• The number of patients with Covid-19 in U.S. hospitals surpassed last winter's peak over the weekend and the U.S. reported a single-day record of 1.5 million Covid cases on Monday.
• A larger share of patients appear to be entering hospitals for something other than Covid and testing positive once they're in a facility.
• Even if the omicron variant causes less severe disease, U.S. hospitals can still be strained due to the high volume of patients and staffing shortages.

The number of patients with Covid-19 in U.S. hospitals surpassed last winter's peak over the weekend and the country reported another single-day record of nearly 1.5 million new cases on Monday, two grim milestones as the nation's health system grapples with the extremely contagious omicron variant. 

There were 144,441 Americans hospitalized with the virus as of Sunday, above the prior high mark of 142,315 patients recorded about a year ago on Jan. 14, according to data tracked by the Department of Health and Human Services, and the count has climbed to 147,000 as of Tuesday.

The country also reported roughly 1.5 million new cases on Monday, according to data compiled by Johns Hopkins University, pushing the seven-day average to 754,000 new cases per day.

To be sure, a large portion of Covid hospitalizations appear to stem from people admitted for other reasons who test positive for the virus once they're in a facility. And while hospitalizations are the highest on record, HHS didn't start collecting the data until August 2020 so it doesn't capture the first early surge of cases that spring.

The daily tally of confirmed infections is also likely artificially high since many states report their weekend Covid testing data on Mondays.

Miami Mayor Francis Suarez told CNBC's "Squawk on the Street" on Monday that about half of the city's hospitalizations are people hospitalized with Covid as opposed to for Covid, for example, and a Monday press release from the New York state Department of Health reported 42% of the state's hospitalized patients were admitted for something other than Covid. National data isn't available since most states don't track that level of detail in their cases.

The number of cases are also likely being undercounted due to the availability of at-home test kits for which results are typically not reported to state or federal agencies.

White House chief medical advisor Dr. Anthony Fauci said last week that a growing body of evidence indicates the Covid omicron variant is less severe than the delta strain. More data is needed to confirm that, he said, cautioning that the sheer volume of infections and hospitalized people could still strain hospital systems. 

"A certain proportion of a large volume of cases, no matter what, are going to be severe," Fauci said. "So don't take this as a signal that we can pull back from the recommendations."

Infections are on the rise in nearly every part of the country and average daily case counts are at record highs in 28 states as of Monday. Fifteen states and the District of Columbia are reporting a record level of current hospitalizations, according to a CNBC analysis of HHS data that dates back to the summer of 2020.

"There is a lot of infection around the country right now, and, at the end of this, probably 30% to 40% of the U.S. population will have been infected by omicron," former FDA commissioner, Pfizer board member and CNBC contributor Scott Gottlieb, told CNBC's "Squawk Box" on Tuesday.

The stress on the health system is compounded by the fact that many hospitals are short-staffed due to labor shortages or health-care workers being forced to quarantine after getting Covid themselves.

"The challenge, and this existed going into this surge as well, is staffing," Louisiana State Health Officer Dr. Joseph Kanter said on the program "Full Court Press with Greta Van Susteren" on Sunday. 

"It's so hard to retain staff for good reasons. It's so hard to recruit new staff," he added. "That's the biggest limiting factor for our hospitals. It's not physical beds, it's not ventilators, it's not PPE right now. It's just keeping enough qualified staff on board."

Illnesses among employees are impacting industries beyond health care. U.S. airlines canceled thousands of flights over the holidays through early this year due to Covid infections among crews and a series of winter storms. United Airlines is trimming its schedule to address a surge in sick calls among employees, CEO Scott Kirby told employees.

United has about 3,000 workers who are currently positive for Covid, Kirby said in a staff memo published Monday, some 4% of its U.S. workforce.

U.S. health officials have warned that the most significant risk from Covid remains for those who are not vaccinated. Roughly 63% of Americans are fully immunized, Centers for Disease Control and Prevention data shows, and 36% of those fully vaccinated have received a booster dose. 

"It is a very wily virus," Fauci told lawmakers at a hearing Tuesday. "It has fooled everybody all the time — from the time it first came in to delta to now omicron — it's very unpredictable and we're doing the best we possibly can."

The U.S. is reporting a seven-day average of about 1,650 Covid deaths per day, according to Johns Hopkins, which is on the rise but roughly half of the peak levels seen at this time last year, before vaccines were widely available.

Disclosure: Scott Gottlieb is a CNBC contributor and is a member of the boards of Pfizer, genetic testing start-up Tempus, health-care tech company Aetion and biotech company Illumina. He also serves as co-chair of Norwegian Cruise Line Holdings′ and Royal Caribbean's "Healthy Sail Panel."

 — CNBC's Jessica Bursztynsky, Spencer Kimball and Leslie Josephs contributed to this article.