Experienced virologists heavily favor the natural-spillover theory. That’s because the phenomenon has been common throughout history, accounting for the spread of most viruses and indeed for most pandemics. The direct animal source hasn’t been identified; it’s not unusual for investigations of that nature to take years. SARS2 has been known to scientists for only about 18 months.
“We cannot prove that SARS-CoV-2 has a natural origin and we cannot prove that its emergence was not the result of a lab leak,” Kristian Andersen of the Scripps Research Institute in La Jolla, lead author of a seminal paper on the origin of the virus, told me by email.
“However, while both scenarios are possible, they are not equally likely,” Andersen wrote. “Precedence, data and other evidence strongly favor natural emergence as a highly likely scientific theory for the emergence of SARS-CoV-2, while the lab leak remains a speculative incomplete hypothesis with no credible evidence.”
Other virologists challenge the assumption by Wade and the assertion by Baltimore that there’s anything unique or especially unusual about the furin cleavage site on SARS2. Such sites have been found in similar viruses, and natural mechanisms for their appearance have been identified.