Coronavirus / No indication of COVID-19 in Wuhan before Dec 2019: WHO team in China

Wuhan: The WHO mission to China to uncover the origins of the coronavirus failed to identify the source of a pandemic which has swept across the world, but the team Tuesday ruled out the Wuhan lab-leak theory propagated by Donald Trump.

Experts believe the disease -- which has killed more than 2.3 million people worldwide -- originated in bats and could have been transmitted to humans via another mammal.

WHO foreign expert Peter Ben Embarek said identifying the animal pathway remains a "work in progress", adding the absence of bats in the Wuhan area dimmed the likelihood of direct transmission.

It was "most likely" to have come from an intermediary species, he said, before backing up China's position that there was no evidence of "large outbreaks in Wuhan" before December when the first official cases have been recorded.

Liang Wannian, head of the China side of the joint mission, said animal transmission remained the likely route, but "the reservoir hosts remain to be identified".

Ben Embarek also quashed the theory that a leak from a virology lab in Wuhan could have caused the pandemic.

"The laboratory incident hypothesis is extremely unlikely," he said, adding it "is not in the hypotheses that we will suggest for future studies".

The mission was a diplomatically knotty one -- presaged by fears of a whitewash -- with the US demanding a "robust" probe and China firing back with a warning not to "politicise" the investigation.

The WHO experts spent a month in China including two weeks in quarantine.

Liang said studies showed the virus could be "carried long-distance on cold chain products", appearing to nudge towards the possible importation of the virus -- a theory that has abounded in China in recent months.

Beijing is desperate to defang criticism of its handling of the chaotic early stages of the outbreak.

It has refocused attention at home -- and externally -- on its handling and recovery, and also floated the theory the virus emerged abroad and was brought into China possibly via frozen foods.

Reporters were largely kept at arms' length from the experts during their closely-monitored visit, but snippets of their findings crept out via Twitter and interviews.

But, already over a year after the virus emerged, some of it was of questionable relevance to their stated aim of finding the virus source -- including a visit to a propaganda exhibition celebrating China's recovery from the pandemic.

The group spent just an hour at the seafood market where many of the first reported clusters of infections emerged over a year ago.

They also appeared to spend several days inside their hotel, receiving visits from various Chinese officials without venturing out into the city.

Deeper research was carried out at the Wuhan virology institute where they spent nearly four hours and said they met with Chinese scientists there including Shi Zhengli, one of China's leading experts on bat coronaviruses and a deputy director of the Wuhan lab.

Scientists at the laboratory conduct research on some of the world's most dangerous diseases, including strains of bat coronaviruses similar to Covid-19.

Former US president Donald Trump frequently repeated a controversial theory that a lab leak may have been the source of the pandemic.