NEW YORK (BLOOMBERG) - After months of warnings that vaccinations would ward off a Covid-19 disaster, the United States is sailing towards a holiday crisis.

Cases and hospital admissions are rising amid a season of family gatherings. Most victims have shunned inoculations.

The situation is especially dire in the chilly north-eastern states, but doctors in many places report a grimly repetitive cycle of admission, intensive care and death.

There are shortages of beds and staff to care for the suffering.

"We're in desperate shape," said Dr Brian Weis, chief medical officer at Northwest Texas Healthcare System in Amarillo, the state's worst hot spot.

In 12 states and the nation's capital, the seven-day average of admissions with confirmed Covid-19 has climbed at least 50 per cent from two weeks earlier, according to US Department of Health and Human Services data.

The areas with the largest percentage upticks included Connecticut, New Jersey and Washington.

A little more than 60 per cent of the US population is considered fully vaccinated, generally meaning two shots, according to data from the Centres for Disease Control and Prevention (CDC).

That still leaves a large pool of highly susceptible people capable of pressuring hospitals.

In the most recent CDC data, from September, unvaccinated people had about 14 times the risk of dying from Covid-19 after adjusting for age - a major factor in Covid-19 outcomes.

In some states in the Midwest and north-east, Covid-19 hospitalisations are mirroring last year's seasonal pattern, said Professor Pinar Karaca-Mandic, one of the leaders of the Covid-19 Hospitalisation Tracking Project at the University of Minnesota.

"With the winter coming, people are being more indoors," she said.

During last year's surge, "everyone was unvaccinated", Prof Karaca-Mandic said.

While most Americans are inoculated now, they are also isolating less than last year.

Officials continue to push vaccine shots. In Boston, Mayor Michelle Wu sent out news of getting her own booster at City Hall on Thursday (Dec 9).

The city is adding vaccination clinics, including at schools, and colleges around the region have begun to let students know that boosters will be required. New Hampshire will hold a "Booster Blitz" on Friday at sites across the state.

But Prof Karaca-Mandic said the wild card is the new variant: "We just don't know what will happen with Omicron."

As the world turns its focus to the strain, which spreads fast but may be no more deadly, cases caused by the Delta variant have continued to mount in the United States.

The waves emerged and receded at different times in different regions, and recent hot spots such as Montana and Colorado are now seeing improvement.

Still, Colorado had just 518 acute-care beds available on Thursday, said the state's Covid-19 incident commander Scott Bookman.

"We have a long way to go before our hospitals empty out," he said in a news briefing.

Even in places coping well, a sense of foreboding prevails.

California has seen relatively steady infection rates in recent weeks, with hospitalisations around the levels they were in July, before Delta took hold.

But the most populous state had 11 confirmed Omicron cases as at Wednesday, which "presumes we'll see dozens more in the next days, hundreds more in the next weeks, thousands more" after that, Governor Gavin Newsom said on ABC television.

In Amarillo, elective surgical procedures have been cancelled and emergency rooms are jammed with virus patients who must wait as long as five days for a hospital bed, Dr Weis said in a phone interview.

Regional hospital officials have petitioned the state for additional staff "but there's little hope they can come through", he said.

In New Jersey, average daily hospital admissions reached a seven-day average of 206, up 78 per cent from two weeks earlier. Even so, at this time a year ago, the pace of admissions was well over twice as fast.

"The overwhelming majority of our new cases, new hospitalisations and new deaths, sadly, are from among the unvaccinated," Governor Phil Murphy said during a Dec 8 briefing.

Geisinger Health System, which has nine hospitals in north-eastern and central Pennsylvania, is over capacity and turning away patients, said Dr Gerald Maloney, chief medical officer for hospital services.

"People are tired," Dr Maloney said in an interview. "It's worse already than it was a year ago, and it may get even worse."

In New York, Governor Kathy Hochul ordered more than 30 hospitals that were filling up with patients to halt some procedures. Mayor Bill de Blasio said that was a grim harbinger for New York City.

"Biggest city in America, densely populated, we cannot let that happen here," he said.

In Michigan, hospitals are hitting a critical point.

The state's 22,883 inpatient beds are more than 85 per cent occupied, said Mr John Karasinski, spokesman for the Michigan Health and Hospital Association.

"The situation is dire and compounded by several factors," he said in a phone interview.

"The Covid-19 surge is stressing hospitals, their workforce and capacity. There are ongoing staffing shortages. It existed before the pandemic and has gotten worse during the pandemic."

The Department of Defence sent three teams of 22 medical professionals to help, each in a different part of the state.

Illinois had 3,178 Covid-19 hospitalisations as at Wednesday, the highest since January, according to the state health department.

Six of the state's 11 regions had 20 or fewer intensive care beds available.

Thanksgiving weekend is a likely driver of the rebound, said hospital coordinating-centre manager for the state's southernmost counties, Mr Arien Herrmann.

"People travelling, visiting friends and family, having gatherings created an opportunity for community spread," Mr Herrmann said in a telephone interview. "It'll be the same thing going into Christmas and then New Year's. This is keeping me up at night."