel O. Wertheim, associate professor in the Division of Infectious Diseases and Global Public Health at UCSD. "By combining these disparate lines of evidence, we were able to put an upper limit of mid-October 2019 for when SARS-CoV-2 started circulating in Hubei province."
Reporting their findings in the journal Science, the researchers described how their epidemiological simulations also found that mutations in the SARS-CoV-2 virus normally resulted in the virus dying out three-quarters of the time, without igniting a pandemic to the scale we have seen in the last year.
The researchers used an array of molecular and analytical tools to assess how the virus may have behaved during the early stages of the outbreak. These included epidemic simulations that tracked known attributes of the virus behavior such as transmissibility factors, as well as taking into account some of the virus's genetic properties.
“Typically, scientists use the viral genetic diversity to get the timing of when a virus started to spread,” said Wertheim. “Our study added a crucial layer on top of this approach by modeling how long the virus could have circulated before giving rise to the observed genetic diversity."
“Our approach yielded some surprising results. 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 100 times, two out of three times, COVID-19 would have fizzled out on its own without igniting a pandemic. This finding supports the notion that humans are constantly being bombarded with zoonotic pathogens.”
The findings also indicated that low levels of SARS-CoV-2 were circulating in China during the fall of 2019, and hence was not widely dispersed during this period and the numbers of infectious remained low up until December 2019.
Therefore, the authors remain skeptical of claims that suggest the virus was spreading outside of China during this period: “Given that, it’s hard to reconcile these low levels of virus in China with claims of infections in Europe and the U.S. at the same time,” Wertheim said. “I am quite skeptical of claims of COVID-19 outside China at that time.”
Nevertheless, the full picture of the virus and its origin currently remains unclear – however, research such as this is providing useful insights into how the virus might have been behaving during the early stages of the outbreak and may help pin down where exactly it originated from to help inform and prepare for future outbreak surveillance.
“Pandemic surveillance wasn’t prepared for a virus like SARS-CoV-2,” Wertheim states. “We were looking for the next SARS or MERS, something that killed people at a high rate, but in hindsight, we see how a highly transmissible virus with a modest mortality rate can also lay the world low.”