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

However a couple of weeks later, Omicron got here to the metropolis once more, this time inflicting an outbreak amongst cleaners at a public-housing property that spiraled uncontrolled. The conflagration of ensuing instances is now killing individuals at a charge exceeding that of just about any nation because the coronavirus emerged.

Over the whole pandemic, Hong Kong’s loss of life toll per capita, as soon as far decrease than these of Western nations, is not distinctive. A month in the past, People had died from Covid at 90 instances the speed of individuals in Hong Kong. By Monday, the cumulative American toll was three and a half instances as excessive.

As the US braces for its personal, much less punishing rise in instances, and mainland China battles its greatest outbreak in two years, scientists have appeared to Hong Kong for clues in regards to the risk Omicron poses in a wholly totally different setting: a dense metropolis the place individuals weren’t solely largely untouched by earlier infections, however whose oldest and most weak residents have been additionally largely unvaccinated.

A number of crucial classes emerged, well being consultants mentioned.

Within the period of Omicron and its much more infectious subvariant, BA.2, vaccinating a broad swath of the inhabitants remained essential, scientists mentioned. However inoculating as many older individuals as potential had develop into far and away the highest precedence.

That message, they mentioned, was most urgent for China, the place vaccinations in older age teams additionally look like lagging and there’s little immunity from earlier infections.

However it was related once more in the US, too, the place subpar vaccination and booster charges amongst older individuals have left scientists involved a few potential surge of BA.2 instances. Partly as a result of so many extra People have been contaminated and killed by the coronavirus throughout earlier waves, scientists don’t anticipate the US to face as severe a scenario within the coming months as Hong Kong.

Hong Kong’s dreadful outbreak additionally alerts the perils of making an attempt to eradicate the virus and not using a plan for what would come subsequent, well being consultants mentioned. Omicron’s excessive transmissibility, they mentioned, made outbreaks nearly inevitable.

Hong Kong, which together with mainland China had been among the many final holdouts of a technique of tight restrictions and border controls to eradicate the virus, was left weak by how few of its residents had any immunity from prior infections: Earlier than the Omicron surge, scientists estimated that just one p.c of Hong Kong’s inhabitants had contracted the virus.

These low ranges of immunity can depart locations weak to waves of instances, as extra contagious variants sneak in or restrictions are lifted. However governments can nonetheless put together for these waves, mentioned Dr. Gabriel Leung, the dean of drugs on the College of Hong Kong.

Lower than one-quarter of individuals aged 80 and over in Hong Kong had been given two doses of a vaccine earlier than Omicron surged, in contrast with greater than 90 p.c of individuals in Singapore and New Zealand.

Due to the variety of unvaccinated older individuals in China, scientists mentioned, it may additionally have some issue lifting “zero Covid” restrictions. Greater than 87 p.c of China’s inhabitants have been vaccinated. However simply over half of individuals 80 and older have had two pictures, and fewer than 20 p.c of individuals in that age group have obtained a booster, Zeng Yixin, a vice minister of the Nationwide Well being Fee, mentioned on Friday.

“I don’t assume it’s fairly prepared for the transition,” Dr. Leung mentioned.

Plenty of Asian and Pacific international locations had largely saved the virus at bay for 2 years, solely to face Omicron outbreaks as a result of the virus was so contagious and their populations had averted earlier infections. However excessive vaccination charges, together with amongst older individuals, have helped a lot of these international locations keep away from extra devastating surges.

In South Korea, for instance, the place 87 p.c of individuals are vaccinated and 63 p.c have booster pictures, the cumulative loss of life toll per capita is one-tenth of America’s, although South Korea has recorded greater than three-quarters as many instances as the US over the whole pandemic.

Well being consultants mentioned that Hong Kong’s difficulties vaccinating older individuals resulted from a mix of complacency, given town’s earlier success in containing the virus, and unfounded fears that older individuals and people sick confronted specific dangers from vaccines.

The town has now vaccinated 39 p.c of residents aged 80 and above, regardless of having inoculated nearly two-thirds of 12- to 19-year-olds.

