Emergency Room nurses and EMTs tend to patients in hallways at the Houston Methodist The Woodlands Hospital on August 18, 2021 in Houston, Texas.Brandon Bell | Getty ImagesFlu cases are on the rise in the United States. That could put pressure on hospitals already battling a surge of Covid.The so-called “twindemic” — what public health experts have dubbed the combination of influenza and Covid — could accelerate during holiday gatherings this week.”The most significant problem we are facing is a complete overwhelm of our health care capacity,” said Shira Shafir, an epidemiology professor at the University of California, Los Angeles.”When we have Covid transmitting, plus flu, plus all the other things for which someone might need to go to the hospital … we potentially no longer have the ability to provide care to all of those individuals,” Shafir added.Hospital admissions for the flu are up nearly 34% over the past week, reaching a seven-day average of nearly 250 per day as of Thursday, according to data from the U.S. Department of Health and Human Services. Average daily admissions never topped 125 last flu season.About 8,400 patients with Covid have been admitted to U.S. hospitals each day on average over the past week, the data shows, up 5% from a week ago but lower than the delta wave’s peak level of more than 12,000 daily admissions over the summer.Last year, the U.S. had virtually no flu season as public health protocols to slow the transmission of the coronavirus also prevented the spread of influenza, Shafir said. Flu cases reached an all-time low last season, according to the Centers for Disease Control and Prevention.Flu season came back as U.S. officials have relaxed restrictions such as mask-wearing and limited gatherings with Covid vaccines available this year.That means Americans may have reduced immunity against influenza, the CDC said earlier this fall.Scientists design flu vaccines each year to match which strains they predict will be most common. Less data from last year means this year’s shot is a poor match, scientists have said.”We do expect that people will have less protection against the flu than they would have in a normal flu situation because we did not have a flu season last year,” Shafir said. “That does leave people particularly vulnerable to influenza this year.”About 48% of U.S. adults have received a flu vaccine, according to a CDC survey conducted between Dec. 2 and Dec. 13. Another 9.5% said they plan to get it.Projected total vaccine coverage for the 2021-22 flu season is 57.2%, the agency said, up 3.4 percentage points from the reported vaccination rate last season.”The concerning part is the immunization rates against flu for this time of year are significantly down from where they usually are this time of year, as we move into this holiday gathering period,” said Dr. Jim Conway, medical director for the immunization program at the University of Wisconsin’s School of Medicine and Public Health.Holiday gatheringsThe holidays could exacerbate the spread of influenza as individuals experiencing symptoms who test negative for Covid may proceed with gatherings and transmit the flu, according to Shafir.”Even if someone has a negative Covid test, they still could have something else that they could transmit to the people in their lives,” Shafir said.The epidemiologist recommended individuals experiencing symptoms like a cough, sore throat or runny nose stay at home. Mask wearing, ventilation and, if possible, gathering outdoors are strategies that could help slow spread, she said.Health officials also recommend Covid vaccines and booster doses, as well as the flu shot to help protect individuals. The rapidly spreading omicron variant of Covid has propelled the surge in cases, but research has shown that it is not as severe as other variants. Vaccines and boosters offer heightened protection, studies have shown.