WASHINGTON (REUTERS) - An unrelenting US coronavirus surge pushed hospitals further to their limits as the United States pressed on with its immunisation roll-out on Thursday (Dec 17) and prepared to ship nearly 6 million doses of a new vaccine on the cusp of winning regulatory approval.

Covid-19 hospitalisations rose to record heights for a 19th straight day, with nearly 113,000 coronavirus patients in US medical facilities nationwide on Wednesday, while 3,580 more Americans perished, the most yet in a single day.

The virus has claimed almost 308,000 lives in the United States to date, and health experts have warned of a deepening crisis this winter as intensive care units (ICUs) filled up and hospital beds overflowed into hallways.

"We expect to have more dead bodies that we have spaces for them," Los Angeles Mayor Eric Garcetti said at a briefing on Thursday, adding that the country's second largest city had fully exhausted its ICU capacity.

The US reported 232,255 more cases on Wednesday, the second-greatest daily tally yet, driving the number of known infections nationally to more than 17 million since the pandemic began.

The tolls mounted as US regulators weighed whether to grant emergency use authorisation for a vaccine developed by Moderna Inc, just a week after an earlier vaccine from Pfizer Inc and German partner BioNTech SE won consent for mass distribution.

A panel of outside advisers to the US Food and Drug Administration overwhelmingly endorsed Moderna's vaccine candidate for emergency use after a daylong meeting on Thursday.

FDA authorisation could come as soon as Friday.

Both vaccines require two doses, given three or four weeks apart, for each person inoculated.

The initial 2.9 million doses of the Pfizer-BioNTech vaccine began shipping on Sunday and were still making their way to hospitals across the country and into the arms of doctors, nurses, and other frontline medical professionals.

Some of the first shots were also going to residents and staff of long-term care facilities. Later vaccine rounds will go to other essential workers, senior citizens and people with chronic health conditions.

It will take several months before vaccines are widely available to the public on demand, and opinion polls have found many Americans hesitant about getting inoculated.

Some are distrustful of immunisations in general, and some are wary of the unprecedented speed with which the first vaccines were developed and rolled out - 11 months from the first documented US cases of Covid-19.

Public health authorities have sought to reassure Americans that the Covid-19 vaccines are safe as well as highly effective at preventing illness.

'Some are on the fence'

But ambivalence has emerged even among pockets of healthcare workers designated as first in line for inoculation.

"Some are on the fence. Some feel that we need to get it done. It's split down the middle," Diego Montes Lopez, 28, a phlebotomist at Martin Luther King Jr. Community Hospital in South Los Angeles, said of co-workers after getting injected himself.

Public service messaging about vaccines has been mixed with urgent pleas for Americans to remain diligent about social distancing and mask-wearing until immunisations become widely available.

They point to data showing infections continuing to spread virtually unabated across much of the country, apparently fuelled by increased transmissions of the virus as many Americans disregarded warnings to avoid social gatherings and unnecessary travel over the Thanksgiving holiday last month.

California has been hit particularly hard in recent weeks, with many of its hospitals reporting intensive care units (ICUs)at or near capacity, a dire situation that triggered a renewal of sweeping stay-at-home orders across much of the state.

Beds in hallways

Health experts have warned that when ICUs become overwhelmed the death rate tends to rise.

"Hospitals and healthcare workers continue to be stretched to the limit, as we continue to surge beyond even what we anticipated. And we're not even through the holidays yet," said Adam Blackstone, a spokesman for the Hospital Association of Southern California.

In San Bernardino County, where all available ICU space has been taken, newly admitted patients at Arrowhead Regional Medical Center were lined up in beds in corridors waiting for care, spokeswoman Justine Rodriguez told Reuters.

With the strain taking a growing toll on medical staff, the race to expand vaccinations is seen as critical to preventing a collapse of healthcare systems.

Health and Human Services Secretary Alex Azar told CNBC on Thursday that 5.9 million Moderna vaccine doses had been allotted for state governments to receive and were ready to distribute nationwide starting at the weekend.

The Moderna vaccine has less onerous cold storage requirements than the Pfizer-BioNTech shot, making it a better option for remote and rural areas. Both were about 95 per cent effective in preventing illness in clinical trials.

With millions of Americans thrown out of work following state and local economic restrictions intended to contain the virus, Congress could soon lend a hand as well.

Top Republicans and Democrats on Capitol Hill on Thursday were closer than they have been in months to approving the first coronavirus economic relief since April, haggling over the details of a US$900 billion (S$1.2 trillion) package.

The legislation was expected to include individual stimulus checks of about US$600, extend unemployment benefits, funds for vaccine distribution and to assist struggling small businesses.