Record-high Covid infections and hospitalisations have been reported in the US, with fears they will not slow in the run-up to Christmas.
The number of people in hospital passed 100,000 for the first time, a figure that has doubled since early November.
New cases rose by a record 195,695 on Wednesday, and the daily death toll of 2,733 was close to a new high.
The city of Los Angeles has reacted to an unprecedented surge there by ordering residents to stay at home.
Nationwide, infections have now surpassed 14 million, with more than 274,000 deaths, according to data from Johns Hopkins University.
Figures have continued to soar in recent weeks, with around a million new infections reported every week in November. - equivalent to 99 every minute.
In response to surging numbers, US authorities have warned that the country's healthcare system faces an unprecedented strain this winter.
"The reality is that December, January and February are going to be the most difficult time in the public health history of this nation," said Robert Redfield, director of the Centers for Disease Control and Prevention (CDC).
California, Texas and Florida - the three most populous US states - are among the worst-affected areas of the country, and have each registered more than one million cases.
California Governor Gavin Newsom on Thursday issued a stay-at-home order for much of his state, saying he was pulling an "emergency brake" as the virus surge threatens to overwhelm hospitals. Regions with less than 15% intensive care capacity - including Southern California and the Sacramento area - will be on lockdown for at least three weeks.
In the city of Los Angeles, Mayor Eric Garcetti issued an emergency order for residents to stay home with immediate affect, following an unprecedented surge of infections. "It's time to cancel everything. If it isn't essential, don't do it," Mr Garcetti said.
A similar order is already in place in Los Angeles county - the current epicentre of America's outbreak - where some hospitals are already approaching full capacity. Authorities reported 5,987 cases on Tuesday, bringing the county's total to 414,185.
US officials have said they expect infection numbers to continue rising over the next few days because many people travelled over the Thanksgiving holiday, ignoring government advice."Travel volume was high over Thanksgiving," said Cindy Friedman, chief of the travellers' health branch at the CDC.
"Even if only a small percentage of those people were carrying the disease and passed it on to other people, that can translate into hundreds of thousands of additional infections."
The CDC has urged people to refrain from travel over Christmas.
But the public health body has also relaxed its guidelines for how long people should quarantine after coming into contact with an infected person - shortening it from 14 days to between seven and 10.
Although the number of deaths is still rising, one set of brighter figures is that of deaths in relation to infections.
The CDC says the share of cases resulting in death fell from 6.7% in April to 1.9% in September, reflecting that health workers are now more successful in treating the disease.
US regulators are expected to meet on 10 December to discuss emergency approval for a vaccine developed by Moderna. They will meet again on 17 December to address another one made by Pfizer, which was approved this week in the UK.
Federal officials at the CDC have agreed that the nation's 21 million healthcare workers should be prioritised, as well the three million elderly Americans living in long-term care homes. But there is less consensus on how states should distribute it to other groups.
There are also concerns regarding how many Americans are willing to get vaccinated. A recent Gallup poll found that 58% said they would be willing, although this is up from a low of 50% in September.
Meanwhile, on Wednesday top Democratic lawmakers signalled support for a $908bn (£677bn) coronavirus relief framework - a major concession following months of deadlock with Republicans over policy disagreements.
Exact details of the framework have yet to be publicly disclosed, but it broadly includes funding for state and local governments, unemployment benefits, small businesses and other areas of the economy impacted by the outbreak.