HONG KONG — Asia's two most globalized cities faced a conundrum when the coronavirus pandemic hit. Among the first to close borders to nonresidents and impose draconian quarantine rules to keep people safe, the business hubs of Singapore and Hong Kong were suddenly cut off from the international connections that underpinned their success.

Now, the longtime rivals are diverging, with implications for thousands of companies with operations in the region.

After a successful vaccination rollout, Singapore is pivoting to living with the coronavirus and reopening to the world, while Hong Kong, like the rest of China, sticks to a “zero-covid” strategy that will keep it isolated. Singapore has begun to drop quarantine restrictions for some travelers and is preparing further easing; Hong Kong moved this week to tighten restrictions, putting most travelers into quarantine hotels for either 14 or 21 days.

The divergence threatens Hong Kong’s competitiveness as an international financial hub, particularly when compounded by escalating political risk from a new China-imposed national security law. Companies are shifting personnel out of the city, business groups say, adding to an outflow of some 90,000 people since mid-2020.

“It has reached a boiling point,” said Tara Joseph, president of the American Chamber of Commerce in Hong Kong. “Watching the rest of the world think forward while Hong Kong doesn’t budge and becomes more draconian is deeply frustrating and, for some people, the last straw.”

In an open letter Thursday to Hong Kong leader Carrie Lam, Frederik Gollob, chairman of the European Chamber of Commerce in the city, said the quarantine rules are “out of proportion.”

“We are of the view that Hong Kong must open itself sooner rather than later or this new quarantine regime could lead many in the international community to question if they want to remain indefinitely trapped in Hong Kong when the rest of the world is moving on,” he wrote.

The Hong Kong government didn’t respond to a request for comment. In remarks this week, Lam said the government is doing everything “to protect Hong Kong from another major outbreak that we have seen and we have suffered.”

“We do not want to reverse our decisions on a frequent basis,” she said. “But sometimes in order to err on the side of caution and to prevent the spread of the disease, we have to do it.”

At stake is not only billions of dollars in business and the fate of two modern, international cities, but the wisdom of competing strategies as the pandemic rages on. Some places that moved quickly to curb coronavirus cases avoided thousands of deaths, but having squashed infections, they are struggling to determine an acceptable level of risk in populations that have yet to reach herd immunity particularly since the delta variant emerged.

Behind the divergent approaches of Singapore and Hong Kong is a yawning gap in vaccination rates, especially among the elderly. Only 10 percent of people over the age of 80 in Hong Kong have received at least one dose of a coronavirus vaccine; another wave of infections, particularly of the delta variant, could devastate this group. Overall, 73 percent of Singaporeans are fully vaccinated — among the world’s highest rates — versus 45 percent in Hong Kong. Both cities began vaccinations at about the same time and offer the shots free of charge.

Public health experts say Hong Kong could boost its rate by dropping its reliance on “zero-covid” — a strategy that works toward not having a single coronavirus case — and instead incentivizing vaccination. So far, this effort has largely been left to businesses, which offer prizes such as free apartments and shopping vouchers.

“In Hong Kong, if we had a strategy and timeline to say once we go past 70 percent [vaccination] we can relax this and that . . . I think there would be a stronger push to get the vaccination coverage to that level,” said Ben Cowling, an infectious-disease epidemiologist at the University of Hong Kong’s School of Public Health, in a podcast episode released Thursday. “But it just seems like right now, if we are thinking about the alternative, like staying in a zero-covid strategy for months or a year, then actually it’s not a priority to get vaccinations up.”

“I think we’ve really got to take a look at strategies elsewhere, like Singapore,” Cowling added.

Officials in Singapore several months ago began priming people for the idea that they must live with the virus. Residents accustomed to low cases in the city-state would get jumpy over each new cluster, like one in May linked to the airport. Through op-eds from ministers and video campaigns featuring comedians and community leaders, the government sent a signal that the virus would be endemic and vaccination was the only way forward.

“All of this needs to be explained to the community,” said Dale Fisher, a senior consultant at the division of infectious diseases in Singapore’s National University Hospital. “Eventually you are going to have to let covid in, and as soon as you open borders and ease restrictions, covid will thrive.”

But, he added, “if we believe in the vaccine — which we do — then we don’t expect to see large numbers of severe disease.”

On Thursday, Singapore dropped quarantine for vaccinated travelers from Germany, Hong Kong, Macao and Brunei. Meanwhile, Hong Kong abandoned a shortened-quarantine measure and reclassified more than a dozen countries as high-risk, necessitating three weeks of confinement, regardless of vaccination status. The changes threw travel plans into disarray and left some begging for a space in quarantine hotels that are largely full.

Anyone in the city can also be forced into a quarantine facility if deemed a close contact of someone who tested positive.

Hong Kong officials have told business leaders the zero-covid strategy is driven by mainland China, said Joseph of the American Chamber of Commerce in Hong Kong. Beijing increasingly dictates policy in the territory, despite its earlier promise to allow Hong Kong to largely run its own affairs until 2047.

China has stuck to this approach, locking down entire provinces, increasing surveillance and offering cash incentives to report on people suspected of carrying the virus. Chinese state media and some Chinese experts have continued to tout the wisdom of this strategy, arguing that deviating from it would overload the health system.

But as Singapore forges ahead, it may look increasingly attractive.

The absence of a clear plan for reopening has eroded Hong Kong’s value proposition to businesses, said Joseph. Hong Kong is already one of the world’s priciest cities and is the most expensive place to rent a luxury apartment. Some companies, because of the political situation and pandemic restrictions, have started to categorize Hong Kong as a hardship posting, adding tens of thousands of dollars to salary offers, she said.

“Even if you take the emotion out of it, it has to be worth the money to be here,” Joseph said.