“It was a perfect storm — we know now that it had to catch a lucky break or two to actually firmly become established,” Michael Worobey, a professor of evolutionary biology at the University of Arizona who worked on the study, told CNN.
“If things had been just a tiny bit different, if that first person who brought that into the Huanan market had decided to not go that day, or even was too ill to go and just stayed at home, that or other early super-spreading events might not have occurred. We may never have even known about it.”
The team employed molecular dating, using the rate of ongoing mutations to calculate how long the virus has been around. They also ran computer models to show when and how it could have spread, and how it did spread.
“Our study was designed to answer the question of how long could SARS-CoV-2 have circulated in China before it was discovered,” said Joel Wertheim, associate professor in the Division of Infectious Diseases and Global Public Health at the University of California, 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. 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.”
The evidence strongly indicates the virus could not have been circulating before that, the researchers said. There have been reports from Italy and other European countries of evidence the virus may have infected people there before October. But Thursday’s study indicates only about a dozen people were infected between October and December, Worobey said.
“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 in a statement. “I am quite skeptical of claims of COVID-19 outside China at that time.”
The study indicates the virus did emerge in China’s Hubei province and not elsewhere, the researchers said.
“Our results also refute claims of large numbers of patients requiring hospitalization due to COVID-19 in Hubei province prior to December 2019,” they wrote.
The study doesn’t show which animal was the source of the virus. Genetic evidence shows bats carry a closely related virus, and also suggests another, intermediate species of animal was likely infected and transmitted the virus to a human being somewhere.
What’s needed is an infected person and a lot of contact with other people — such as in a densely packed seafood market. “If the virus isn’t lucky enough to find those circumstances, even a well-adapted virus can blip out of existence,” Worobey said.
“It gives you some perspective — these events are probably happening much more frequently than we realize. They just don’t quite make it and we never hear about them,” Worobey said.
And that could have happened with Covid-19.
In the models the team ran, the virus only takes off about 30% of the time. The rest of the time, the models show it should have gone extinct after infecting a handful of people.
“What may have happened here was that the virus was sputtering along in a very low number of people in October, November, into December and then it got into this Huanan seafood market,” Worobey said.
It’s likely the market was not where the virus first infected people, but just the place where it got amplified.
Given how little time the virus was around, it’s remarkable that it was identified so quickly, Worobey said.
“It was pretty clearly some time in December before there were was a sizable enough group of people infected that there was a chance of discovering a new virus,” he said. By January of 2020, it had been sequenced and characterized.
Nonetheless, it was too late — perhaps because Covid-19 isn’t quite deadly enough. The first SARS virus killed close to 10% of its victims in 2002 to 2004 before it was stopped via a concerted global effort.
“As a scientific community, we were certainly aware of the pandemic potential of a highly transmissible, moderately virulent pathogen. But our system of reporting illness is contingent on detecting spikes in hospitalizations and deaths. Clearly that wasn’t enough to stop Covid-19,” Wertheim told CNN.