While all eyes are on the new and little-understood Omicron variant, the Delta form of the coronavirus isn't finished wreaking havoc in the US, sending record numbers of patients to the hospital in some states, especially in the Midwest and New England.
"Omicron is a spark that's on the horizon. Delta variant is the fire that's here today," said Dr Nirav Shah, director of the state Center for Disease Control and Prevention in Maine, where a record 334 people were in the hospital with Covid-19 as of midweek.
The US recorded its first known Omicron infection in a fully vaccinated person who had returned to California from South Africa, where the variant was first identified just over a week ago.
And a second US case was later confirmed in Minnesota, involving a vaccinated man who had been in New York City. That would suggest the variant has begun to spread within the country.
But there is much that is unknown about Omicron, including whether it is more contagious than previous versions, makes people sicker or more easily thwarts the vaccine or breaks through the immunity that people get from a bout of Covid-19.
For now, the extra-contagious Delta variant accounts for practically all cases in the US and continues to inflict misery at a time when many hospitals are struggling with shortages of nurses and a backlog of patients undergoing procedures that had been put off early in the pandemic.
The fear is that Omicron will foist even more patients, and perhaps sicker ones, onto hospitals.
"For me, it's really just, I can't imagine," said Dr Natasha Bhuyan, a family physician in Phoenix. "Are we going to see another surge in cases that's even higher than what we're seeing now? What will that do to our health system? What will that do to our hospitals?"
Two years into the outbreak, Covid-19 has killed more than 780,000 Americans, and deaths are running at about 900 per day.
Covid-19 cases and deaths in the US have dropped by about half since the Delta peak in August and September, but at about 86,000 new infections per day, the numbers are still worryingly high, especially heading into the holidays, when people travel and gather with family.
With the onset of cold weather sending more people indoors, hospitals are feeling the strain.
"Delta is not subsiding," said Dr Andre Kalil, an infectious-disease physician at the University of Nebraska Medical Center. On Tuesday, Nebraska reported 555 people in the hospital with Covid-19 — the highest number since last December, when the vaccine rollout was just beginning.
Vermont recorded its highest total since the start of the pandemic: 84. New Hampshire, once an early vaccination leader, is now second only to Michigan in the most new cases per capita over the past two weeks.
In Minnesota, which ranks third for most new cases per capita, the Pentagon sent medical teams last month to two major hospitals to relieve doctors and nurses swamped by Covid-19 patients.
"This fourth wave, I can pretty clearly state, has hit Minnesota harder than any of the previous ones," said Dr Timothy Johnson, president of the Minnesota chapter of the American College of Emergency Physicians.
He said hospitals are struggling because of a combination of a lack of nurses, fatigue and patients undergoing treatments that had to be postponed earlier in the crisis. "Now those chickens are coming home to roost a little bit," he said.
At Hennepin County Medical Center in Minneapolis, where one of the military medical teams was sent, the number of Covid-19 patients has doubled since September, although it remains below pandemic highs, spokeswoman Christine Hill said.
"And it's concerning with the holidays coming up," she said.
Dr Pauline Park, who takes care of critically ill patients at the University of Michigan Health in Ann Arbor, called the latest surge "heartbreaking." One Covid-19 patient, a woman in her 20s, died the week of Thanksgiving. Another, a mother with young children, is on a machine built to take over for her lungs.
Arizona, where students in dozens of classrooms have been forced into quarantine, reported over 3100 new Covid-19 cases Wednesday, numbers similar to the disastrous summer of 2020. Hospital bed space has fallen to pandemic lows.
Bhuyan said a patient of hers with a blood clot in a lung was discharged instead of being admitted. Other patients are waiting hours in the emergency room.
"It's just hard because it does feel like that we are actually going backwards in time, even though we have these vaccines, which are such a great weapon for us," she said.