HONG KONG (BLOOMBERG) - Hong Kong is at a Covid-19 tipping point.

The once-vibrant gateway to China has sacrificed its status as an international hub to "Covid Zero", its strategy for eliminating the virus by isolating itself from a world awash in the pathogen.

It worked for nearly a year, keeping residents safe and largely unfettered while raising the tantalising possibility of reopening the border with China, the city's economic lifeblood.

Now it is living with the worst of both worlds, after a couple of imported infections caused by the highly transmissible Omicron variant started spreading in the under-vaccinated city, triggering renewed curbs.

Residents can no longer go to the gym or the cinema, and the once-ubiquitous banquets where people gathered to celebrate the Chinese New Year are cancelled for another year.

Also gone is the sense of security that stemmed from the city being virus-free, as are scores of expatriates who went back to their home countries for the holidays and now cannot return due to flight bans slapped on eight places, including the United States and Britain.

The local economy that boomed in 2021, thanks to grounded residents spending more in the city, will also take a hit as domestic curbs ramp up.

"It does feel like we made no progress in the past two years," said Mr Christopher Mark, the co-founder of Black Sheep Restaurants, a high-end hospitality group that offers everything from traditional Chinese meals to Argentinian steaks.

He anticipates that 60 per cent to 70 per cent of his business will vanish, a deja vu moment that brings him back to the early days of the pandemic when Covid-19 was a new and mysterious disease.

Two years on, it is a familiar foe, with most of the world adjusting to living with the virus.

But Hong Kong, once known as the most globalised city in Asia, has stood still with a zero-tolerance strategy aligned with mainland China's.

While waves of death and the Delta variant have been avoided, the more transmissible Omicron is bringing a reckoning.

"I've never thought that Covid Zero was a feasible strategy," said Dr Amesh Adalja, senior scholar at the Johns Hopkins University Centre for Health Security in Baltimore, adding that it is impossible to eliminate a respiratory virus that spreads efficiently.

"Omicron is the variant that's really going to shatter the idea of Covid Zero for those people who are irrationally holding on to it."

Already, the tentacles of the virus are slithering through the city.

About two dozen currently known transmissions in the city have been linked back to international travellers, mainly aircrew from Cathay Pacific Airways who were allowed to skip strict quarantine requirements.

Healthcare workers are tracking at least three separate chains of transmission.

Despite the low number of cases by Western standards, every strata of society from construction workers to government ministers have been touched due to officials' aggressive tracing and tracking.

Hundreds are being sent to spartan quarantine camps, as fear rises about Omicron's exponential spread seen in countries and hospitals already nearing capacity.

The scientific logic of some of the new rules remains unclear.

Hong Kongers are no longer able to eat inside restaurants after 6pm though two clusters of known Omicron transmission happened at breakfast and lunch.

Children's playgrounds and some walking trails are closed, despite the lower risk when outdoors, while schools and shopping malls remain open.

Their combined effect is nonetheless widespread upheaval - which can be attributed at least in part to lost opportunities during the seven months of virus-free life enjoyed in 2021, when the chance to boost vaccination rates and plug some loopholes were largely squandered.

Hong Kong is the third least inoculated place among the International Monetary Fund's 39 advanced economies, above Latvia and Slovakia, with just 62 per cent of its population having received two doses. More significantly, only 23 per cent of those aged over 80 in the population have taken a first dose due to rampant misinformation and complacency.

While the arrival of Omicron and a looming vaccine requirement to enter most public venues have spurred demand over the past week, the shuttering of vaccine centres has made it difficult to get an appointment.

The government is not pushing it. The mandate will go into force only on Feb 24.

Meanwhile, Israel and European countries have used such strategies since early 2021, resulting in strong immunity shields when Omicron hit late last year.

"We will look back on this initiative and wonder what could have been achieved if it had been brought in much earlier," said Dr Nicholas Thomas, an associate professor in health security at the City University of Hong Kong.

The decision to require immunisation was not easy when there was no virus circulating, said Dr Lam Ching Choi, a member of Hong Kong Chief Executive Carrie Lam's advisory Executive Council.

"People felt the outbreak would never come," he said. "Thus it requires a bit of more political power to introduce the Covid-19 vaccine passport."

But it is not just a question of getting more shots in arms. People need effective protection to avoid skyrocketing hospitalisations and deaths.

