ROYAL OAK, MICHIGAN (NYTIMES)- At Beaumont Hospital, Royal Oak, in one of America's worst coronavirus hot spots, entire units are still filled with Covid-19 patients.

People weak with the virus still struggle to sit up in bed. And the phone still rings with pleas to transfer patients on the verge of death to units with higher-tech equipment.

But unlike previous surges, it now is younger and middle-aged adults - not their parents and grandparents - who are taking up many of Michigan's hospital beds.

A 37-year-old woman on a ventilator after giving birth. A 41-year-old father. A 55-year-old autoworker who has been sick for weeks.

"We're getting to the point where we're just so beat down," said Ms Alexandra Budnik, an intensive care nurse who works in a unit with lifesaving machines, or circuits, that are in short supply.

"Every time we get a call or every time we hear that there's another 40-year-old that we don't have a circuit for, it's just like, you know, we can't save them all."

Across Michigan, which is experiencing by far the country's most dangerous outbreak, more younger people are being admitted to hospitals with the coronavirus than at any other time in the pandemic.

Michigan hospitals are now admitting about twice as many coronavirus patients in their 30s and 40s as they were during the fall peak, according to the Michigan Health & Hospital Association.

The shifting demographics come as a majority of Michigan residents age 65 or older have been fully vaccinated, greatly reducing the risk to the most vulnerable and leading to fewer hospitalisations among the oldest age groups.

But the vaccinations of older people do not explain rising hospitalisations among people younger than 60, including those in their 20s and 30s.

Public health experts say the outbreak - driven by the B117 variant of the virus, which is more contagious and more severe - is spreading rapidly in younger age groups.

And across the state, doctors and nurses are increasingly reporting a concerning trend: Younger patients are coming in more often with serious cases of Covid-19.

"I am putting more patients in their 20s and 30s and 40s on oxygen and on life support than at any other time in this pandemic," said Dr Erin Brennan, an emergency room physician in Detroit.

As Michigan has struggled to control another surge, the national outlook has improved. Cases nationwide have drastically decreased since a peak in January, and there are fewer than half as many people hospitalised.

With about two-thirds of the country's oldest residents fully vaccinated, younger adults, many of whom only recently became eligible for vaccines, now make up a higher share of total hospitalisations.

But the number of people in their 30s and 40s hospitalised with Covid-19 nationally has also increased somewhat since March, according to recent CDC data, with a more significant rise for people in their 50s. Hospitalisations among younger people have been up most notably in the Great Lakes states, with rises seen as well in the Mid-Atlantic.

The director of the Centres for Disease Control and Prevention, Dr Rochelle Walensky, was among those who warned that more younger adults were coming to the hospital with severe forms of Covid-19.

The B117 variant - first identified in Britain and now the most common source of new infection in the United States - is believed to be about 60 per cent more contagious and 67 per cent more deadly than the original form of the coronavirus.

Public health experts attribute the hospitalisations among younger people to a number of factors, including the B117 variant and the lifting of pandemic restrictions around the country.

Younger people are among those most likely to be out and about socialising and in the workforce, at a time when just one-third of American adults are fully vaccinated.

"The restrictions were our pause button," said Dr Jennifer Nuzzo, a public health researcher at Johns Hopkins Bloomberg School of Public Health. "As soon as you press play, you are going to see the virus race back as quickly as it can."

Some health experts said it was conceivable that more younger people were being hospitalised because some hospitals had lowered their standards for admission.

"That's one explanation that you have to think about," said Dr Amesh Adalja, an infectious disease expert and hospital doctor in the Pittsburgh area, who said he sent borderline cases to be treated at home when hospitals were full.

"I have a lower threshold to admit when I'm not worried about hospital capacity."

But at Beaumont, Royal Oak, in suburban Detroit, where case numbers remain high, doctors said they had not lowered the bar for admissions. The younger people in their care often had fewer chronic health problems and a good chance to recover, but they exhibited serious symptoms that required immediate intervention.

"Some of them have kids that are younger than my kids, and you think about these people and the circumstances for their family if they don't survive," Dr Felicia Ivascu said as she looked out over a unit where the sickest of the hospital's coronavirus patients were hooked up to machines in glass-walled rooms.

