The mission to Wuhan, where the first cases were spotted, failed to identify the source of the virus but poured cold water on the theory that it leaked from a virology laboratory in the Chinese city.

At a press conference in Geneva alongside Wuhan mission leader Peter Ben Embarek, WHO director-general Tedros Adhanom Ghebreyesus said the team had conducted a "very important scientific exercise in very difficult circumstances".

"Some questions have been raised as to whether some hypotheses have been discarded. Having spoken with some members of the team, I wish to confirm that all hypotheses remain open and require further analysis and studies," Tedros said.

"Some of that work may lie outside the remit and scope of this mission. We have always said that this mission would not find all the answers, but it has added important information that takes us closer to understanding the origins of the virus."

- Lab theory -

At a press conference in Wuhan on Tuesday concluding the team of experts' visit, Ben Embarek quashed the theory that a leak from a virology lab in Wuhan could have caused the pandemic -- a notion propagated by former US president Donald Trump.

"The laboratory incident hypothesis is extremely unlikely," Ben Embarek had said. It "is not in the hypotheses that we will suggest for future studies."

In Geneva on Friday, he said the team had been told by the Wuhan laboratories they visited and spoke to that none of them had been working on or had SARS-CoV-2 -- the virus that causes Covid-19 disease -- in their collections.

"It's of course always possible that the virus is and was present in samples that have not yet been processed, or among viruses that have not yet been characterised," he said.

"But knowingly, apparently from all the labs we've talked with, nobody has seen this virus before."

Ben Embarek added that the common practice for scientists discovering new viruses was to publish their findings immediately.

- Not a chase mission -

Experts believe the disease originated in bats and could have been transmitted to humans via another mammal.

The first Covid-19 cases were reported in Wuhan in December 2019. More than 2.3 million people worldwide have since been killed by the virus.

The international investigation in the city only began in January 2021.

Ben Embarek said Friday that had the team gone much earlier, "we wouldn't have had the same material to look at", that researchers had put together in the interim.

"It was not a mission to go and chase an animal in the market or chase a patient somewhere.

"That would have been something that could have been done perhaps back in December when the outbreak... was detected."

He said when disease outbreaks occur, the first reaction is to treat patients, not to try to figure out how it happened.

"But maybe that's something we should look at in the future on how better to respond to emerging disease outbreaks," he added.

- Complacency 'dangerous' -

Tedros said he hoped a summary report from the mission would be published next week, with the full final report to follow in the coming weeks.

Tedros meanwhile welcomed that the global number of reported Covid-19 cases had fallen for a fourth consecutive week, which he put down to countries implementing public health measures more stringently.

But, he warned, "complacency is as dangerous as the virus itself. Now is not the time for any country to relax measures, or for any individual to let down their guard".

He said new Covid-19 deaths were all the more tragic as vaccines begin to be rolled out, and said people who think it will be impossible to vaccinate the whole world are "dead wrong".