The origin of the novel coronavirus is still unclear and there is not yet enough evidence to say conclusively if it occurred naturally or was caused by a laboratory leak, a group of senior scientists said in a letter.
"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 journal Science.
The Covid-19 pandemic has killed 3.34 million people worldwide since it emerged in China in late 2019.
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