Chan and her colleague Dr Shing Hei Zhan scrutinised the evolutions of SARS-CoV-1 (the cause of the 2003 SARS epidemic) and SARS-CoV-2 (the cause of the current pandemic) in the early months of their respective outbreaks and found that while the former mutated rapidly in early human cases, as the virus adapted to its new host, the latter did not. This implies that the virus causing Covid-19 was already well adapted to infecting human beings, a point that was also suggested by the World Health Organization in its global study on the origins of Covid-19, published in November.
There are three possible explanations for this. The first is that the virus had circulated, undetected, in people for months. The second is that the virus was already highly adept at human-to-human transmission, even while it was still in bats or other animals. The third is that it had become adapted in human cells, or humanised animals, in a laboratory.