Returning traveler tests positive for Omicron after completing two-week quarantine in China

China has detected its second case of the Omicron coronavirus variant in a traveler more than two weeks after he arrivedin the country from overseas, posing a fresh challenge to the government’s zero-Covid strategy.

The case, reported in the southern city of Guangzhou on Tuesday, came a day after health authorities in the northern port city of Tianjin said they detected mainland China’s first Omicron infection – also in a traveler who arrived from overseas.

The Tianjin case was identified as an asymptomatic carrier on arrival. The individual was already in quarantine while genome sequencing confirmed he had the Omicron variant – suggesting the variant had not been directly exposed to local residents.

The second case, a 67-year-old man, entered China on November 27 in Shanghai and underwent two weeks of centralized quarantine, during which he repeatedly tested negative. The man then flew from Shanghai to Guangzhou on AirChina flight CA1837. AirChina staff confirmed to CNN the flight was nearly full, with all economy seats taken and only six seats remaining in business class.

The man then entered into home quarantine. He was tested again on December 12 – a full 15 days after first arriving in China, with the result coming back positive in the early hours of December 13. Subsequent genome sequencing reviewed by the Chinese Center for Disease Control and Prevention confirmed it was the Omicron variant, according to authorities.

Unlike the Tianjin case, the man has been diagnosed as a confirmed case – meaning he has symptoms. He is now being treated in isolation in hospital, officials said. Following his diagnosis, 10,544 people connected to the man have been tested for the virus – so far all results have returned negative.

A picture shows a vial of the Pfizer/BioNTech Covid-19 vaccine during a vaccination drive at the al-Manahel School in the Druze village of Majdal Shams in the Israel-annexed Golan Heights, on December 12, 2021. (Photo by JALAA MAREY / AFP) (Photo by JALAA MAREY/AFP via Getty Images)

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Chinese authorities did not reveal when, where or how the man caught the virus. On average, it takes between 5 to 6 days from when someone is infected with the virus for symptoms to show, however it can take up to 14 days, according to the World Health Organization.

Much remains unknown about the fast-spreading Omicron variant, including its incubation period. The variant carries an unusually large number of mutations that scientists worry could potentially make it more transmissible and less susceptible to existing vaccines.

Last month, omicron was identified in two returning travelers in separate rooms on the same floor of a designated quarantine hotel in Hong Kong, leading scientists to believe it may have passed through the air in the hallway.

Jin Dongyan, a virologist at the University of Hong Kong, told CNN “there are several possibilities” as to how the the Guangzhou man contracted the virus, including that he became infected in China.

“There have been many instances of infections acquired at quarantine hotels,” Jin said. He added that the virus could have an incubation period of 14 days but said this was “unusual.”

“It’s also probable that the Covid test was not sensitive enough to detect the virus,” he said.

“It’s very important to find out whether the case in Shanghai was infected outside China or in a local quarantine hotel. Tracing the source of infection is more important,”Jin said.

Chinese public health experts and state media have previously voiced confidence in the country’s ability to tackle the new variant, citing China’s strict border control measures and its ability to swiftly identify and isolate infected cases and their close contacts.

In China, all overseas arrivals must be tested for Covid before they leave the airport, followed by at least two weeks of centralized quarantine. This is often followed by another lengthy period of home isolation. Throughout the process, international arrivals are prevented from coming into contact with the wider community until they have completed the required quarantine process.

But China’s ambitious zero-tolerance strategy – composed of mass testing, snap lockdowns and extensive quarantines – also comes with great economic costs, as well as no shortage of disruption to daily life.

Beijing is due to host the Winter Olympics in February. As the event approaches, authorities are resorting to ever more stringent measures to curb local outbreaks, which have continued to flare up with increasing frequency. The country has now reported locally transmitted cases everyday for the past eight weeks.

TOPSHOT - This photo taken on December 1, 2021 shows residents undergoing nucleic acid tests for the Covid-19 coronavirus in Hulun Buir, in China's Inner Mongolia Region. - China OUT (Photo by AFP) / China OUT (Photo by STR/AFP via Getty Images)

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On Tuesday, eastern Zhejiang province, home to the country’s key manufacturing and export hubs, reported 44 new cases, bringing the total caseload over the past week to above 200.

Authorities in virus-hit areas swiftly quarantined tens of thousands of residents, suspended businesses, events and tour groups, and canceled flights, ferries and bus services.

The travel restrictions in medium- and high-risk areas will be in place until March 15 next year – well after the Winter Olympics is over, and around the time the country’s annual legislative meetings are completed in Beijing.

Keeping infections away from the Chinese capital is a top priority for the government, as the city gears up for the Games. All flights from the city of Ningbo, a major industrial hub, to Beijing have been canceled, while only one daily flight from Hangzhou, the provincial capital, to Beijing is allowed.

In other parts of the country, infections are also being reported. In the north, Inner Mongolia reported 5 symptomatic cases and 4 asymptomatic infections, while the provinces of Heilongjiang and Shaanxi each reported one confirmed case, according to the National Health Commission.

In the southern province of Guangdong, two people in the manufacturing hub of Dongguan were tested positive Monday, prompting a local lockdown.

CNN’s Cheryl Ho contributed reporting.