(Karachi) A coronavirus strain that infected more than 300 people in Beijing since start of June could have originated in Southeast Asia, a study by Harvard University researchers transpired.

The Covid-19 outbreak in Beijing has raised concerns about China's vulnerability to a "second wave" of infections. The virus found in Beijing cases is an imported strain of COVID-19, according to the China Centre for Disease Control and Prevention.

The Harvard study, published on the preprint website medRxiv.org took three of the SARS-CoV-2 genome sequences collected in Beijing last month and compared them to 7,643 samples worldwide. The three genomes showed the greatest resemblance to cases in Europe from February to May, and to cases in South and Southeast Asia from May to June.

They were also similar to a small number of infections seen in China in March, suggesting the strain could have appeared first in China and then returned to the country three months later, the study suggested.

The outbreak traced to Beijing's huge Xinfadi wholesale market on June 11 had infected 329 people by the end of Wednesday.

Quarantine restrictions and large-scale testing of residents began soon after the first cases were identified, and China also required all shipments of imported meat to be tested for COVID-19 before they could leave its ports.