The theory that the coronavirus outbreak was caused by a laboratory leak should be taken seriously until proven wrong, leading scientists said.
Covid-19, which emerged in Wuhan, China in late 2019, has killed 3.34 million people, cost the world trillions of dollars in lost income and upended normal life for billions of people.
Eighteen scientists from prestigious universities including Cambridge and Stanford, wrote in a letter to Science magazine that "more investigation is still needed to determine the origin of the pandemic".
"Theories of accidental release from a lab and zoonotic spillover both remain viable," they wrote.
“Knowing how Covid-19 emerged is critical for informing global strategies to mitigate the risk of future outbreaks.”
The authors of the letter said the World Health Organisation's investigation into the origins of the virus had not made a "balanced consideration" of the theory that it may have come from a laboratory incident.
In its final report, written jointly with Chinese scientists, the WHO-led team that spent four weeks in and around Wuhan between January and February said the virus had probably been transmitted from bats to human beings through another animal, and that a lab leak was "extremely unlikely" as a cause.
But there are myriad different ideas about the origin of the virus, including numerous conspiracy theories.
The scientists, who include researchers Jesse Bloom and Ravindra Gupta, said “we must take hypotheses about both natural and laboratory spillovers seriously until we have sufficient data”.
"A proper investigation should be transparent, objective, data-driven, inclusive of broad expertise, subject to independent oversight, and responsibly managed to minimise the impact of conflicts of interest," they wrote.
"Public health agencies and research laboratories alike need to open their records to the public. Investigators should document the veracity and provenance of data from which analyses are conducted and conclusions drawn, so that analyses are reproducible by independent experts."
The scientists called on the international community to rally behind the cause.
"In this time of unfortunate anti-Asian sentiment in some countries, we note that at the beginning of the pandemic, it was Chinese doctors, scientists, journalists and citizens who shared with the world crucial information about the spread of the virus – often at great personal cost,” they said.
"We should show the same determination in promoting a dispassionate science-based discourse on this difficult but important issue."