Centre for Health Protection controller Dr Ronald Lam Man-kin put the city on alert by warning of the higher transmissibility, severity of illnesses and viral load associated with the Delta variant, first found in India, and urged residents to get vaccinated for protection.

“We can see the threat from variants is increasing by the day, because their transmissibility is extremely high,” Lam told a regular press briefing. “The patient and his family have no record of vaccination, therefore their risk of catching and spreading the disease is higher.”

Health officials also confirmed six imported Covid-19 cases on Thursday, involving four overseas students returning from Britain, and two arrivals from Indonesia. They all contained the L452R mutation linked to several coronavirus variants including the Delta one.

The latest cases take the city’s Covid-19 infection count to 11,905, with 210 related deaths.

A day after the 27-year-old airport worker tested preliminary-positive for the coronavirus, the centre on Thursday confirmed the infection and tentatively listed it as a local case.

Hong Kong detects first suspected local case of Delta Covid-19 variant

His case involved the more contagious L452R mutant strain and was likely to be a Delta variant given many past test results of that strain found in Hong Kong eventually came to that conclusion, said the centre’s communicable disease branch head Dr Chuang Shuk-kwan.

She said it would take two or three days for full genome sequencing to reveal whether it was indeed a Delta variant, which contained that mutation.

Lam warned the dreaded variant appeared to be quickly replacing others and becoming the dominant one globally, adding it seemed to be able to infect many unvaccinated people, especially the youth.

According to Chuang, research in Britain showed the Delta variant was 60 per cent more transmissible than the Alpha one first detected in that country, and in Singapore, more Delta patients ended up in intensive care or with pneumonia than other Covid-19 sufferers.

She also highlighted that two jabs of the BioNTech vaccine showed in British studies 90 per cent protection against hospitalisation, and 80 per cent against symptomatic infections.

After reviewing the contact tracing and epidemiological information of the 27-year-old patient, Chuang said his infection was more likely to be imported, due to his regular contact with passengers, pilots and flight crews. He was also judged unlikely to have been infected by his family, as his father, mother and two sisters all tested negative for the virus.

“Because our community doesn’t have these [Delta] cases, we are worried this case is linked with importation,” the health official said.

Nevertheless, the authorities have cast a wide net in quarantining 180 people, comprising both the 27-year-old’s close contacts and also their close contacts, with the latter group being given a shorter, three-day isolation order, according to existing rules.

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Hong Kong to shorten mandatory quarantine to 7 days for fully vaccinated residents, travellers

Hong Kong to shorten mandatory quarantine to 7 days for fully vaccinated residents, travellers

Those included colleagues of the patient’s mother at a Tai Po restaurant and at the Sha Tin MTR station where she worked at night as a cleaner. Also in Tai Po, a secondary school where the patient’s sister taught and a kindergarten where his niece attended had to suspend classes for cleaning and testing.

Apart from his job at the airport for Hong Kong Aviation Ground Services, the patient also worked part-time in customer service at Uptown Plaza in Tai Po. Other recent activities included a visit to Tung Chung for a meal with family, and farewell photo-taking with about 50 to 80 colleagues at the airport.

On the coming changes to Covid-19 rules, some medical experts have warned that reducing the length of quarantine for eligible travellers returning from high- and medium-risk countries could possibly lead to more imported cases leaking into the community.

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But Lam said the plan was the result of joint deliberations between government experts and was broader than simply imposing seven days of confinement, pointing to the requirement for travellers to self-monitor for another week after their release from quarantine.

Returnees must also test positive for coronavirus antibodies and negative for the virus itself.

Another strategy of locating Covid-19 patients through sewage testing has so far failed to detect signs of a wider spread in the area.

Dr Samuel Chui Ho-kwong, deputy director of environmental protection, said the technology was now being used across Hong Kong covering as many as 5 million residents, and in the latest case in Tai Po, it first picked up a “very high viral load” of the coronavirus at Yat Nga Court.

Then on Wednesday, officials moved upstream to collect more samples in that drainage basin, and issued separate mandatory testing orders there for residents of eight residential sites.

Chui said, as of Thursday, only sewage samples from Wan Hang House in Wan Tau Tong Estate, where the 27-year-old airport worker lived, returned positive readings for Covid-19.

Sewage monitoring would continue over the next few days in the area around.

Residents and visitors from Yat Nga Court, King Nga Court, Tak Nga Court, Classical Gardens, Dynasty View, Grand Dynasty View, The Balmoral and Ha Wun Yiu Village have been required to undergo mandatory testing by Friday.

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Professor Gabriel Leung, dean of the University of Hong Kong’s medical school, called the discovery of the possible local infection with the Delta variant “one of the most worrying inflection points or watershed moments” for Hong Kong in the ongoing Covid-19 pandemic.

He warned each infected person with the Delta variant could spread the virus to between six and eight people on average compared with the strain first detected in Wuhan, which could be transmitted to two or three others.

“This is a very, very important reason why we should be hyper, hyper concerned and vigilant,” he said. “Once you let it go it will completely predominate ... so we must be very careful.”

He said his university’s regular monitoring of sewage in the area first picked up the coronavirus on Monday – before the patient’s infection was detected on Wednesday – and found a “very high” viral load of 58,000 viral copies in one sample.

Speaking on a radio programme earlier on Thursday, respiratory medicine specialist Dr Leung Chi-chiu said the case revealed a loophole in the city’s border defences.

“We have never detected this L452R virus strain in our community before, so all such cases must be linked to importation,” he said.