The seasonal flu has returned to Minnesota, presenting clinics and hospitals with sick patients when COVID-19 is creating stress on the health system.

One year ago, the flu was virtually nonexistent in the state as well as most regions of the globe, as travel restrictions and COVID-19 prevention measures helped tamp down what is typically a season of fever, chills, muscles aches and even serious complications like pneumonia.

Although flu infections are not yet swamping the system, health officials report a noticeable uptick in cases that began in early December and continues to escalate.

"We are seeing people sick enough with the flu that they are in our emergency rooms and our hospitals already which is a huge change from what we saw last year," said Dr. Hannah Lichtsinn, a Hennepin Healthcare physician who specializes in internal medicine and pediatrics.

Holiday gatherings sometimes serve as incubators for the flu virus as travelers can import the disease from parts of the country with higher rates of illness, which currently include North Dakota, Indiana, Nebraska, New Mexico and New Jersey, according to the Centers for Disease Control and Prevention.

Flu vaccinations in Minnesota are below last year's levels, providing fertile ground for infections to develop. Overall, flu vaccination rates are down nearly 15%, while for children it is 24% lower than the same time last year.

Ten weeks into the flu season, Minnesota has recorded five flu-related deaths, including one death of a child. By comparison, there were seven influenza deaths during last year's flu season, which typically runs from October to May.

In the week before Christmas, 39 patients were admitted to Minnesota hospitals with flu-related complications, bringing the season's total to 87, more than double the 35 hospitalizations recorded last year. The Minnesota Department of Health is set to release updated numbers on Thursday.

"I do think we are in for more weeks of cases," said Jeff Sanders, an epidemiologist with the state Health Department who tracks flu activity.

Hennepin Healthcare went from finding 11 flu cases through testing in November to nearly 200 in December.

Allina Health also is among systems seeing increased flu activity.

"We saw a sharp increase in influenza cases at our clinics and hospitals the beginning of December," spokeswoman Conny Bergerson said.

At Mayo Clinic, the testing positivity rate for the flu has been doubling each week, going from 2% in early December to 15% last week.

"We are seeing a pretty large surge in influenza cases," said Matthew Binnicker, who specializes in respiratory viral infections at Mayo. "Over the last three weeks the number of flu cases in the state has increased pretty substantially."

For now, flu-related hospitalizations are below previous flu seasons that preceded last year, but Minnesota is seeing strains that tend "to be a little bit more associated with more severe disease in older adults and young children," Sanders said.

Public health officials hope to avoid a situation in which hospitals are swarmed with flu patients as well as COVID-19 patients. It is unclear how the fast-spreading omicron variant will affect patient admissions.

Hospitals have been struggling for months with high patient demand and a worker shortage that combined have led to full intensive care units and patients waiting in emergency rooms for hours before they can get admitted.

Before the COVID-19 pandemic, flu seasons often put pressure on hospitals.

"A typical flu season on its own with a fully staffed hospital system is enough to put us off the edge typically with the number of people who are seeking and needing hospitalization and care," Lichtsinn said.

The state's hospitals already faced bed shortages and busy emergency rooms earlier this year because of a surprising number of RSV cases, or respiratory syncytial virus, an upper respiratory infection that more typically emerges in the winter.

As of Tuesday, 97% of the state's adult ICU beds were occupied while 94% of adult non-ICU beds were filled. Pediatric bed space was also tight.

"One of the differences between influenza and COVID is that influenza is really dangerous for young kids, especially kids under the age of 1 are susceptible to illness from influenza," Lichstinn said. The elderly and pregnant also are at high risk, she said.

There were 24 school outbreaks of the flu and other flu-like illnesses for the week ending Dec. 18, outpacing the 2017-18 season, which was the most deadly among the last five seasons. There have been 87 outbreaks in schools this season.

Long-term care facilities largely have been spared, with nine outbreaks reported to the state, including three most recently.

Public health officials are also monitoring whether this year's flu vaccine is a good match for the circulating strains amid some reports that parts of the vaccine formula might not provide adequate protection. Typically, vaccine effectiveness ranges between 40 and 60%, but medical professionals say that the vaccine tends to prevent serious illness even if it cannot prevent infection.

Some flu seasons peak in January, while others see the most activity in March.

"There is lots of unpredictability," Sanders said.