The escalating situation in Michigan has upended the lives of people like Mr Matthew Kirschner of Clinton Township, north of Detroit.

After hosting a small lunch in his backyard late last month, when vaccines were not yet open to everyone in Michigan, several family members tested positive for the virus. Kirschner, 36, who transports Covid-19 patients in his job as a firefighter and had survived a bout with the virus last fall, thought he knew what to expect: He grew most worried for his mother, who is in her 70s, has underlying conditions and fits the profile of people most affected by the disease.

But it was his 37-year-old sister, Ms Cara Kirschner Estrada, who ended up seriously ill.

Ms Kirschner Estrada, who was seven months pregnant when she got sick, checked herself into an emergency room this month with a bevy of symptoms: fever, chills, a cough, difficulty breathing. Doctors did an emergency C-section to deliver her son, Angelo, who is doing well.

Ms Kirschner Estrada's condition, however, grew precarious. She has been sedated at Beaumont, Royal Oak, and on life support, according to her family.

"It is shocking. It is traumatic," Mr Kirschner's wife, Lauren, said of her sister-in-law, who she described as an active young mother working as a nurse at a wellness spa and raising a toddler. "It's kind of rocked my view. I can't believe Cara. Why is it her?"

Nationwide, more than 45,000 people were in hospitals with Covid-19 last week, far below the winter peak but up from about 39,000 a month ago. The hospitalisation numbers have been largely stagnant for the past week.

The risk of hospitalisation remains low for younger adults. According to state data from Maryland, where overall hospitalisations are up from last month, people in their 30s have a 5 per cent chance of being hospitalised if they learn they have the virus, far lower than the 20 per cent chance of someone in their 60s.

But as more younger people get infected, experts say, more will inevitably be admitted to hospitals.

"I tell everyone how bad it was and how scary it is," said Mr Nic Cabrera, 26, of Oxon Hill, Maryland, who was hospitalised for five days this month and had to be put on oxygen.

The rise in hospitalisations has been numbing, if familiar, for doctors in Michigan. At Beaumont, Royal Oak, where nearly 200 coronavirus patients were hospitalised Thursday (April 22), doctors discussed contingency plans to open more beds for Covid-19 patients if needed.

"We were very hopeful in December when they rolled out the vaccines," Dr Barbara Ducatman, the hospital's chief medical officer, said before leading her staff through a slideshow of discouraging statistics. "We didn't want to be here. It's like déjà vu."

The vaccines only recently became available to all adults, and the rollout has not yet reached many younger people. In hallway after hallway at the sprawling hospital, doctors and nurses donned extra masks to go inside the rooms of Covid-19 patients and spoke somberly about seeing people of their own generation in dire shape.

"You take it home a lot more," said Ms Budnik, 32, the intensive care nurse. "Your mindset is a little different when you look at them and you think, this could be my friend, this could be my sister, this could be me."

Dr Olusola Ogundipe, an infectious disease fellow, said he noticed that some of his younger patients also had a tougher time emotionally with their condition.

"They have a feeling of immortality," he said, "and so I think it does take younger people by surprise."

Reports of new cases in Michigan have finally started to decline in recent days, but that progress is not yet evident inside the hospital, where emergency room doctors see as many as 10 new coronavirus patients per shift and where some of those who are admitted will stay for weeks.

Mr James Dyer, 55, who works at a Ford plant, was just starting to feel better Thursday after three weeks in the hospital with Covid-19. Mr Dyer said he had previously been daunted by the stiff competition to get a vaccine but now was eager for a shot.

"I would definitely encourage other people to do it," Mr Dyer said. "And before I leave Beaumont, if I can, I'm going to get one."

Not far away, Ms Eleanor Wilson, 53, was on her fourth day in the hospital with a nasty case of Covid-19 that had left her gasping for breath and struggling to walk even short distances.

"I think we all kind of think we're superheroes, like we'll be all right," Ms Wilson, a day care provider, said. "Once you get it, you're like, 'Whoa, this is not a joke.'"

But after a rough few days and a course of steroids, Ms Wilson got good news: Doctors had cleared her to go home.