NEW YORK (NYTIMES) - Despite strong levels of vaccination among older people, Covid-19 killed them at vastly higher rates during this winter's Omicron wave than it did last year, preying on long delays since their last shots and the variant's ability to skirt immune defences.

This winter's wave of deaths in older people belied the Omicron variant's relative mildness. Almost as many Americans 65 and older died in four months of the Omicron surge as they did in six months of the Delta wave, even though the Delta variant, for any one person, tended to cause more severe illness.

While overall per capita Covid-19 death rates have fallen, older people still account for an overwhelming share of them.

"This is not simply a pandemic of the unvaccinated," said Dr Andrew Stokes, an assistant professor in global health at Boston University who studies age patterns of Covid deaths. "There's still exceptionally high risk among older adults, even those with primary vaccine series." Covid-19 deaths, though always concentrated in older people, have in 2022 skewed towards older people more than they did at any point since vaccines became widely available.

That swing in the pandemic has intensified pressure on the Biden administration to protect older Americans, with health officials in recent weeks encouraging everyone 50 and older to get a second booster and introducing new models of distributing antiviral pills.

In much of the country, though, the booster campaign remains listless and disorganised, older people and their doctors said. Patients, many of whom struggle to drive or get online, have to manoeuvre through an often labyrinthine health care system to receive potentially lifesaving antivirals.

Nationwide US Covid-19 deaths in recent weeks have been near the lowest levels of the pandemic, below an average of 400 a day. But the mortality gap between older and younger people has grown: Middle-aged Americans, who suffered a large share of pandemic deaths last summer and fall, are now benefiting from new stores of immune protection in the population as Covid-19 deaths once again cluster around older people.

And the new wave of Omicron subvariants may create additional threats: While hospitalisations in younger age groups have remained relatively low, admission rates among people 70 and older in the Northeast have climbed to one-third of the winter Omicron wave's towering peak.

"I think we are going to see the death rates rising," said Dr Sharon Inouye, a geriatrician and a professor of medicine at Harvard Medical School. "It is going to become more and more risky for older adults as their immunity wanes."

Mr Harold Thomas, 70, of Knoxville, Tennessee, is one of many older Americans whose immunity may be waning because he has not received a booster shot. The Covid States Project, an academic group, recently estimated that among people 65 and older, 13 per cent are unvaccinated, 3 per cent have a single Moderna or Pfizer shot, and another 14 per cent are vaccinated but not boosted.

When vaccines first arrived, Mr Thomas said, the state health department made getting them "convenient" by administering shots at his apartment community for older people. But he did not know of any such effort for booster doses.

Deaths have fallen from the heights of the winter wave in part because of growing levels of immunity from past infections, experts said. For older people, there is also a grimmer reason: So many of the most fragile Americans were killed by Covid-19 over the winter that the virus now has fewer targets in that age group.

But scientists warned that many older Americans remained susceptible. To protect them, geriatricians called on nursing homes to organise in-home vaccinations or mandate additional shots.

In the longer term, scientists said that policymakers needed to address the economic and medical ills that have affected especially nonwhite older Americans, lest Covid-19 continue cutting so many of their lives short.

"I don't think we should treat the premature death of older adults as a means of ending the pandemic," Dr Stokes said. "There are still plenty of susceptible older adults - living with comorbid conditions or living in multigenerational households - who are highly vulnerable."

The pattern of Covid deaths this year has re-created the dynamics from 2020 - before vaccines were introduced, when the virus killed older Americans at markedly higher rates. Early in the pandemic, mortality rates steadily climbed with each extra year of age, Stokes and his collaborators found in a recent study.

That changed last summer and fall, during the Delta surge. Older people were getting vaccinated more quickly than other groups: By November, the vaccination rate in Americans 65 and older was roughly 20 percentage points higher than that of those in their 40s. And critically, those older Americans had received vaccines relatively recently, leaving them with strong levels of residual protection.

