The first time the Omicron variant breached Hong Kong’s coronavirus defenses, in late 2021, the city stamped it out, cementing its status as one of the world’s most formidable redoubts of “zero Covid.”

But a few weeks later, Omicron came to the metropolis again, this time causing an outbreak among cleaners at a public-housing estate that spiraled out of control. The conflagration of resulting cases is now killing people at a rate exceeding that of almost any country since the coronavirus emerged.

Over the entire pandemic, Hong Kong’s death toll per capita, once far lower than those of Western nations, is no longer exceptional. A month ago, Americans had died from Covid at 90 times the rate of people in Hong Kong. By Monday, the cumulative American toll was three and a half times as high.

As the United States braces for its own, less punishing rise in cases, and mainland China battles its biggest outbreak in two years, scientists have looked to Hong Kong for clues about the threat Omicron poses in an entirely different setting: a dense city where people were not only largely untouched by previous infections, but whose oldest and most vulnerable residents were also largely unvaccinated.

Several critical lessons emerged, health experts said.

In the era of Omicron and its even more infectious subvariant, BA.2, vaccinating a broad swath of the population remained important, scientists said. But inoculating as many older people as possible had become far and away the top priority.

That message, they said, was most pressing for China, where vaccinations in older age groups also appear to be lagging and there is little immunity from earlier infections.

But it was relevant again in the United States, too, where subpar vaccination and booster rates among older people have left scientists concerned about a potential surge of BA.2 cases. Partly because so many more Americans have been infected and killed by the coronavirus during earlier waves, scientists do not expect the United States to face as serious a situation in the coming months as Hong Kong.

Hong Kong’s dreadful outbreak also signals the perils of trying to eliminate the virus without a plan for what would come next, health experts said. Omicron’s high transmissibility, they said, made outbreaks almost inevitable.

Hong Kong, which along with mainland China had been among the last holdouts of a strategy of tight restrictions and border controls to eradicate the virus, was left vulnerable by how few of its residents had any immunity from prior infections: Before the Omicron surge, scientists estimated that only 1 percent of Hong Kong’s population had contracted the virus.

Those low levels of immunity can leave places vulnerable to waves of cases, as more contagious variants sneak in or restrictions are lifted. But governments can still prepare for those waves, said Dr. Gabriel Leung, the dean of medicine at the University of Hong Kong.

Less than one-quarter of people aged 80 and over in Hong Kong had been given two doses of a vaccine before Omicron surged, compared with more than 90 percent of people in Singapore and New Zealand.

Because of the number of unvaccinated older people in China, scientists said, it might also have some difficulty lifting “zero Covid” restrictions. More than 87 percent of China’s population have been vaccinated. But just over half of people 80 and older have had two shots, and less than 20 percent of people in that age group have received a booster, Zeng Yixin, a vice minister of the National Health Commission, said on Friday.

“I don’t think it’s quite ready for the transition,” Dr. Leung said.

A number of Asian and Pacific countries had largely kept the virus at bay for two years, only to face Omicron outbreaks because the virus was so contagious and their populations had avoided earlier infections. But high vaccination rates, including among older people, have helped many of those countries avoid more devastating surges.

In South Korea, for example, where 87 percent of people are vaccinated and 63 percent have booster shots, the cumulative death toll per capita is one-tenth of America’s, even though South Korea has recorded more than three-quarters as many cases as the United States over the entire pandemic.

Health experts said that Hong Kong’s difficulties vaccinating older people resulted from a combination of complacency, given the city’s earlier success in containing the virus, and unfounded fears that older people and those in poor health faced particular risks from vaccines.

The city has now vaccinated 39 percent of residents aged 80 and above, despite having inoculated almost two-thirds of 12- to 19-year-olds.

Many people in Hong Kong have been given the Chinese vaccine Sinovac, which appears to offer relatively little protection from Omicron infections but a better defense against severe disease. Scientists noted that almost 90 percent of people who died during the latest wave were not fully vaccinated, suggesting that getting shots to the most vulnerable is more important than the particular brand.

“The problem in Hong Kong is, we haven’t succeeded in vaccinating our most vulnerable population — the elderly, especially those staying in elderly-care homes,” said Dr. Siddharth Sridhar, a clinical virologist at the University of Hong Kong. “And as a result, we are in a very bad situation.”

The United States has vaccinated many more of its older residents than Hong Kong but fewer than Western Europe and has seen a high death rate. And as immunity from early vaccinations wanes and booster shots become critical for shoring up protection against Omicron among older people, the United States finds itself exposed on that count, too. About 41 percent of people 65 and over have not received a booster shot.

Unlike other parts of Asia that had gradually lifted restrictions in recent months, Hong Kong was not ready for its defenses to fail, scientists said.

“From the government’s point of view, there was such a strong fixation on ‘zero Covid’ that as long as that worked, vaccination was not necessarily the priority,” said Ben Cowling, a professor of epidemiology at the University of Hong Kong.

Many older residents and their families adopted the same view, public health experts said. If Hong Kong’s rigid social-distancing measures and careful border controls were going to keep the virus out anyway, the conventional thinking went, was getting a vaccine worth the trouble?

“If you’re telling people that the disease is never going to get in, then there’s less of an incentive to go and get vaccinated,” said Dr. David Owens, a family doctor in Hong Kong. “To an extent, the messaging around elimination confounded the need to vaccinate.”

Dr. Cowling, of the University of Hong Kong, said that his city could have responded in one of two ways to signs that cases would surge: either double down on “zero Covid” through measures like building better quarantine facilities for overseas arrivals, or acknowledge that outbreaks are unavoidable and raise vaccination rates.

“Zero Covid is a really good strategy if you can stay at zero,” Dr. Cowling said. “But as we found in Hong Kong, it doesn’t last forever.”

Hong Kong eventually took steps to persuade older people to become vaccinated, once earlier inducements like vaccine passes proved ineffective. In January, the government announced that it would ban unvaccinated people from restaurants that serve dim sum, which are popular among older residents. But it was too late.

With cases and deaths now declining, Hong Kong announced on Monday that it would lift certain restrictions.

Singapore began abandoning “zero Covid” policies in the summer. Dr. Ooi Eng Eong, an infectious disease expert at Duke-National University of Singapore Medical School, said that it took a wave of the Delta variant to raise vaccination rates and disabuse people of the notion that they did not need protection.

Now, cases in Singapore have surged, but deaths are relatively low.

“It’s so much more transmissible that I think wearing a face mask and all — that helps but not to the extent that it has impacted the epidemiology,” Dr. Ooi said of Omicron. “The trends are really driven by vaccination.”

Still, even after five or six waves of the pandemic, the reasons that some countries have succeeded while others have suffered remain unclear.

Japan, for example, has tamped down on cases throughout the pandemic without resorting to full-fledged lockdowns, scientists said.

The country benefited from its government sharing sound public health advice early in the pandemic. As much as residents tired of precautions, they largely took the advice seriously, said Taro Yamamoto, a professor at the Institute of Tropical Medicine at Nagasaki University.

Roughly 80 percent of people in Japan have had their initial vaccine series. But even though the country is lagging in administering booster doses and had a surge of Omicron infections, death rates during Omicron have remained considerably lower than in nearby South Korea.

“Partly it’s a mystery,” Professor Yamamoto said. “We cannot explain it all.”