Six markets have been shut down in Beijing and 11 residential districts forced into lockdown amid fears of a second wave after more than a month in the clear.
More than 50 people have tested positive in the Chinese capital with the source of the outbreak believed to be the Xinfadi wholesale food market.
Beijing officials said 45 of the infected people worked in the market, though they showed no symptoms.
That was in addition to seven earlier cases of people with symptoms, including six who had visited or worked at the market.
Overall, China has reported its biggest one-day jump in coronavirus cases in two months.
The nationwide total of 57 new confirmed coronavirus infections represents the highest daily number since April 13 and has heightened fears that a dreaded ‘second wave’ of is underway.
Beijing authorities, who had not seen a locally transmitted infection in more than 50 days, immediately reversed some recent moves to relax coronavirus restrictions.
The Beijing News newspaper said Communist Party members and volunteers were being organised to shop for food and other daily necessities for affected residents in the quarantine zone.
Beijing authorities had earlier halted beef and mutton trading at the market while major supermarkets removed salmon from their shelves after the virus was discovered on chopping boards, the state-owned Beijing Youth Daily reported.
Nearly 2000 samples have been taken from meat and surfaces such as rubbish bins at the markets.
Beijing authorities said more than 10,000 people at the market will take nucleic acid tests to detect coronavirus infections.
The city government also said it had dropped plans to reopen schools on Monday for students in grades one through three because of the new cases.
China was the original epicentre of COVID-19 when it was first detected in people connected to a wet market in the city of Wuhan.
The country which recorded more than 84,000 infections and 4638 deaths, had managed to contain the spread while other nations grappled with devastating infection rates and deaths.
Lab experiments have revealed a genetic change in the SARS coronavirus 2 that has made it much more infectious to humans.
The mutated strain now circulating throughout Europe and the US has up to five times more “functional spikes” on its surface, meaning it was better at binding.
The study at Florida’s Scripps Research may explain the fast spread of the pandemic in places like New York and Italy but more work is needed.
The Scripps Research study claims the early variant of the virus which emerged in China did not contain the genetic variant D614G which is “now dominating in much of the world”.
The study has raised questions about the severity of outbreaks outside China and whether that may be connected to the mutation.
“Viruses with this mutation were much more infectious than those without the mutation in the cell culture system we used,” said virologist Professor Hyeryun Choe, PhD, senior author of the study, in a media statement.
“The number—or density—of functional spikes on the virus is four or five times greater due to this mutation,” Professor Choe said.
“Our data are very clear, the virus becomes much more stable with the mutation.”
While ICU data from New York and elsewhere reported a preponderance of the new D614G variant, much more data and studies were needed, Professor Choe said.
It is still unknown whether this small mutation affects the severity of symptoms of infected people, or increases mortality, the scientists say.
Australia coronavirus update
A Victorian doctor who worked at three medical clinics while potentially infectious with coronavirus is in isolation.
The GP is asymptomatic and caught the virus from a close contact, who also showed no symptoms..
He was one of eight of new cases of COVID-19 in Victoria on Saturday.
The doctor worked at Lilydale Medical Clinic on June 11 and Cedars Medical Clinic in Coburg and Croydon Family Practice on June 9.
The department is contacting all potentially affected patients and the clinics are being cleaned.
Six other virus cases were detected in returned travellers in hotel quarantine, and one is a household contact linked to the outbreak at the Rydges on Swanston hotel.
Victorians can now meet up to 20 people in their home or in a public space.
Restaurants, cafes and pubs can have up to 20 customers dine in, and businesses such as beauty therapists can see clients.
Meanwhile another Sydney school employee has been confirmed to have COVID-19, with all students considered close contacts and directed to self-isolate.
Laguna Street Public School in southern Sydney will stop on-site learning until June 24 following the positive test result.
It comes after a staff member at Rose Bay Public School in Sydney’s eastern suburbs was confirmed to have tested positive for the coronavirus on Friday.
Happy birthday Queen
It was a quiet celebration for the Queen who marked her official birthday by viewing a socially distanced military ceremony at Windsor Castle, after coronavirus forced the cancellation of Trooping the Colour.
The gun salutes in London that normally celebrate the occasion were also cancelled.
It was the Queen’s first official public appearance since the country went into lockdown in late March.
The 94-year-old monarch, whose actual birthday is April 21, watched a series of drills by the 1st Battalion Welsh Guards in the castle’s quadrangle, instead of the Trooping the Colour event through central London that traditionally marks the occasion.
The Queen, who has been living with 99-year-old husband Prince Philip at the castle west of London, has issued a number of rallying messages to the nation in the past three months, including televised addresses that have been a rarity during her 68-year reign.
But she had not been seen in public until Saturday, when dressed in a jade coat and wearing a diamond Welsh Guards brooch, she observed soldiers adhering to the two-metre social distancing rule, and listened to music performed by a Band of the Household Division.