Assessing the theory that covid-19 leaked from a Chinese lab

IT IS POSSIBLE that the chain of infections which spread SARS-CoV-2 around the world began, as most new diseases do, when an animal virus found its way unaided into humans, whether in field or farm, cave or market. It is also possible that the chain began in a Chinese government laboratory. These two possibilities have been recognised by many of those studying the covid-19 pandemic for a long time. But the fact that two things are both possible does not mean they are equally likely.

For most of 2020 scientists and the media tended to treat the likelihood of a leak from a lab as a very small one, with everyday contact—“zoonotic spillover”—overwhelmingly more probable. That has now changed. Tedros Adhanom Ghebreyesus, director-general of the World Health Organisation, said in March that assessment of the laboratory hypothesis had not yet been extensive enough. On May 26th President Joe Biden ordered America’s intelligence agencies, which have not as yet reached a conclusion either way on the subject, to go away and try harder.