BEIJING (REUTERS, CHINA DAILY/ASIA NEWS NETWORK) - Scores of flights to and from Beijing were cancelled, schools shut and some neighbourhoods blocked off as officials ramped up efforts to contain a coronavirus outbreak that has fanned fears of wider contagion.

The resurgence of the Covid-19 disease in the Chinese capital since last Thursday (June 11) has upended daily life for many, with some fearing the entire city is headed for a lockdown as the number of new cases mounts.

Health officials reported 31 new confirmed infections for Tuesday, bringing the total tally to 137, the worst resurgence of the disease in Beijing since early February.

Here are some questions and answers about the latest outbreak:

Who is patient zero?

After 56 days, Beijing reported a new locally transmitted case on June 11 - a 52-year-old man surnamed Tang in Xicheng district. He visited a hospital on June 10 because of a recurring fever and he had not travelled outside of Beijing in the past two weeks.

Chief epidemiologist Wu Zunyou of the Chinese Centre for Disease Control and Prevention said Mr Tang started having symptoms on June 6.

On June 12, six more confirmed cases were reported. They included two in Fengtai district who were coworkers at a meat products research institute, and one surnamed Liu who said he had travelled to Qingdao in Shandong province in the past two weeks.

Epidemiological investigation found both Mr Liu and Mr Tang had been to Xinfadi, a major farm products wholesale market in Fengtai district.

The Qingdao health commission said disease control experts in Qingdao had ruled out the possibility that Mr Liu, who was on a business trip to Qingdao from May 29 to June 2, got infected there.

Is salmon the coronavirus carrier?

Chinese and Norwegian authorities have concluded that Norwegian salmon was likely not the source of the coronavirus found on cutting boards in Xinfadi market, the Norwegian fisheries and seafood minister said on Wednesday.

Following a meeting between officials from both sides the day before, the countries concluded that the source of the outbreak did not originate with fish from the Nordic country, Mr Odd Emil Ingebrigtsen said. "We can clear away uncertainty and the halt in salmon export to China," he told a video conference.

Since the discovery of the virus, many markets have stopped selling salmon products.

Dr Wu, the chief epidemiologist, said fish in their natural habitat cannot catch coronavirus. He said the new outbreak could have two possible explanations. The first is that some of the products may have been contaminated by workers during processing and transportation.

The second possibility is person-to-person transmission. "The infected person who brought the virus into the market might be asymptomatic or might have very mild symptoms, and the hustle and bustle of the market led to the cluster of new infections," he said.

"Our seafood products are typically stored and transported in cold containers. Thus it is possible for the virus to be preserved for a long time and this increase the likelihood of infection," he added.

Is the virus from Europe?

Senior researcher Zeng Guang from the Chinese Centre for Disease Control and Prevention said the coronavirus strain in the latest outbreak is different from the ones found earlier in China, and early data suggests it is a mutated variety from Europe. He stressed that more testing is needed to confirm the origin of the virus.

Dr Wu said whole genome sequencing will help researchers learn about the virus' mutations in different places and thus identify spread routes. For example, British Columbia, a Canadian province, announced recently that the virus spreading in the province was local, he said.

Although the virus in Beijing is a mutated variety of that in Europe, this does not prove it comes directly from European countries, according to Dr Wu.

Why did the outbreak happen in Xinfadi?

Covering a total area of 112ha, the Xinfadi market provides 80 per cent of Beijing's vegetables and fruits. Every day, about 10,000 people and 3,000 trucks arrive at the market.

In 2019, the market's trade volume reached 131.9 billion yuan (S$25.9 billion), and 17.49 million tonnes of produce were traded there. Among more than 4,600 wholesale markets of farm produce across China, Xinfadi has been the biggest for 17 years.

Dr Wu said that before the new outbreak, Beijing had no local cases for more than 50 days, which means there should be no virus locally. It needs to investigate all products and visitors to the market to better understand the situation.

Since the Wuhan outbreak, researchers have focused on animals and products to find the origin of the virus. Many experts believe the outbreak in Beijing will provide an opportunity to find the intermediate host of the virus.

Professor Wang Liming from the College of Life Science of Zhejiang University said few samples were collected from Huanan Market in Wuhan before it was closed and disinfected, but Xinfadi provided the opportunity.

How many people should be tested?

According to experts, anyone who visited the market or had close contact with personnel at the market should receive tests, said Mr Pang Xinghuo, deputy director of the Beijing Centre for Disease Control and Prevention.

More than 8,000 workers and tenants have gone through nucleic acid tests so far and been sent to centralised isolation points for medical observation, according to Beijing government official Xu Ying.

The city has surveyed about 200,000 people who visited the market from May 30 and asked them to stay home for observation, Mr Xu said.

How will farm produce supplies to Beijing be maintained?

The overall supply of farm produce was increased after the closure of Xinfadi market, said Mr Wang Hongcun, an official with the Beijing Municipal Commerce Bureau on Monday.

Six temporary trade markets have been set up, he said. On Monday morning, a total of 1,800 tonnes of vegetables were traded in those markets, nearly double the amount on Sunday.

In addition, six other large wholesale markets across the city moved fast to increase their vegetable supplies. Supermarket chains also responded quickly, Mr Wang said, citing Jingkelong which tripled its direct purchases from vegetable bases in Hebei and Shandong provinces.