(CAIXIN GLOBAL) - Southern China's Guangzhou has imposed travel restrictions and stepped up mass coronavirus testing, as the giant metropolis battles a dangerous variant of Covid-19 in one of the most alarming flare-ups in the country since last summer.

The industrial and trade hub - the capital of China's most populous province, Guangdong, and the home to almost 19 million people - reported 64 new infections in two weeks since May 21, plus 13 asymptomatic cases.

The cluster, which has been traced to a woman from the city's Liwan district, marks the first community outbreak in China of a highly transmissible strain first detected in India.

As the virus continues spreading in many parts of the world, Guangzhou's efforts to contain the outbreak will serve as an important test for China for addressing the mutating virus, experts said.

Caixin has learnt that the State Council, China's Cabinet, dispatched a special expert team to Guangzhou to study the outbreak and virus control measures.

The Guangzhou outbreak is also a real-time trial for Covid-19 vaccines.

Some of the patients in Guangzhou were infected after receiving the first shot of vaccines, sparking concerns over the vaccines' effectiveness against the new variant.

"We have to admit that even with the inoculation, there is still chance to be infected with Covid-19," said Dr Cai Weiping, chief infectious diseases expert at Guangzhou No. 8 People's Hospital, which treats Covid-19 patients.

"But based on current cases, the vaccines worked well to reduce the severity of disease," Dr Cai said, adding that even with one shot, the vaccines still offered some protection against the virus.

As at May 31, Guangzhou has administered more than 13 million Covid-19 shots. About 3.3 million people in the city have completed both shots, official data showed.

While stepping up virus testing, Guangzhou also imposed a quasi-lockdown to restrict people from leaving the city.

Starting from May 31, everyone leaving the city has to take a virus test before departure.

Meanwhile, residents of 37 residential compounds where infections have appeared were told to stay home, affecting 140,000 people.

These measures are unprecedented for a trade hub like Guangzhou. Even during the worst days of the domestic outbreak last year, Guangzhou remained one of the few Chinese cities open to the world.

The city's airport received 43.8 million travellers last year, making it the busiest airport in the world amid the pandemic.

The World Health Organisation has named the variant involved in the Guangzhou outbreak Delta, listing it as a variant of concern because of its high transmissibility.

Since it was first detected in October last year, the variant has spread to more than 62 countries and regions.

"The variant has a shorter incubation period, faster transmission speed, and higher viral load, making the situation in Guangzhou completely different from the past," said Mr Zhang Zhoubin, deputy head of the Guangzhou Centre for Disease Control (CDC), at a news briefing last week.

China, the first epicentre of the Covid-19 pandemic, has quelled the virus and largely returned to normal since last summer, although sporadic flare-ups have been reported.

The new variant is a stronger enemy. Contact tracing showed that some of the earliest cases in the Guangzhou outbreak had breakfast in the same restaurant. Some of the patients did not have direct contact with each other and sat apart in the room, an indication of the variant's highly contagious nature.

It shows that the virus can be transmitted over longer distances, Dr Cai said, adding that more patients in the current outbreak have presented as asymptomatic.

"Many of them didn't suffer fever, cough or other symptoms, even as their lungs showed infections in CT scans," he said.

That makes nucleic acid testing the most important means for detecting the virus.

On May 25, the Liwan district, the centre of the latest outbreak, kicked off mass testing of all 730,000 residents.

On May 30, the testing was expanded to more districts.

The next day, Guangzhou temporarily suspended all public vaccinations to concentrate medical resources on virus testing.

"Speeding up virus testing to cut off the transmission of the virus as soon as possible is the top task for Guangzhou to control the outbreak," said Dr Zhong Nanshan, the 84-year-old pulmonologist who led China's fight against severe acute respiratory syndrome in 2003 and who plays a prominent role in the country's response to the Covid-19 pandemic.

At a May 31 briefing, Guangzhou Deputy Mayor Li Ming said that they "have basically figured out the transmission routes", as most of the new cases were traced to the Liwan district.

Tracking 'patient zero'

But the origin of the outbreak remains unclear, as experts are still debating who was Patient Zero, or the originating case in the flare-up. Several government documents suggested that the cluster started with an imported infection.

Guangdong receives about 90 per cent of all inbound travellers arriving in China every day, as the authorities limit the arrival of international flights to selected airports.

