In hospitals around the country, doctors are taking notice: This wave of Covid seems different from the last one.

Once again, as they face the highly contagious Omicron variant, medical personnel are exhausted and are contracting the virus themselves. And the numbers of patients entering hospitals with the variant are surging to staggering levels, filling up badly needed beds, delaying nonemergency procedures and increasing the risk that vulnerable uninfected patients will catch the virus.

But in Omicron hot spots from New York to Florida to Texas, a smaller proportion of those patients are landing in intensive care units or requiring mechanical ventilation, doctors said. And many — roughly 50 to 65 percent of admissions in some New York hospitals — show up at the hospital for other ailments and then test positive for the virus.

“We are seeing an increase in the number of hospitalizations,” said Dr. Rahul Sharma, emergency physician in chief for NewYork-Presbyterian/Weill Cornell hospital. But the severity of the disease looks different from previous waves, he said. “We’re not sending as many patients to the I.C.U., we’re not intubating as many patients, and actually, most of our patients that are coming to the emergency department that do test positive are actually being discharged.”

Though it’s still early for firm predictions, the shift in hospital patterns fits with emerging data that Omicron may be a variant with inherently milder effects than those that have come before, less prone to infecting the lungs, where it can cause serious disease. But the lower proportion of severe cases is also happening because, compared with previous variants, Omicron is infecting more people who have some prior immunity, whether through prior infection or vaccination. The vast majority of Omicron patients in I.C.U.s are unvaccinated or have severely compromised immune systems, doctors said.

Hospitals, facing staff shortages, are under enormous strain. In New York City, hospitalizations have exceeded the peak of last winter’s surge. And Governor Larry Hogan of Maryland declared a state of emergency on Tuesday, noting that the state had more hospitalized Covid-19 patients at that time than at any previous point during the pandemic.

“We’re in truly crushed mode,” said Dr. Gabe Kelen, director of the Johns Hopkins University School of Medicine’s emergency department.

The number of I.C.U. patients is a lagging indicator, likely to rise in the coming weeks, experts said. What’s more, some states are still struggling under the crush of hospitalizations from Delta, a previous version of the virus that may be more virulent. (Hospitals are frequently in the dark about which variant newly admitted patients are infected with.)

Still, several reports suggest that Omicron is a foe different from the variants that came before. And the challenges hospitals face — at least so far — are less about stockpiling equipment and more about staffing and contagion, doctors said.

“Early on in the pandemic, we were worried about running out of things, like ventilators,” said Dr. Ryan Maves, an infectious disease and critical care physician at the Wake Forest School of Medicine in Winston-Salem, N.C. “Now, the real limitations are obviously physical bed space, but even more so, it’s staffing.”

When reports emerged in early December that hospitals in South Africa were handling relatively few severe Omicron cases, experts stressed that the findings should be interpreted with caution. South Africa has a relatively young population, and a large proportion had been infected by previous waves, leaving the affected people with some pre-existing immunity.

But now that the virus has spread across the world and the United States, there is more evidence that many people who have been infected with Omicron in recent weeks seem to be faring better than those who were infected with other variants or during earlier surges.

In Britain, people with Omicron were about half as likely to require hospital care and one-third as likely to be admitted to the hospital from emergency rooms as those infected with Delta, according to a government report released last week. Early reports from Canada suggest a similar pattern.

And a new report from the Houston Methodist health care system, which has been sequencing the vast majority of viral samples from its patients since February 2020, found broadly the same thing.

By Dec. 20, the new variant was causing more than 90 percent of new Covid cases at Houston Methodist. In the new analysis, researchers compared 1,313 symptomatic patients who had been infected with Omicron by that date to Houston Methodist patients who had been infected with the Delta or Alpha variants beginning earlier in the pandemic.

The numbers of Omicron cases examined in Houston are small, and it takes time for the worst outcomes to manifest. But fewer than 15 percent of those early Omicron patients were hospitalized, compared with 43 percent of the Delta patients and 55 percent of the Alpha patients, the study found.

Among those who were admitted, Omicron patients were also less likely to require mechanical ventilation and had shorter hospital stays than did those infected with the other variants.

“On average — and I’m stressing on average — the Omicron cases are less severe,” said Dr. James Musser, the chair of pathology and genomic medicine at Houston Methodist, who led the research. He added, “And that’s obviously good news for our patients.”

The Omicron patients were also younger, and more likely to be vaccinated, than were those with previous variants, which may partially account for the milder illness.

