The announcement came as the Hong Kong government once again classified Britain, which has been hit by a surge of Covid-19 cases driven by the more transmissible Delta variant, as a “very high-risk” country for the virus, moving it from the “high-risk” category. The change will come into effect on Monday.

Arrivals from “very high-risk” countries, dubbed group A2, will not qualify to have their quarantine period reduced even if they are fully vaccinated.

Aside from undergoing a mandatory 21-day quarantine, they will also need to take four tests during isolation, another on the 26th day after arriving in Hong Kong, and self-monitor for another seven days after being released.

Currently, Ireland and Indonesia are the only A2 countries, though the latter is to be downgraded to A1 on Friday. People from A1 countries are not allowed to fly into the city at all.

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In Cathay’s memo to staff on Thursday, chief operations and service delivery officer Greg Hughes said the realities of global coronavirus restrictions made the vaccine mandate necessary.

“It is becoming clear that only fully vaccinated aircrew will be able to return without quarantine from most places. Furthermore, only fully vaccinated crew have been permitted to operate to certain high-risk places and only fully vaccinated crew are able to operate quarantine-free ‘bubble flights’,” Hughes said.

While the airline is not the first major company to insist staff be vaccinated, it is the largest in Hong Kong to do so. Airlines globally have increasingly been asking prospective hires to be vaccinated to secure employment.

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Hong Kong’s Cathay Pacific Airways posts record loss of HK$21.6 billion for 2020

Hong Kong’s Cathay Pacific Airways posts record loss of HK$21.6 billion for 2020

Also on Thursday, Hong Kong confirmed what is likely to be its first community coronavirus case involving the Delta variant, first found in India, ending a 16-day run of zero local infections. Authorities were racing to identify the source and contain its spread, putting as many as 180 people into quarantine.

After a sudden overnight lockdown and mandatory testing in Tai Po, where the 27-year-old patient lives, ended on Thursday morning without uncovering any cases among 2,100 residents screened, officials later in the day confirmed his infection and tentatively listed it as a local case.

However, authorities believed there was a good chance the infection was imported, given the patient’s regular contact with passengers, pilots and flight crews during his work as an airline ground staff member.

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.

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Nevertheless, officials have cast a wide net in quarantining 180 people, including 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.

“We can see the threat from variants is increasing by the day, because their transmissibility is extremely high,” Dr Ronald Lam Man-kin, the Centre for Health Protection’s controller, said.

There were also six imported Covid-19 cases on Thursday, involving four overseas students returning from Britain, and two arrivals from Indonesia. This brought the city’s total number of confirmed infections to 11,905, with 210 related deaths.

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 coronavirus pandemic.

He said each infected person with the Delta variant could spread the virus to between six and eight people on average compared to the Wuhan strain, which can spread to two or three people.

Speaking on a radio programme 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.

The infectious disease expert said the L452R strain was not only more contagious, but also had a mean incubation period of just two to four days, shorter than other strains.

Because of that, he said authorities needed to speed up mandatory testing of the suspected patient’s potential contacts to contain a possible spread of the Delta variant into the community.

“If you do it slowly, you are bound to fall behind the virus,” he said, citing the recent example of the Delta cluster in Guangzhou, which he said took just 10 days to have a five-generation transmission.