Via the South China Morning Post: Coronavirus outbreak may have started in September, say British scientists. Excerpt and then a comment:
The first outbreak of the coronavirus could have happened further south than the central Chinese city of Wuhan as early as September, according to a team of scientists led by the University of Cambridge.
Researchers investigating the virus’ origin analysed a large number of strains from around the world and calculated that the initial outbreak occurred in a window between September 13 and December 7.
“The virus may have mutated into its final ‘human-efficient’ form months ago, but stayed inside a bat or other animal or even human for several months without infecting other individuals,” University of Cambridge geneticist Peter Forster said on Thursday.
“Then, it started infecting and spreading among humans between September 13 and December 7, generating the network we present in [the journal] Proceedings of the National Academy of Sciences [PNAS].”
The team analysed the strains using a phylogenetic network – a mathematical algorithm that can map the global movement of organisms through the mutation of their genes.
They were still trying to pinpoint the location of patient zero, and were hoping for help from scientists in China, but some early signs were prompting them to look into areas to the south of Wuhan, where coronavirus infections were first reported in December.
“What we reconstruct in the network is the first significant spread among humans,” Forster said.
The Cambridge team recently made international headlines with a paper about the virus’ evolutionary history. Published in PNAS this month, it found that most of the strains sampled in the United States and Australia were genetically closer to a bat virus than the strains prevalent in patients from across East Asia, and the major European type of the virus was a descendant of the East Asian variant.
But that paper looked only at the first 160 strains collected after late December. The small sample size limited the researchers’ ability to determine when and where the first outbreak actually started.
In their new study, which has not been peer-reviewed, Forster and his colleagues from several institutes including the Institute of Forensic Genetics in Munster, Germany, expanded the database to include 1,001 high-quality full genome sequences released by scientists across the globe.
The more strains analysed, the more precisely they could trace the origin of the virus’ global spread. By counting the mutations, they could get closer to working out when the first person was infected by a strain that was closest to bat virus.
Sars-CoV-2, the virus that causes Covid-19, originated from bats. It has been found to share 96 per cent identical genes with a coronavirus isolated by Chinese scientists from bat droppings in the southwestern province of Yunnan in 2013.
But there were hundreds of mutations between Sars-CoV-2 and the one in Yunnan, and a coronavirus usually acquires one mutation per month. Some scientists have therefore suspected the virus may have been spreading quietly in host animals and humans for years to gradually evolve to a highly adaptive form that could infect humans.
The first outbreak could be a recent event involving the last few mutations that completed the leap from harmless strain to deadly pathogen, according to the Cambridge team.
I can't find the medRxiv paper (yet), but I'll keep looking.