Many individuals in Hong Kong have been given the Chinese language vaccine Sinovac, which seems to supply comparatively little safety from Omicron infections however a greater protection towards extreme illness. Scientists famous that just about 90 p.c of people that died in the course of the newest wave weren’t absolutely vaccinated, suggesting that getting pictures to essentially the most weak is extra essential than the actual model.

“The issue in Hong Kong is, we haven’t succeeded in vaccinating our most weak inhabitants — the aged, particularly these staying in elderly-care houses,” mentioned Dr. Siddharth Sridhar, a scientific virologist on the College of Hong Kong. “And because of this, we’re in a really dangerous scenario.”

America has vaccinated many extra of its older residents than Hong Kong however fewer than Western Europe and has seen a excessive loss of life charge. And as immunity from early vaccinations wanes and booster pictures develop into crucial for shoring up safety towards Omicron amongst older individuals, the US finds itself uncovered on that depend, too. About 41 p.c of individuals 65 and over haven’t obtained a booster shot.

In contrast to different components of Asia that had step by step lifted restrictions in latest months, Hong Kong was not prepared for its defenses to fail, scientists mentioned.

“From the federal government’s perspective, there was such a robust fixation on ‘zero Covid’ that so long as that labored, vaccination was not essentially the precedence,” mentioned Ben Cowling, a professor of epidemiology on the College of Hong Kong.

Many older residents and their households adopted the identical view, public well being consultants mentioned. If Hong Kong’s inflexible social-distancing measures and cautious border controls have been going to maintain the virus out anyway, the standard pondering went, was getting a vaccine definitely worth the bother?

“Should you’re telling people who the illness is rarely going to get in, then there’s much less of an incentive to go and get vaccinated,” mentioned Dr. David Owens, a household physician in Hong Kong. “To an extent, the messaging round elimination confounded the necessity to vaccinate.”

Dr. Cowling, of the College of Hong Kong, mentioned that his metropolis may have responded in one in all two methods to indicators that instances would surge: both double down on “zero Covid” via measures like constructing higher quarantine amenities for abroad arrivals, or acknowledge that outbreaks are unavoidable and lift vaccination charges.

“Zero Covid is a very good technique in the event you can keep at zero,” Dr. Cowling mentioned. “However as we present in Hong Kong, it doesn’t final eternally.”

Hong Kong ultimately took steps to influence older individuals to develop into vaccinated, as soon as earlier inducements like vaccine passes proved ineffective. In January, the federal government introduced that it will ban unvaccinated individuals from eating places that serve dim sum, that are in style amongst older residents. However it was too late.

With instances and deaths now declining, Hong Kong introduced on Monday that it will carry sure restrictions.

Singapore started abandoning “zero Covid” insurance policies in the summertime. Dr. Ooi Eng Eong, an infectious illness professional at Duke-Nationwide College of Singapore Medical College, mentioned that it took a wave of the Delta variant to lift vaccination charges and disabuse individuals of the notion that they didn’t want safety.

Now, instances in Singapore have surged, however deaths are comparatively low.

“It’s a lot extra transmissible that I believe carrying a face masks and all — that helps however to not the extent that it has impacted the epidemiology,” Dr. Ooi mentioned of Omicron. “The traits are actually pushed by vaccination.”

Nonetheless, even after 5 or 6 waves of the pandemic, the explanations that some international locations have succeeded whereas others have suffered stay unclear.

Japan, for instance, has tamped down on instances all through the pandemic with out resorting to full-fledged lockdowns, scientists mentioned.

The nation benefited from its authorities sharing sound public well being recommendation early within the pandemic. As a lot as residents uninterested in precautions, they largely took the recommendation severely, mentioned Taro Yamamoto, a professor on the Institute of Tropical Drugs at Nagasaki College.

Roughly 80 p.c of individuals in Japan have had their preliminary vaccine sequence. However although the nation is lagging in administering booster doses and had a surge of Omicron infections, loss of life charges throughout Omicron have remained significantly decrease than in close by South Korea.

“Partly it’s a thriller,” Professor Yamamoto mentioned. “We can not clarify all of it.”