The World Health Organisation announced the preliminary findings from a fact-finding mission to Wuhan in China, where the coronavirus was first detected.
Peter Ben Embarek, a WHO virologist, said that their findings had uncovered information but did not radically change the picture of the outbreak from what was already known.
Here are six of the key findings of the WHO Wuhan investigation:
The four origins of Covid-19
The team looked at the four main hypotheses on where coronavirus came from.
These were: the direct transmission from an animal to a person; the transmission from one animal to another and then to a human; frozen food being a surface for the transmission to humans, or the virus escaping a laboratory.
“We took a systematic approach to look at all these and assessed the likelihood using a standardised set of parameters,” Mr Embarek said.
While they did not find a definitive explanation, the team said the findings suggested that the virus moved from one animal to humans through an intermediary. That process remains unclear.
“It will require more targeted research,” Mr Embarek said.
Where did Covid come from?
The disease is likely to have gone from a secondary animal to humans, investigators said.
Liang Wannian, an expert with China's Health Commission, said that bats and pangolins are potential candidates for transmission, but coronavirus samples found in those species were not identical with Sars-Cov-2 – the official name for the coronavirus.
But he said that samples from the bat caves in Wuhan, and from other animal sites, have so far failed to identify the presence of the virus that causes Covid-19.
Mr Embarek said it was not yet possible to pinpoint the intermediary animal.
Mr Liang said they had conducted PCR tests on 11,000 types of animals, including livestock and poultry in provinces across China, and found no positive cases.
They have collected 50,000 samples of the wild animals from 30 species and found no positive cases.
The pathway, Mr Embarek said, from the original animal to the Wuhan market, could have been very convoluted and could have crossed borders.
The lab breakout theory is ‘unlikely’
Since the early days of the pandemic last year, there was speculation and reports that claimed that the coronavirus escaped from a laboratory in Wuhan.
Mr Embarek said last week that the WHO team had held “very frank” discussions with Chinese scientists about the source of the pandemic, including the theories it leaked from the Wuhan Institute of Virology.
Scientists at the laboratory conduct research on some of the world's most dangerous diseases, including strains of bat coronaviruses similar to the one that leads to Covid-19.
But, there is no evidence that the virus did escape the facility and the team concluded that it was “extremely unlikely” to be the origin of the pandemic.
Mr Embarek said their visit to the Wuhan Institute of Virology indicated it was very unlikely to have escaped the lab and there was no evidence that it was being studied at the site.
“There was no publication or research on this virus in any laboratory in the world. … We also looked at the Wuhan Institute of Virology and the state of that laboratory and it was very unlikely that anything could escape from such a place,” he said. “We also know that when lab accidents happen, they are extremely rare.”
Mr Liang said there are two scenarios to consider with a lab escape hypothesis. The first was that the virus was engineered by humans and got out. This was refuted by scientists all over the world who studied the virus's make-up.
A second was that it was being researched and got out.
There is no evidence, he said, that the virus that leads to Covid-19 was being studied anywhere prior to December 2019.
“Without the previous existence of the virus there can be no leak,” he said.
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So did it start at the ‘Wuhan market’?
Peter Daszak, a New York-based zoologist assisting the WHO-sponsored mission to focus on the animal side of the inquiry, said his trip to the fresh produce market in central Wuhan was useful.
The market sold mostly seafood, as well as meat that included freshly prepared wildlife.
It was a focus early in the outbreak, when cases occurred among workers and shoppers, suggesting it might have been where the virus jumped from animals to humans.
But subsequent research found earlier cases among people not linked to the market, undermining that theory.
“The seafood market may not be the first place to have an outbreak – it was not even necessarily the place with the first case. The case with the onset of December 8, [2019] has no relationship with the seafood market which saw its first case on December 12,” Mr Liang said.
The team admitted they do not know the exact role of the seafood market played in the pandemic.
Could frozen food have spread the virus?
Mr Embarek said the hypothesis that coronavirus is transmitted through trade of frozen cold chain products is possible.
They are still trying to map the transmission routes, “but frozen food is not ruled out as some of the frozen wild animals are known reservoirs of covid”.
“We also have to do much more for understanding the cold chain and frozen products. We know the virus can persist and survive in these cold and frozen environments, but we don't understand if the virus can then transmit to humans and we don't understand how it transmits,” Mr Embarek said.
Did it start in Wuhan at all?
Mr Liang said there had been no substantial spread of the virus in the city of Wuhan before the late 2019 outbreak. But, it is possible the coronavirus was circulating in other regions before Wuhan – although there is no specific evidence.
“The path to the [Wuhan] market could have taken a very long and convoluted route that includes movement and borders before it ended up in the market. It is important to follow up on clues,” Mr Embarek said.
