The window is closing for key research into the origins of coronavirus, authors of the World Health Organisation (WHO) report into how Covid-19 emerged have warned.

They say that further delay makes this crucial inquiry biologically difficult.

The designated independent international members of the joint WHO–China team is calling on the scientific community and country leaders to accelerate the follow-up research needed to identify how the virus came about.

In a comment piece in the Nature journal, they write: “Our report was published this March.

“It was meant to be the first step in a process that has stalled.”

The authors add: “The window of opportunity for conducting this crucial inquiry is closing fast: any delay will render some of the studies biologically impossible.

“Understanding the origins of a devastating pandemic is a global priority, grounded in science.”

The team sets out six priorities for a second phase of studies.

These include the critical trace-back of people and animals in regions inside and outside China that have the earliest evidence for circulation of the virus, targeted surveys of possible reservoir or intermediate hosts, and following up credible new leads.

The authors say understanding the origins of the pandemic is a global priority.

They explain that Covid-19 antibodies wane, so collecting further samples and testing animals or animal handlers who might have been exposed before December 2019 may be less fruitful as time passes.

The article says: “The search for the origins of Sars-CoV-2 is at a critical juncture.

“There is willingness to move forward from both the WHO international team and the Chinese team.

“Crucially, the window is rapidly closing on the biological feasibility of conducting the critical trace-back of people and animals inside and outside China.

“Sars-CoV-2 antibodies wane, so collecting further samples and testing people who might have been exposed before December 2019 will yield diminishing returns.”

The authors conclude: “Therefore, we call on the scientific community and country leaders to join forces to expedite the phase two studies detailed here, while there is still time.”

Tasked by the WHO in October 2020 with understanding the origins of the virus, the team reported in March that the introduction of the virus from an intermediate host is the most likely scenario – which means the virus “jumped” from one species to another, and then moved from the second species to humans.

It also suggested the virus was circulating for several weeks before it was initially detected.

China has faced claims that the Wuhan Institute of Virology could be the source of the Covid-19 virus. But the team of experts from WHO and China said in February the virus is “extremely unlikely” to have entered the human population as a result of a laboratory-related incident.

However, at the time Dr Tedros Adhanom Ghebreyesus, director general of the WHO, said he did not believe that assessment had been extensive enough and that further data and studies would be needed to reach more robust conclusions.