A group of leading scientists has announced that the origin of COVID-19 is still unclear and the theory that it resulted from a laboratory leak needs to be taken seriously until there is a rigorous data-led probe that proves it wrong.

The coronavirus, which emerged in China in late 2019, has claimed 3.34 million lives, inflicted upon the world costs worth trillions of dollars in lost income and upended normal life for billions of people.

“More investigation is still needed to determine the origin of the pandemic,” said the 18 scientists, including Jesse Bloom, who studies the evolution of viruses at the Fred Hutchinson Cancer Research Center, and Ravindra Gupta, a clinical microbiologist at the University of Cambridge.

“Theories of accidental release from a lab and zoonotic spillover both remain viable,” the scientists including David Relman, professor of microbiology at Stanford, said in a letter to the journal Science.

The authors of the letter said the WHO’s probe into the origins of the coronavirus had not made a “balanced consideration” of the theory that it may have come from a laboratory incident.

In its final report, drafted jointly with Chinese scientists, a WHO-led team that spent 4 weeks in and around Wuhan in January and February stated that the coronavirus had probably been transmitted from bats to humans through another animal, and that a lab leak was “extremely unlikely” as a cause.

But the scientists said: “we must take hypotheses about both natural and laboratory spillovers seriously until we have sufficient data,” pointing out that an intellectually rigorous and dispassionate probe needed to be carried out.