“We need to take this case in the most serious manner,” Lam told a regular briefing, adding that 21 days of compulsory quarantine had been ordered for all of the patient’s about 100 close contacts.

“With this new variant case, we cannot lower our guard. We need to get vaccinated to possibly stop the next wave from happening.”

The girl’s mother was the sole local case among fewer than 10 people who tested preliminary-positive on Saturday.

The variant was different from a strain previously circulating locally since the fourth wave, and from the one first found in South Africa that was later detected in a Dubai arrival and some of his contacts in Hong Kong last month.

With a cluster of cases involving the B.1.1.7 variant recently found in Shenzhen’s Yantian district, questions have been raised on whether the source of the 17-year-old’s infection originated across the border.

The CHP’s Dr Chuang Shuk-kwan said the Hong Kong health authorities would seek details of the Shenzhen cases and see if they shared similar DNA to the virus found in the teenager.

Lam would not be drawn when asked if the city should tighten the “Return2HK” scheme, which allows Hongkongers in mainland China to come back without compulsory quarantine. He said only that there was a mechanism in place for the authorities in Guangdong province, Macau and Hong Kong to share details on outbreaks.

Asked whether the government would suspend flights from Britain again, Lam said Hong Kong already had some of the most stringent measures in place to stop the virus from entering, including making everyone get tested before flying into the city.

The authorities would first need to confirm the transmission chain for the girl before looking into the possibility of tightening epidemic measures, he added.

Hours before the briefing, Dr Gilman Siu Kit-hang of the PolyU team that studied the case warned about a “loophole” that allowed the coronavirus to enter the city.

“This is a moment of concern, because it shows a new variant somehow creeping through the border and reaching our community. Somehow there is a loophole somewhere we don’t know of yet,” said Siu, an associate professor in PolyU’s health, technology and informatics department.

Hong Kong has detected dozens of B.1.1.7 variants in arrivals, but these were all imports found during testing at the airport or in quarantine, and the patients were quickly isolated.

Siu said genome sequencing by his team showed the variant carried by the 17-year-old contained at least 10 mutations from previously imported B.1.1.7 strains, meaning her infection could be from a different source.

Covid-19: low vaccination rates spell trouble for Hong Kong restaurants, bars

He added it was not clear yet whether the mutations would make the variant even more transmissible, but the important thing was the existence of a possible border loophole.

“It could be at the quarantine hotel, which could be ground for spreading the virus again,” he said. “The 10 mutations could also suggest the virus has been spreading in the community for some time.”

Asked how the variant might have entered the city, Siu pointed to the cluster of B.1.1.7 infections reported on May 21 in Yantian.

“Mainland authorities said they found and isolated all cases there, but there could still be infections at large,” Siu said.

He also suggested that Tin Shui Wai, where the girl lives, was geographically near Yantian, adding more epidemiological research was needed to ascertain the exact transmission path.

An overnight lockdown for testing of residents in Tin Shing Court’s Shing Yu House in Tin Shui Wai found no infections among the 850 screened. Residents will need to take four further tests in the next 19 days.

Authorities also ordered mandatory testing for 4,000 attendees of the International Education Expo, held at the Convention and Exhibition Centre in Wan Chai, where the schoolgirl visited. All pupils and staff of Queen Elizabeth School Old Students’ Association Tong Kwok Wah Secondary School, where she studies, must also undergo screening.

The girl had also recently visited the Little Boss pet shop in Mong Kok and the nearby Men Wah Bing Teng restaurant. Her mother had been to the food court in the Fortune Metropolis shopping centre in Hung Hom, Tai Po Mega Mall, Tin Shui Wai public library, and Tin Shing market in Tin Shui Wai, among other places.

The authorities have ordered mandatory testing for everyone who visited these places at around the same time as the girl or her mother.

The city’s overall infection tally now stands at 11,851, with 210 related deaths.