Hong Kong’s high death rate shows the importance of vaccinating the elderly.

As the death toll from Covid-19 surges in Hong Kong, some scientists said that, in the era of Omicron, vaccinating as many older people as possible should be a top priority.

Hong Kong, which was once one of the world’s most formidable redoubts of “zero Covid,” has offered scientists a case study about the threat Omicron poses in an entirely different setting: a dense city where people were not only largely untouched by previous infections, but also where the oldest and most vulnerable residents were largely unvaccinated.

Several critical lessons emerged, health experts said.

With two Omicron subvariants circulating, including the highly contagious BA.2 subvariant, 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 most critical thing to do.

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, 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.

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.

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.