Omicron isn't so mild:

Regina Perez, 57, had never been hospitalized for her lifelong asthma condition until she came down with Covid this month.

She started having difficulty breathing, even after taking her usual medications. “It kind of took over, almost,” she said. She wound up at St. Luke’s Hospital in Allentown, Pa., for most of a week at a time when nearly all the Covid patients sampled had contracted the Omicron variant.

The episode frightened her. While doctors were able to get her asthma and breathing under control, “I’ll probably be scared for the rest of my life,” she said. Ms. Perez, who was fully vaccinated and is now recovering at home, said she had spent the last two years doing everything she could to avoid infection, including working from home and rarely going out. She has not yet gotten a booster shot.

Throughout the pandemic, people like Ms. Perez have been at higher risk for serious illness from Covid because they have underlying medical conditions, like asthma, diabetes, heart or lung disease. More than half of American adults have at least one underlying chronic condition, and for many of them, the Omicron wave hasn’t been as mild as it has for the larger, healthier populations around the world.

Omicron has indeed caused far lower rates of severe illness and death in the U.S. population, especially among those who are vaccinated and have received booster shots. Still, the variant’s high transmissibility did lead to record-setting case counts that resulted in pandemic-high hospitalizations.

Dr. Rochelle Walensky, the director for the Centers for Disease Control and Prevention, warned on Wednesday that this surge was still imposing a heavy burden. “Importantly, ‘milder’ does not mean ‘mild,’” she said. “And we cannot look past the strain on our health systems and substantial number of deaths — nearing 2,200 a day as a result of the extremely transmissible Omicron variant.”

In the last few weeks, the rate of hospitalization has declined considerably in some regions, where Omicron first arrived and sent case counts soaring. While a smaller share of people with the variant are being hospitalized, according to a recent report from researchers at the C.D.C., the soaring number of Omicron infections has led to higher admissions than in previous surges. Nationally, hospitalizations are still averaging about 150,000 people, including many rural regions where facilities are stretched thin.

“Our experience is that it’s worse right now than it’s ever been,” said Craig Thompson, chief executive of Golden Valley Memorial Healthcare, a small rural hospital in Clinton, Mo. This month, the Covid, heart attack and stroke patients that the hospital would typically transfer to larger facilities were boarded in the emergency room for days. Staff members made about 200 phone calls to get beds for patients — at times up to 400 miles away.

The majority of those hospitalized with severe illness during the Omicron surge are unvaccinated, public health experts say. But some who were vaccinated and have underlying conditions have also been at risk for more serious illness caused by the virus, and for the infection potentially worsening their existing diseases, increasing their chances of hospitalization.

While they may not be hospitalized for respiratory illnesses, “we are seeing some exacerbation of other conditions in individuals who are vulnerable,” said Dr. Sandra Nelson, an infectious disease specialist at Massachusetts General Hospital and an assistant professor of medicine at Harvard Medical School. In some cases, patients were dehydrated from the effects of a virus infection and came in with kidney failure.

Doctors say that it is not always clear what role Omicron plays, but there is a plausible biological explanation for a virus causing patients to develop systemic issues. “You’re going to see kidneys get worse, etc.,” said Dr. Panagis Galiatsatos, a critical care specialist and assistant professor of medicine at Johns Hopkins Medicine.

At St. Luke’s, where Ms. Perez was admitted, roughly two-thirds of the coronavirus-positive patients in the system’s network had a primary diagnosis of Covid, but an additional 15 to 20 percent were diagnosed with other illnesses, like sepsis or acute kidney failure, that doctors said were clearly related to a virus infection.

“It isn’t an incidental diagnosis,” said Dr. Jeffrey Jahre, an infectious disease specialist who is senior vice president for medical and academic affairs at St. Luke’s University Health Network, which operates 11 hospitals in Pennsylvania and New Jersey.

In some cases, these patients may have had a “smoldering” case of diabetes or hypertension that a Covid case pushed over into serious illness, said Dr. Nicholas Kman, an emergency physician at the Ohio State University Wexner Medical Center in Columbus. In other cases, individuals who had successfully been managing their conditions before becoming infected, like Ms. Perez, are coming in with high blood sugar levels or worrying hypertension.

Others, like transplant or cancer patients — although fully vaccinated — are not able to mount a sufficient immune response to protect themselves from serious disease when they become infected.

Doctors say these admissions — often categorized as “with” Covid rather than “for” Covid — have had significant effects on stressed hospitals. “All those patients add to the surge and the volume,” Dr. Kman said, adding that “one or two extra patients can push a health system over the edge.”

Hospitals at maximum capacity have also been dealing with nationwide shortages of basic supplies that are needed to care for patients with complicated conditions, including intravenous bags of saline solution, small syringes of saline solution and small plastic tubes to take blood samples.

