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A post-Thanksgiving uptick in COVID-19 patients at U.S. hospitals is arriving even as health systems contend with waves of feverish, coughing people stricken with RSV and influenza infections.

COVID hospitalizations last week reached their highest level in three months, with more than 35,000 patients being treated, according to Washington Post data tracking. National hospitalizations had stagnated throughout fall but started rising in the days leading up to Thanksgiving. All but a few states reported per capita increases in the past week.

Public health authorities are concerned that the increase in the number of COVID patients will worsen the strain on hospitals already under pressure from the effects of two other viral ailments, influenza and respiratory syncytial virus, widely known as RSV.

Nearly 20,000 Americans were hospitalized with influenza during Thanksgiving week, the most for that week in more than a decade and almost double the previous week’s count.

Nancy Foster of the American Hospital Association said members are still mostly raising concerns about RSV and flu rather than COVID.

“It could be in a week or two we are seeing many more COVID patients than we are seeing RSV or flu, but the real concern is we will see a large influx of all of them really stressing out the hospitals’ capacity to care for these very sick patients,” said Foster, the association’s vice president of quality and patient safety policy.

Experts warn that holiday gatherings are a prime time for the coronavirus to spread as millions of Americans travel and get together. The increase in hospitalizations probably reflects a combination of patients who were infected before the Thanksgiving holiday rush and those exposed during Thanksgiving week, health experts said.

Daily new COVID hospital admissions are now above 9,000 after hovering between 5,000 and 7,000 for much of autumn.

Winter is typically hectic for hospitals, especially in cold-weather states where people are more likely to congregate indoors, providing ample opportunities for respiratory viruses to flourish.

In late 2020 and early 2021, before the widespread availability of coronavirus vaccines, COVID patients put the nation’s hospital wards under enormous strain, with deaths peaking at more than 3,000 a day. And last winter, the explosion of cases fueled by the omicron variant of the coronavirus left hospitals struggling to provide basic care as they endured staffing shortages when the virus tore through their workforces.

This winter, most people in the United States have some degree of immunity because of vaccination, prior infection or both, which should reduce the severity of infections. And those who do become sick have a broader suite of therapeutics to accelerate recovery and keep them out of the hospital.

“If we are going to see a big surge, it’s going to start to ramp up now, and it’s going to extend and probably peak in late December and early January,” said Columbia University epidemiologist Jeffrey Shaman. “The hope will be it will be somewhat mild, of course, and enough boosting and prior exposure is going to keep a large chunk of people out of the hospital.”

At Banner Health, Arizona’s largest health system, COVID hospitalizations have doubled in the past month to nearly 600 patients. They make up fewer than 10% of all patients, compared with nearly 40 percent at earlier stages of the pandemic. But the relatively smaller COVID caseload is balanced out by an early spike in influenza and RSV cases exceeding five-year averages.

“What is happening this winter already and what we can continue to expect is influenza and RSV are not going to be at unprecedented low levels,” said Banner Chief Clinical Officer Marjorie Bessel. “We are going to have a high-volume winter like we have had previously in the pandemic. How much of a high volume due to all this coming together is an unknown.”

For hospitals already strained by other viruses, the coronavirus is an additional complication.

At the Norton Healthcare system in Louisville, coronavirus cases are flat, RSV is declining and flu is rising.

“Some winters are more challenging than others, and I think we are in a more challenging winter this year,” said Norton’s senior vice president and chief clinical and strategy officer, Steven Hester. “We are going to see COVID as part of the regular challenge we have.”

RSV cases have slammed Children’s Hospital of Orange County in California, prompting the county to declare and extend an emergency. The hospital’s emergency department had a record influx of 12,000 patients in November and, at times, had to divert patients to other medical centers. Now, the children’s hospital also is seeing a rise in COVID hospitalizations, although nowhere near to the number of cases related to RSV. Some children arrive infected with multiple viruses.

“It’s forcing us to be organized and thoughtful, and we have to be innovative in space. We have to always seek out supplies,” said Sandip Godambe, the system’s chief medical officer.

Although children’s hospitals and pediatric wings have been facing the greatest strain because of other respiratory viruses, older patients account for most new COVID hospitalizations, according to Department of Health and Human Services data.

Experts say the trend drives home the importance of people 65 and older receiving updated booster shots tailored to the omicron variant. Less than a third of that age cohort has received the latest shot, according to estimates from the Centers for Disease Control and Prevention.

“The principal issue we are grappling with is the waning of immunity,” said Eric Topol, a professor of molecular medicine at Scripps Research Institute. “We are under-vaccinated and under-boosted, particularly in people of advanced age.”

To reduce the strain on hospitals, California’s state epidemiologist said it is essential to increase booster uptake among older people and ensure that medically vulnerable patients are receiving antiviral treatments such as Paxlovid soon after they test positive.

“Even when you have mild or moderate symptoms, if you are over the age of 50 or if you have any sort of medical problems, you can likely benefit from treatment for a few days so you don’t need to go to hospital,” said Erica Pan.

Carmela Coyle, the president and chief executive of the California Hospital Association, said the public should still be concerned about the ability of hospitals to deliver care even if there are fewer coronavirus patients this winter compared with earlier in the pandemic. A health care workforce shortage has left hospitals understaffed, and hospitals are financially strapped nearly three years into the pandemic, she said.

“COVID for hospitals was like a flood,” Coyle said. “And while the floodwaters may have receded if you look at the last two winters compared to where we are now, the damage is still here.”