Although the reports are encouraging, it is still too early, and there is not yet enough detailed data, to draw firm conclusions about Omicron’s inherent severity, said Natalie Dean, a biostatistician at Emory University in Atlanta.

“There hasn’t been really quite enough time,” Dr. Dean said. It took months for numerous large studies of Delta’s hospitalization risks to appear.

In New York City, cases have been steadily rising since December and are now overwhelmingly accounted for by Omicron. Covid hospitalizations have also increased sharply, and I.C.U. admissions have been rising more slowly.

At New York University’s Langone Health, for example, around 65 percent of patients admitted with Covid were “incidentally” found to have the virus, and their hospitalizations were not primarily because of the illness. At NewYork-Presbyterian, just under half of Covid admissions were incidental.

Hospitals in other cities have also been reporting higher rates of incidental infections. Across Jackson Health System hospitals in Florida, 53 percent of the 471 patients with Covid were admitted to the hospital primarily for other reasons. At Johns Hopkins Medicine in Maryland, 20 percent of patients seeking treatment for non-Covid complaints are testing positive for infections, said Dr. Kelen of Johns Hopkins.

Incidental infections can still pose significant risks for people who are hospitalized for other health problems. And the high number of hospitalized patients with asymptomatic Covid presents an additional challenge for infection control.

“You still need to put them in isolation,” said Dr. Carlos del Rio, an infectious disease specialist at the Emory University School of Medicine. “You still need to treat them as patients who could potentially transmit Covid in the hospital. And when you have less staff, then you really have a problem.”

At NYU Langone, intensive care admissions are 58 percent lower among people hospitalized for Covid than they were in January 2021, said a spokeswoman, Lisa Greiner. At Mount Sinai South Nassau, doctors are also seeing fewer patients requiring critical care compared with previous peaks, but the sheer number of cases means that there are higher numbers of people getting very sick than in recent months.

“I would say on the whole, the illness is less severe,” said Dr. Aaron Glatt, chief of infectious diseases and epidemiologist at the hospital. But, he added, “We have had deaths from Covid, which we haven’t seen in a long time. And we’re seeing patients in the I.C.U. and on ventilators, which we haven’t had in a long time.”

The majority of people who are going into I.C.U.s are unvaccinated or are vaccinated people who are in higher risk groups, experts said. And among people who make it into I.C.U.s, cases can still be as severe as with prior variants.

The increase in hospitalizations has put further stress on overburdened hospitals.

Many hospitals were already struggling with staffing shortages before Omicron emerged. Even when hospital beds are available, an exodus of health care professionals over the course of the pandemic has made it more difficult to deliver care.

“There’s just no capacity,” said Dr. Megan Ranney, an emergency physician and the academic dean of Brown University’s School of Public Health. “There’s not adequate staff for the beds that exist.”

The rise in hospitalized Covid cases has happened alongside a rise in hospitalizations for other conditions, said Dr. Sharma of NewYork-Presbyterian/Weill Cornell, putting further strain on hospitals. At the peak of the pandemic in 2020, those admissions plummeted as people without Covid avoided hospitals.

“People aren’t scared to come to the hospitals like they were in 2020. Our volumes in our E.R.s are almost back to, if not above, prepandemic numbers,” Dr. Sharma said. “That means that we’re busy — capacity becomes an increased challenge.”

The staff shortages are leading many hospitals to consider reducing elective surgeries.

“We’re never going to crowd out strokes, and we’re never going to crowd out heart attacks,” said Ed Jimenez, chief executive of the University of Florida Health Shands hospital system. “But if this keeps going the way it’s going, we’re going to have hospitals that have to start considering slowing down their planned admissions.”

At Grady Hospital in Atlanta, “we’re hoping not to cancel elective surgeries, but we’ve considered it,” Dr. del Rio said. “The reality is that we’re finding that some of these elective surgeries are canceling themselves because people are coming in and testing positive for Covid.”

It has been about six weeks since the world first learned about Omicron, and hospital personnel are still waiting nervously to see how the coming weeks unfold.

As of Tuesday morning, Houston Methodist had 630 inpatients with the virus across its eight hospitals, Dr. Musser said, the vast majority of whom most likely have Omicron. That figure remains below the system’s Delta peak — in which there were between 850 and 900 inpatients with the virus at once — but the numbers of new cases are still rising, he said.

“How high will it go?” he said. “Can’t tell you. Don’t know. We’re all watching it, obviously, very, very closely.”

Gina Kolata contributed reporting.