A strain of COVID-19 that has infected more than 300 people in Beijing since early June may come from South or Southeast Asia, according to a study by Harvard University researchers.
The Beijing outbreak raised concerns about China's vulnerability to a "second wave" of infections. The virus found in cases in Beijing is an imported COVID-19 strain, according to the China Center for Disease Control and Prevention.
The Harvard study, which was published on the preprint website medRxiv.org on Tuesday and is yet to be peer-reviewed, removed three of the SARS-CoV-2 genome sequences collected in Beijing last month and included 7,643 samples worldwide compared.
The three genomes were most similar to cases in Europe from February to May and to cases in South and Southeast Asia from May to June.
They also resembled a small number of infections that were observed in China in March, suggesting that the strain may have first appeared in China and then returned to the country three months later, the authors said.
"Since the most recent cases in these branches come almost exclusively from South (East) Asia, this could indicate that the new cases in Beijing have been reintroduced through broadcasts from South (East) Asia," they wrote.
The outbreak, which was due to Beijing's huge Xinfadi wholesale market on June 11, infected 329 people by the end of Wednesday.
Quarantine restrictions and large-scale testing by residents began shortly after the first cases were identified, and China also requested that all shipments of imported meat be tested for COVID-19 before they could leave their ports.
The SARS-CoV-2 virus was thought to have originated in a market in the central Chinese city of Wuhan in December last year. It has now infected more than 10 million people and killed more than 500,000 people worldwide.
However, some studies suggest that it may circulate much sooner after crossing the species barrier of horseshoe bats that are native not only to southwest China but also to Laos and Myanmar.
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