BEIJING - Experts from the World Health Organisation and China have quashed the controversial theory that the coronavirus had leaked from a Wuhan laboratory, suggesting instead that the Covid-19 virus most likely infected humans from an intermediate animal host.

The possibility that Covid-19 could have resulted from a laboratory incident was one of four hypotheses that investigators probed during their just-concluded mission to Wuhan, where Covid-19 was first discovered in December 2019.

But it was "extremely unlikely", and further research into this theory was not required, said Dr Peter Ben Embarek, head of the WHO-led mission, at a press briefing wrapping up the team's 28-day visit yesterday.

Instead, further studies looking at whether the virus could have made the jump to humans from frozen animal products was needed, said Dr Embarek, in an indication of the direction of future investigations probing the origins of Covid-19.

Professor Liang Wannian, an infectious diseases expert from Tsinghua University and a member of the WHO-China joint team, said laboratories in Wuhan did not possess specimens of the Sars-CoV-2 virus prior to the outbreak, refuting conspiracy theories posited by some including former US President Donald Trump.

"If there is no existence of this virus, then there is no way it could be leaked," he said.

The WHO team had arrived in Wuhan in the middle of last month and had visited key locations in the city, including the Wuhan Institute of Virology and the Huanan seafood market, where many of the first cases were discovered.

Apart from the laboratory leak hypothesis, the other three scenarios that the WHO-led team explored include the virus jumping directly from an animal host; infecting humans via an intermediate animal species; and its transmission to the human population via frozen food products.

Referring to the Huanan market, Dr Embarek said the virus could have been introduced there "by a person who was infected and then spread to other people in the market - could be a trader or visitor - but it also could be the introduction of a product".

He added that among the "more interesting products" at the market were frozen wildlife animals, including species known to be susceptible to such viruses.

"We know that the virus can persist and survive in these cold and frozen environments, but we don't really understand if the virus can then transmit to humans and under which conditions this could happen," said Dr Embarek, adding that further studies were required.

He added that work so far suggested that bats are a "natural reservoir" of the coronavirus, but it is unlikely that the virus made the jump directly to humans in Wuhan, since the city is not located near these bat habitats.

The team had examined other intermediate animal species that could have introduced the virus to the city, but no exact animal reservoir had been found, they said.

The team's full findings will be published in a report to come, but their initial statements will vindicate Chinese officials, who have long bristled at the possibility that the virus could have come from a lab and countered that Covid-19 could have instead been brought into Wuhan via cold-chain logistics and frozen food products.

Prof Liang also said that the team had examined hospital data and patient and drug records, which do not indicate that there was widespread transmission of Covid-19 in Wuhan before the first cases were discovered in December 2019.

The team had also reviewed unpublished studies from different countries that suggested the virus was circulating in other countries weeks before it was discovered in Wuhan, said Prof Liang.

Team member Marion Koopmans, a virologist at Erasmus University Medical Centre in the Netherlands, said these studies did not provide "full evidence" that the virus could have been circulating elsewhere, but could give an indication for further investigations.

"So in the next step... we should really go and search for evidence for earlier circulation, wherever that is indicated," Professor Koopmans said.

Key findings

Investigators from the World Health Organisation and China explored four hypotheses involving the origin of Covid-19:

1. Direct zoonotic spillover of the coronavirus from an animal reservoir to humans.

2. Introduction of the virus through an intermediate animal host, where the virus could adapt or circulate before making the jump to humans.

3. Food-related routes of transmission, or cold chain and frozen food products acting as a surface for transmission of the virus to the human population.

4. A leak or escape from a laboratory.

Their key findings

- The WHO-led team said the possibility of Covid-19 resulting from a laboratory leak was "extremely unlikely".

- Initial spread of Covid-19 in Wuhan in December 2019, with a few sporadic cases at first then small outbreaks in clusters, is a "classic picture" of the start of an emerging outbreak.

- There is no evidence for widespread community transmission of Covid-19 before December 2019, when the first cases were discovered in Wuhan.

- The virus was most likely introduced to humans through an intermediate animal host, although investigators were not able to pinpoint an exact species.

What's next?

A full report outlining the team's findings and recommendations will be released. But the team gave indications for further lines of study:

- Studying how the virus can infect humans from frozen food products.

- Tracing the supply chain of frozen animal products present at the Huanan market, to pinpoint a possible animal host for the coronavirus.

- Expanding the search globally to look for evidence of circulation of Covid-19 before Dec 2019.