As a result, older people suffered from Covid-19 at lower rates than they had been before vaccines became available. Among people 85 and older, the death rate last fall was roughly 75 per cent lower than it had been in the winter of 2020, Dr Stokes' recent study found.

At the same time, the virus walloped younger and less vaccinated Americans, many of whom were also returning to in-person work. Death rates for white people in their late 30s more than tripled last fall compared to the previous winter. Death rates for Black people in the same age group more than doubled.

The rebalancing of Covid-19 deaths was so pronounced that, among Americans 80 and older, overall deaths returned to pre-pandemic levels in 2021, according to a study posted online in February. The opposite was true for middle-aged Americans: Life expectancy in that group, which had already dropped more than it had among the same age range in Europe, fell even further in 2021.

"In 2021, you see the mortality impact of the pandemic shift younger," said Dr Ridhi Kashyap, a lead author of that study and a demographer at the University of Oxford.

By the time the highly contagious Omicron variant took over, researchers said, more older Americans had gone a long time since their last Covid-19 vaccination, weakening their immune defences.

As of mid-May, more than one-quarter of Americans 65 and older had not had their most recent vaccine dose within a year. And more than half of people in that age group had not been given a shot in the past six months.

The Omicron variant was better than previous versions of the virus at evading those already weakening immune defences, reducing the effectiveness of vaccines against infection and more serious illness. That was especially true for older people, whose immune systems respond less aggressively to vaccines in the first place.

For some people, even three vaccine doses appear to become less protective over time against Omicron-related hospital admissions. A study published recently in The Lancet Respiratory Medicine found that trend held for people with weakened immune systems, a category that older Americans were likelier to fall into.

Dr Sara Tartof, the study's lead author and a public health researcher at Kaiser Permanente in Southern California, said that roughly 9 per cent of people 65 and older in the study were immunocompromised, compared with 2.5 per cent of adults younger than 50.

During the Omicron wave, Covid-19 death rates were once again dramatically higher for older Americans than younger ones, Dr Stokes said. Older people also made up an overwhelming share of the excess deaths - the difference between the number of people who actually died and the number who would have been expected to die if the pandemic had never happened.

Dr Jeremy Faust, an emergency physician at Brigham and Women's Hospital in Boston, found in a recent study that excess deaths were more heavily concentrated in people 65 and older during the Omicron wave than the Delta surge. Overall, the study found, there were more excess deaths in Massachusetts during the first eight weeks of Omicron than during the 23-week period when Delta dominated.

As older people began dying at higher rates, Covid-19 deaths also came to include higher proportions of vaccinated people. In March, about 40 per cent of the people who died from Covid-19 were vaccinated, according to an analysis of figures from the Centres for Disease Control and Prevention.

Fewer older Americans have also been infected during the pandemic than younger people, leading to lower levels of natural immunity. As of February, roughly one-third of people 65 and older showed evidence of prior infections, compared with about two-thirds of adults younger than 50.

Long-ago Covid-19 cases do not prevent future infections, but reinfected people are less likely to become seriously ill.

A drop-off in Covid-19 precautions this winter, combined with the high transmissibility of Omicron, left older people more exposed, scientists said. It is unclear how their own behaviour may have changed.

An earlier study, from scientists at Marquette University, suggested that while older people in Wisconsin had once been wearing masks at rates higher than those of younger people, that gap had effectively disappeared by mid-2021.

Antiviral pills are now being administered in greater numbers, but it is difficult to know who is benefiting from them. Scientists said that the wintertime spike in Covid-19 death rates among older Americans demanded a more urgent policy response.

Dr Inouye, of Harvard Medical School, said she had waited for a notice from her mother's assisted living facility about the roll-out of second booster shots even as reports started arriving of staff members becoming infected. But still, the facility's director said that a second booster shot drive was impossible without state guidance.

Eventually, her family had to arrange a trip to a pharmacy on their own for a second booster.

"It just seems that now the onus is put completely on the individual," she said. "It's not like it's made easy for you."