More than 300 quarantine hotels across the province receive inbound travellers, who are required to serve a 14-day mandatory quarantine once they arrive in China.

About 30,000 people are currently in quarantine in the province, with nearly 20,000 staffers serving at the sites, according to official data.

A 75-year-old woman surnamed Guo living in Liwan is currently known to be the first confirmed patient in Guangzhou's latest outbreak. But how Madam Guo became infected puzzles health experts.

According to government information, Madam Guo felt a sore throat on May 18 and later developed a low fever. She went to a hospital on May 20 and was confirmed the next day to have Covid-19. Her husband later tested positive for the virus as well.

On May 26, a cluster of four family members was confirmed, including a woman surnamed Song who had breakfast May 19 at the same restaurant as Madam Guo. In the following days, more cases were reported among contacts of the Song family.

New infections surrounding Madam Guo and Ms Song soon spread beyond the Liwan district.

As at June 1, six cases have been reported in neighboring city Foshan, all of which were linked to cases in Guangzhou.

Infections were also found in Maoming in Guangdong province and Nanning in Guangxi.

"The patients have formed a roughly completed infection chain and had close epidemiological links with previous cases," Mr Zhang of the Guangzhou CDC said. "There has not yet been community infections with entirely unknown sources."

Although all of the cases following Madam Guo's infection have been traceable, it is still unclear how she contracted the virus. The woman had not travelled outside her community before she was infected.

Liwan's disease control authority said on May 23 that it was likely Madam Guo was accidently exposed to an inbound traveller with the virus.

The Guangzhou authorities first targeted a 34-year-old man surnamed Song, who returned to China from Rwanda in late April.

Mr Song completed the mandatory 14-day quarantine upon arrival at a hotel in Liwan and returned to Nanjing, where he was later confirmed as an asymptomatic patient, according to the Nanjing authorities.

But Guangzhou disease control officials later ruled out the possibility that Mr Song was the origin of the latest outbreak, saying an epidemiological investigation did not find enough evidence.

Experts also suspected that another inbound traveller, who arrived in Guangzhou from Saudi Arabia early last month, could be the origin of the latest flare-up.

The 47-year-old man showed symptoms during quarantine in Liwan and was sent to a local hospital that Madam Guo also visited the same day, Caixin has learnt. Samples from the hospital's air-conditioning tested positive for the coronavirus, said sources.

Guangdong's provincial health authority deputy head Zhou Zixia said at a summit that the virus could have found its way to spread when the patient was transferred from the quarantine hotel to the hospital for treatment.

But the health authorities have yet to reach any conclusions.

Mr Zhang, in a recent interview with state broadcaster CCTV, said that the search for Patient Zero in Guangzhou is still under way.

A tougher battle

Deputy Mayor Li said that all of the patients were infected with the variant that was first found in India.

Dr Cai said that, compared with previous cases, the new variant shows a faster speed of transmission and stronger viral load.

According to Mr Zhang, the new variant's R0, which measures the number of people a sick person can infect, ranged between five and six when the outbreak started, compared with previous variants' R0 of two to three.

The variant's R0 dropped to 4.19 on May 30 as disease control steps were implemented, indicating the effectiveness of the measures, Mr Zhang said.

Dr Cai said the contagious power of the variant is alarming, noting that the only contact between Madam Guo and Ms Song was their presence in the same restaurant for breakfast, although they were several metres apart.

As the virus spreads, its viral load does not show a significant decline, indicating that the virus is much stronger than it was last year, Dr Cai said.

Many of the patients in the latest outbreak showed no clear symptoms or did not feel anything even though their lungs were already infected, he added.

That makes it even more difficult to detect the virus among the public, because the previous practice of screening potential patients based on body temperature is no longer useful, Dr Cai said.

Since Guangzhou started mass nucleic acid testing May 26, all of the city's 11 districts have announced plans to test all residents within three days, meaning the city will test nearly six million people every day.

Guangzhou CDC official Yang Zhicong said that "if all the measures can be implemented effectively, the effects will show after 14 days" of the virus' incubation period.

• This story was originally published by Caixin Global.

Note: This story has been updated to clarify that the vaccines used in Guangzhou offer some protection against Covid-19 after the first dose.