LONDON, May 15 -- The origin of the novel coronavirus is still unclear and the theory that it was caused by a laboratory leak needs to be taken seriously until there is a rigorous investigation that proves it wrong, a group of scientists said.

Covid-19, which emerged in China in late 2019, has killed 3.34 million people so far.

"More investigation is still needed to determine the origin of the pandemic," said the 18 scientists, including Ravindra Gupta, a clinical microbiologist at the University of Cambridge, and Jesse Bloom, who studies the evolution of viruses at the Fred Hutchinson Cancer Research Center. "Theories of accidental release from a lab and zoonotic spillover both remain viable," the scientists said in a letter to the journ...