The man who had been considered to be the world’s first known COVID-19 case actually became ill later than initially reported, a key shakeup in the early timeline of the pandemic that could help determine the origin of the novel coronavirus, a new scientific analysis found.

In a peer-reviewed article in the journal Science, Michael Worobey, a University of Arizona professor who is an expert in tracing the evolution of viruses, instead points to a female seafood vendor at the Huanan Seafood Market in Wuhan, China, as the earliest known case.

Worobey’s analysis, which examined medical journal articles as well as video interviews in the Chinese news media, is sure to rekindle the debate about how the COVID-19 pandemic, which has killed more than 5 million people globally, started, but it does not definitively answer the question.

Worobey, however, concludes that there is “strong evidence of a live-animal market origin of the pandemic.”

That was a popular theory when COVID-19 first began to spread around the world. However, that hypothesis took a hit in March when a World Health Organization team of investigators identified a 41-year-old man who lived nearly 20 miles away from the market and had no link to the market as being the first known human case.

The man, an accountant, reportedly began to feel sick on Dec. 8, 2019, but, according to Worobey’s paper, the man said in an interview that his COVID-19 symptoms actually began with a fever on Dec. 16, after the virus was already spreading through the Huanan Market. 

Worobey says the Dec. 8 date was related instead to a dental emergency. Those accounts are backed up by hospital records and a scientific paper, Worobey wrote.

Peter Daszak, a member of the WHO team, told The New York Times that the investigators never asked the man the date his symptoms began, instead relying on the date provided by doctors at Hubei Xinhua Hospital, who handled early COVID-19 cases but did not treat the accountant.

“So the mistake lies there,” Daszak said.

Worobey says the first human case was “most likely one of the ~93% who never required hospitalization and indeed could have been any of hundreds of workers who had even brief contact with infected live mammals.”

But he argues that a “preponderance of early cases” connected to the Huanan Market makes it the most likely source of the virus in humans. More precisely, he points to early symptomatic cases connected to the market’s western section, where live racoon dogs, which were found to have SARS nearly 20 years ago, were caged.

Complicating matters, Worobey says, is that as a concerning number of mysterious pneumonia cases were being seen at hospitals, they were not being reported in a national reporting system until Jan. 3, resulting in valuable time being lost to trace back the virus.

“If Huanan Market was the source, why were only one- to two-thirds of early cases linked to the market?” Worobey writes. “Perhaps a better question is why would one expect all cases ascertained weeks into the outbreak to be confined to one market? Given the high transmissibility of SARS-CoV-2 and the high rate of asymptomatic spread, many symptomatic cases would inevitably soon lack a direct link to the location of the pandemic’s origin. And some cases counted as ‘unlinked’ may have been only one or two transmissions away.”

The prevailing alternate theory about COVID-19’s origins is that it leaked out of the Wuhan Institute of Virology, which conducts research on viruses. Worobey does not discuss the lab in his paper, but on Twitter he said the proponents of the leak theory have often used the case of the accountant to argue that, because the earliest known infection had no link to the market, the virus could not have emerged from there.

The WHO report in March concluded the virus most likely started in an animal, such as a bat, that transmitted it to a human. The investigators did not rule out the possibility of a lab leak but called it “extremely unlikely.”

In May, President Joe Biden said the U.S. intelligence community was split on how it believed the pandemic started — two agencies were leaning toward animal contact and another toward the lab explanation. Biden ordered intelligence officials to “redouble” their investigative efforts and issue a report within 90 days, which still failed to reach a firm conclusion.

Worobey wrote on Twitter than he also believes the Dec. 8 case was a key factor in Biden’s order to conduct the review.

In his paper, Worobey wrote conclusive evidence that the market is the origin of the pandemic could be “obtainable through analysis of spatial patterns of early cases and from additional genomic data, including (COVID-19) positive samples from Huanan Market, as well as through integration of additional epidemiologic data.”

He added that it was unfortunate that no live animals from the market were ever screened for COVID-19 and the market was closed and disinfected on Jan. 1, 2020, before its potential role came into focus.