Frontline nurses, already parceling out a few minutes an hour to each patient under their care, say juggling unfamiliar products or adjusting to workarounds makes their jobs even more fraught.

The caseloads have had a rippling effect, far more pronounced in this wave than in others. Severe staffing shortages at nursing homes and dialysis clinics have made it difficult to discharge patients from the hospital who were still positive for the coronavirus, said Dr. David Margolius, an internal medicine specialist at MetroHealth in Cleveland. Some facilities do not accept Covid patients, and others have been so short-staffed that there are no openings.

“With Covid, for Covid, it’s putting so much stress on the health care system because of the implications of having Covid for placement,” he said.

Other patients at high risk — including pregnant women — have also become seriously ill. Alex Chandler, 27, a teacher in Killeen, Texas, who was vaccinated and had received a booster shot, was diagnosed with Covid when she gave birth on Jan. 9, according to her mother, Jenny Clay. That week, Omicron made up 99.7 percent of the Covid cases in Texas and surrounding states, federal data show.

Initially her throat felt as if she had swallowed broken glass, and her chills were hard to shake. But her symptoms gave way to the consuming care of her firstborn child, Beau.

Five days after giving birth, Ms. Chandler sought follow-up care for her son, and staff members noticed that she was breathing heavily, Ms. Clay said. Her oxygen saturation read 76 percent, far lower than the typical 95 to 100 percent.

A C.D.C. spokeswoman, Belsie González, said that women have higher heart rates, lower lung capacity and immune system changes during pregnancy. Nearly all of the pregnant women admitted to critical care in Europe were unvaccinated, according to a study published Friday.

Her mother said she was admitted to the hospital on Jan. 14, and developed pneumonia and a pneumothorax, or punctured lung, a known Covid-19 complication. By the following morning, she had been put on a ventilator, and she is in the intensive care unit at AdventHealth Central Texas in Killeen.

Ms. Clay has been helping care for her grandson. She noticed that he has his mother’s eyes and like his mother as a newborn, he eats well and rarely cries.

“I’m just thinking, ‘His mom should be here and sharing this with me,’” Ms. Clay said. She documented the first precious days, planning to share with her daughter later. At nearly two weeks on the ventilator, Ms. Clay said, her daughter showed some signs of progress but then lost ground. “They said when it’s time, we’ll know,” Ms. Clay said. “There’s not really a lot of hope right now.”

Others who have trouble mounting an immune response are among those hospitalized in this Omicron wave. Dr. Craig Bunnell, the chief medical officer at the Dana-Farber Cancer Institute in Boston, said the strikingly high number of infections — even with Omicron’s reduced virulence — had led to the increased rate of hospitalizations among cancer patients. “The I.C.U.s are still full,” Dr. Bunnell said. “The beds are still full.”

He said the newly available treatments, including monoclonal antibodies and antiviral pills, remained in very short supply. Those who are getting very sick resemble the patients from previous waves.

Some of the patients seen by Dr. Natalia Solenkova, an intensive care physician who works at hospitals in Florida and Tennessee, are organ transplant recipients who she said “did what they were supposed to do” by getting vaccines and booster shots. They are typically on immune-suppressing drugs that keep their bodies from rejecting a donor organ, which makes them highly vulnerable to Covid.

“They are very sick,” Dr. Solenkova said. “Many are on ventilators and on life support and they are dying.”

Amanda Halks, 36, of Tampa, Fla., who was vaccinated and had a booster, had succeeded in avoiding Covid until the last day of December, when Omicron was responsible for 95 percent of the cases in Florida. She had survived a difficult eight-day hospitalization with pneumonia in 2011 and feared any residual damage would make her susceptible to a severe case.

Her fears came to pass on Jan. 11, when she was admitted to a Tampa hospital with her blood oxygen level just below 80. While her hospital stay was brief, her oxygen levels have continued to drop with mild exertion. She was diagnosed with post-Covid MIS-A, or multisystem inflammatory syndrome.

She bristles at the notion that the Omicron variant is mild. She recounted joking with a friend that it is “hot and spicy.”

Dr. Mark Lewis, a cancer specialist at Intermountain Healthcare in Salt Lake City, said he had found that Omicron presents in patients as “a completely different beast.” While the variant appears less likely to descend into a patient’s lungs and cause pneumonia, it may still result in a patient’s condition flaring up. “It’s a profoundly inflammatory state,” Dr. Lewis said.

Ms. Clay said her daughter’s precarious condition was a clear sign that Omicron is not always mild, a notion she views as dismissive and stemming from those fatigued by the lengthy pandemic.

“I know we all want to be done, but it’s not time to be done,” Ms. Clay said. “This is serious and it’s not over.”