The novel coronavirus was probably circulating for around two months before the first human cases of covid-19 were described in Wuhan, in the Hubei province of China, according to researchers in the US. In fact, SARS-CoV-2 could have emerged in mid-October 2019, almost two months before the Chinese authorities enacted the first measures to limit its spread.
As the months go by, scientists are able to shed more light on the origins of covid-19 and the resulting pandemic that has turned the world upside down. A team of researchers at the University of California San Diego School of Medicine -- in collaboration with researchers at the University of Arizona and Illumina Inc. -- have investigated how long SARS-CoV-2 may have been circulating in China before it was officially discovered. For that, they used molecular dating and epidemic simulation tools, and drew on three important pieces of information: a detailed understanding of how SARS-CoV-2 spread in Wuhan prior to lockdown, the genetic diversity of the virus in China, and reports of the earliest covid-19 cases in China.
Published in the journal Science , their research suggests that SARS-CoV-2 started circulating around mid-October 2019 in Hubei province. The virus would therefore have been circulating for around two months without being detected. While the first cluster of covid-19 cases were associated with the Huanan Seafood Wholesale Market, the study authors say that the market cluster is unlikely to have marked the beginning of the pandemic. They explain this by the fact that the earliest documented cases of covid-19 had no connection to the market, originally considered the epicenter of the epidemic.
According to the study, several regional newspaper articles suggest that the first covid-19 diagnoses in Hubei province date back to at least November 17 -- another observation that suggests the virus was already actively circulating by the time public health measures were enacted to limit its spread. However, the researchers point out that SARS-CoV-2 was likely only circulating at low levels until December 2019.
In fact, the researchers estimate that the median number of people infected with SARS-CoV-2 in China was less than one until November 4, 2019, rising to four individuals almost two weeks later, then to nine on December 1, 2019 -- a few days before the first known hospitalizations in Wuhan.
Not much risk of causing a pandemic?
Epidemic simulations based on the virus' known biology, such as its transmissibility and other factors, have also allowed the researchers to estimate that, in most cases, zoonotic viruses die out naturally before causing a pandemic -- in 70.3% of cases.
"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 one hundred 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," explains senior author Joel O. Wertheim, associate professor in the Division of Infectious Diseases and Global Public Health at UC San Diego School of Medicine.