Not all immunisations are the same. Hong Kong offers a choice between vaccines from Sinovac Biotech and Pfizer-BioNTech.

Most newly vaccinated people get Sinovac, even though studies show full immunisation and a booster do not provide sufficient antibodies against Omicron.

Many fear side effects from the more powerful shot. The city government appears to be of a view that a weaker shot is better than nothing. Mrs Lam received three doses of Sinovac.

"It's far, far better than not getting vaccinated," said Dr Ivan Hung, a University of Hong Kong professor who sits on a committee advising the government on vaccinations and believes Sinovac effectively averts serious disease and death.

"After three doses of Sinovac, you will have the option of getting the fourth dose maybe within three months, then that will also help in improving the overall herd immunity."

A major reason Hong Kong officials have been so focused on eliminating Covid-19 is to reopen the mainland border, its most crucial economic link.

The arrival of Omicron threw a wrench into plans to restart free movement, with Mrs Lam saying this week that talks will not resume until the Omicron situation in Hong Kong comes under control.

Prior to the Omicron outbreak, a key loophole in Hong Kong's system that China disliked was the city's travel quarantine exemption for aircrew. Instead of the up to 21 days of hotel quarantine served by leisure travellers, crew members just had to stay home for three days.

Indeed, new Omicron chains of transmission soon emerged from these airline employees, one of whom had lunch at an upscale restaurant while he was supposed to be in home isolation.

A handful of infected people, including a 13-year-old girl, eventually were tied to that meal.

A female crew member followed the rules but passed on the infection to her mother whom she lived with, who in turn passed it to several others, including a group she danced with on New Year's Eve.

The authorities recalled a cruise ship of 2,500 passengers because some close contacts of the mother were onboard.

Yet the very aircrew exemptions that started the outbreak raise the question of whether Hong Kong, a city of 7.4 million that relies on imported goods, could ever have withstood a Covid-19-zero strategy like the vast mainland can.

The aircrew were originally exempted because of much-needed cargo routes that carry everything from food to medicine to Hong Kong. Now that the loophole has been plugged, retailers and businesses predict shortages and inflation to follow, bringing further misery.

Even government leaders have found the recommendations hard to follow.

Numerous officials attended a large party in a restaurant, even as Omicron was spreading and the city's health experts urged residents to avoid mass gatherings. At least two guests who attended the event at different times were unknowingly infected.

Twenty per cent of the newly elected "patriots-only" legislature and a handful of senior ministers, as well as a number of lower-level officials, are now in quarantine in a facility without Wi-Fi and where the mattresses are wrapped in plastic.

Companies from Goldman Sachs Group to JPMorgan Chase are allowing employees to work from home and splitting up teams coming into the office.

While avoiding Covid-19 is a key goal, they also want to reduce the risk that workers - perhaps entire teams or floors - could be sent to the government's dour Penny's Bay quarantine facility.

Desire to avoid the camp - as well as the North Lantau Hospital where those who test positive are kept several to a room - has many in the city debating how forthcoming they would be should they develop symptoms or know of an exposure.

Now it is time for Hong Kong to consider its options, said University of Hong Kong epidemiology professor Benjamin Cowling in a Bloomberg TV interview.

"We put a lot of eggs in the basket of zero Covid-19," he said.

"Maybe we haven't thought so much about plan B, which is relying on vaccines to mitigate a larger epidemic. Now we really do need to look at that as an urgent priority."

The government should continue to urge vaccination, with a goal of reaching 90 per cent of the eligible population by April, said Dr Hung, the Hong Kong University professor.

At that rate, with widespread booster doses given by June, the city may be able to reopen to the rest of the world by the summer and put the virus behind it, he said.

Simply staying locked away - from the world and one another - is not a viable approach, according to Johns Hopkins' Dr Adalja.

"It's not like Noah's Ark, where you wait for everything to pass and then come back out," he said.

"There is a lot of damage that's done. Covid-19 is a major factor in the world, but it's not the only factor in the world."

Still, it is unlikely that Hong Kong can untether itself from the increasingly obsolete approach as long as Beijing remains resolutely behind Covid-19-zero.

As the city peers over the Omicron abyss, a likelier view is months of near-lockdown life, and an exodus of expatriates and global businesses from a city once their first stop in Asia.