China releases genome of virus that caused Beijing outbreak, saying it could be linked to Europe

China has released the genetic sequence of the coronavirus believed to have caused Beijing's new outbreak, saying it could be linked to strains that originated in Europe.

Data released by government researchers suggest the virus is an older version than the one currently infecting Europe, giving credence to the idea that it had been lurking for a while – possibly as early as May – before erupting in a wholesale food market in Beijing last week.

Many traces of the virus were found at Xinfadi market, which "indicates it has been around for some time", said Zhang Yong, an assistant director at China's Centre for Disease Control and Prevention, adding: "If it had only been present in the city for a short time, there may not have been so many positive samples found."

Outside experts, however, have pushed back on China's claims.

Genetic virus data that allows for comparing the number of mutations is not enough "to confidently assign a geographic origin to the lineage. They could have originated from essentially anywhere", Francois Balloux, the director of the UCL Genetics Institute, wrote online.

At best, he said, "proper analysis may allow for some 'educated guesses' for a plausible geographic source".

China is experiencing the worst resurgence of coronavirus since February, with more cases being discovered in Beijing daily. The capital reported 25 new cases on Friday, increasing the total to 183 over the last week, ending a nearly two-month streak of zero cases.

The flare-up underscores the challenges facing governments to balance between lifting lockdowns and curbing coronavirus, even in a country that has instituted strict containment measures including installing cameras and sensors at people's doors to monitor their movements.

Authorities are still tracing the origin of the outbreak, which came as a surprise. For weeks before, China had clocked almost no local transmissions, with all new illnesses coming via citizens returning from abroad.

Chinese officials originally indicated that foreign fish was to blame after finding traces of coronavirus on a cutting board used by a vendor of imported salmon.

It was a convenient line politically, as Beijing has sought to defend itself against global calls for an investigation into its cover-up of the pandemic in the early stage by claiming "foreign forces" such as the US military had brought the virus into China.

While the government later stepped back on the claim that imported salmon was to blame for the latest outbreak, the damage was already done. Cuts of salmon had been pulled off grocery shelves, and many Chinese, concerned about consuming the fish, stopped visiting restaurants serving salmon dishes.

Authorities found that the seafood and meat sections of Xinfadi market were seriously contaminated with the virus, and have yet to rule out the possibility that it remained on food packaging.

"The virus may have remained on imported frozen foods, not mutating due to the cold environment throughout the whole storage and transportation process from overseas to China," said Mr Zhang.

Low temperatures and high humidity could have contributed to the spread at the wholesale market, heavily used by buyers, vendors and suppliers.

Beijing remains on partial lockdown, with residents from areas with high numbers of infections banned from leaving the city. A number of outbound travel links have been cancelled, while entire housing compounds have been sealed as authorities raced to contain the outbreak.

Local officials have also engaged in house calls, visiting people to check their travel and contact history in aggressive efforts to round up all those who may have been exposed for mass testing and quarantine.