Scientists have reconstructed the early "evolutionary paths" of COVID-19 as the infection spread from Wuhan, out to Europe and North America, an advance that sheds light on the mutations undergone by the novel coronavirus since its origin.
According to the researchers, including those from the University of Cambridge in the UK, there are three distinct "variants" of COVID-19, consisting of clusters of closely related lineages, which they labelled 'A', 'B' and 'C'.
By analysing the first 160 complete virus genomes to be sequenced from human patients, the study, published in the journal PNAS, mapped some of the original spread of the new coronavirus, SARS-CoV-2, through its mutations into the lineages.
"There are too many rapid mutations to neatly trace a COVID-19 family tree. We used a mathematical network algorithm to visualise all the plausible trees simultaneously," said geneticist Peter Forster, lead author of the study from the University of Cambridge.
"These techniques are mostly known for mapping the movements of prehistoric human populations through DNA. We think this is the first time they have been used to trace the infection routes of a coronavirus like COVID-19," Forster said.
The scientists used data from virus genomes sampled from across the world between 24 December 2019 and 4 March 2020.
According to Forster and his team, the closest type of COVID-19 to the one discovered in bats—type 'A', the "original human virus genome"—was present in Wuhan, but surprisingly was not the city's predominant virus type.
They said mutated versions of 'A' were seen in Americans reported to have lived in Wuhan, and a large number of A-type viruses were found in patients from the US and Australia.
Wuhan's major virus type, 'B', was prevalent in patients from across East Asia but the variant didn't travel much beyond the region without further mutations.
Based on this, the scientists said there could have been a "founder event" in Wuhan, or "resistance" against this type of COVID-19 outside East Asia.
The 'C' variant, they said, is the major European type, found in early patients from France, Italy, Sweden and England.
It is absent from the study's Chinese mainland sample, but seen in Singapore, Hong Kong, and South Korea.
The study also suggested that one of the earliest introductions of the virus into Italy came via the first documented German infection on January 27, and that another early Italian infection route was related to a "Singapore cluster".