The much-anticipated report from the international mission to Wuhan to investigate Covid-19's origins is set to be published this week, following intense US and Chinese pressure over its contents.

The coronavirus pandemic has engulfed the planet, killing more than 2.6 million people and shredding the global economy since the first cases emerged in the Chinese city in December 2019.

In the 15 months since then, science has miraculously developed multiple vaccines to fight the disease -- but the mystery at the very heart of the pandemic remains unsolved.

It was only in January 2021 that a team of international experts assembled by the World Health Organization finally visited Wuhan to start a month-long investigation on the ground.

The WHO mission was aimed at finding clues as to how the virus originally jumped from animals into humans.

Now, another month on after leaving Wuhan, the team and its Chinese counterparts are set to issue their findings -- which should help to identify the most likely pathways, while relegating other less probable hypotheses.

Hunt for clues

While global leaders want immediate answers, uncovering the exact origin of an epidemic takes time -- and is sometimes never found.

Nonetheless, the mission members, drawn from a range of fields and disciplines, are upbeat.

"I'm convinced we're going to find out fairly soon. Within the next few years, we're going to have real significant data on where this came from and how it emerged," British zoologist Peter Daszak, one of the team members, said on Wednesday.

On February 9, the team held a lengthy news conference in Wuhan before departing, giving a taster of what might appear in the report.

Experts believe that SARS-CoV-2, the new coronavirus that causes Covid-19, originally came from bats, and jumped into humans via an intermediate animal.

However, samples from tens of thousands of wild, domestic and farm animals in the region revealed no trace of the virus.

The scientists are also uncertain as to where and when the outbreak started, though the Wuhan cases remain the earliest known.

Likeliest route

That said, the mission has produced a number of hypotheses.

"There was conduit (back) from Wuhan to the provinces in south China where the closest relative viruses to SARS-CoV-2 are found in bats," Daszak told an event hosted by Britain's Chatham House think-tank last week.

"It provides a link and a pathway by which a virus could convincingly spill over from wildlife into either people or animals farmed in the region and then shipped into a market. "That's a really important clue."

The team also did not rule out transmission through frozen meat -- imported frozen food packages being Beijing's favourite theory.

Dutch virologist Marion Koopmans, also a member of the team, said that while transmission of the virus could potentially happen through infected people touching frozen food products, "the origin most likely is not the outside of the package".

However, she and her colleagues said frozen wild meat from neighbouring provinces remained a "very valid option".

The idea of a lab leak from the Wuhan Institute of Virology -- a hypothesis promoted by former US president Donald Trump's administration -- is "the least likely on the list of our hypotheses", said Koopmans.

It was downplayed at the Wuhan news conference.

However, back in Geneva, in the face of clouds of suspicion that continued to hang over the mission, WHO boss Tedros Adhanom Ghebreyesus insisted all hypotheses remained on the table and promised transparency over the report.

Tug of war

Behind the scenes and in public, China and the United States have been engaged in a tug-of-war over the report.

The plan to publish a summary first, with the main report to follow, was ditched in late February, without any real explanation from the WHO.

The White House, which had had concerns about a summary report being issued first that would not contain the underlying data behind it, voiced its delight and claimed credit for the change of plan.

US and Chinese diplomats have waded in more than once while awaiting the report, with one side calling for greater transparency; the other insisting that the mission was only possible with scientific cooperation from Beijing.

Trump had accused the WHO of being China's puppet.

Though his successor Joe Biden has changed the tone towards the UN health agency since becoming US president in January, Washington has continued to voice serious concerns about the WHO investigation, and has pushed Beijing to provide more information.

The pressure is emanating not just from the United States.

Walter Stevens, the European Union's ambassador to the UN in Geneva, recently called for the report to be "completely transparent" and answer the questions "that we all have".

The mission insists it got access to all the sites and people it wanted to.

However, team leader Peter Ben Embarek asked for more data, in order to go deeper into the investigation.