How dangerous is flu? Symptoms and signs of complications to know

The U.S. is experiencing the highest number of flu hospitalizations in a decade, and there’s no sign that the virus is going to peak or go away in the coming weeks. With the busy holiday travel season approaching and a comparatively low percentage of adults vaccinated for flu, that leaves millions at potential risk for severe complications.

While most people who get flu will recover in a few days, some can develop life-threatening complications. There have been at least 7,300 deaths from flu, including 21 children, since October, according to the latest data from the Centers for Disease Control and Prevention. 

Approximately 120,000 people have been hospitalized for the flu in the last couple of months, the CDC reported. Nine out of 10 adults hospitalized with flu had at least one underlying medical condition.

People most at risk of serious illness from the flu include children under age 5, people over age 65, immunocompromised people and people who are pregnant.

People with complications of the flu end up in the hospital most often because the virus develops into pneumonia, said Dr. Peter Chin-Hong, professor of medicine and an infectious disease physician at the University of California, San Francisco. 

Bacterial pneumonia may develop when the flu virus spreads to the lower respiratory tract, leading to breathing difficulties that may require supplemental oxygen.

Warning signs of pneumonia 

While many of the symptoms of influenza, such as fever and body aches, overlap with pneumonia, there are some clues that may indicate a more serious lung infection may be brewing. 

A cough with yellow or green sputum, increasing fever, and pain in the chest when taking a deep breath or coughing are warning signs of pneumonia.

Some patients with pneumonia may also go on to develop sepsis, a complication that could lead to organ failure and death, especially if not treated promptly. 

Pneumonia from the flu is not limited to just older people, said Dr. Jonathan Grein, an infectious disease physician and director of hospital epidemiology at Cedars-Sinai Medical Center in Los Angeles. 

“Unfortunately, we do sometimes see young healthy people presenting with really severe pneumonia as well,” Grein said. “It can occur in anybody.”

Influenza and pneumonia combined are the ninth leading cause of death in the U.S., killing tens of thousands of people a year, according to the CDC.

Flu virus can raise heart attack risk

A bout of flu is also associated with increased risk of heart attack, as well as rare cases of inflammation in the brain or muscles.

“Flu can cause all of these non-lung problems that people don’t usually think about,” Chin-Hong said.

A 2018 study found patients were six times more likely to experience a heart attack the week after influenza infection than they are at any point during the year prior, or the year after the infection.

Chin-Hong said he has treated people with flu who developed encephalitis — a dangerous inflammation of the brain that can be triggered by viral infections — or myositis, which causes a painful weakening of the muscles.  

Being sick with the flu can also aggravate flare-ups of chronic diseases such as diabetes or asthma, Grein said.

“The flu triggers an immune response in your body to help fight off that infection, but sometimes that response can be a little overwhelming,” he said. “In a patient with diabetes, their blood sugars increase, or you can see patients with underlying lung disease, if they get infected with influenza, that could make their breathing more difficult.”

Pregnant with flu

One of the most at-risk groups for flu complications includes pregnant people, in part because of the weakening of immune systems during pregnancy, said infectious diseases expert Dr. Carlos del Rio, an executive associate dean at the Emory University School of Medicine and Grady Health System in Atlanta.

Even for a healthy woman, changes to the heart and lung functions during pregnancy can make them more likely to get severely ill from flu. Despite this, many patients remain unvaccinated during their pregnancy. 

A recent CDC survey found that only half of all pregnant women got their flu vaccines as recommended, leaving many at risk for severe disease from the flu. 

“The most severe complication is respiratory failure, but there are other complications, such as inflammation of the heart,” del Rio said.

He worries most about respiratory failure, which can require intubation in the intensive care unit.

And there is a possibility that a harsh flu infection could lead to “long flu.” Similar to Covid, there is concern that those infected with the flu may have lingering long-term effects as well. Akiko Iwasaki, professor of immunobiology at Yale University, told NBC News in November that after the flu, it’s not unheard of to experience symptoms, especially lingering fatigue and brain fog. 

More research is needed to understand the scope of the problem, however. 

Is it worth it to get a flu shot? 

While the flu shots are not perfect, getting vaccinated is the best way to help prevent these complications, experts agree. Full protection takes about two weeks, and although it’s still possible to become infected with the flu after getting the shot, you are less likely to get severely ill because of that added protection.

While there isn’t data yet on how effective this year’s flu vaccine is, it appears to be a “very good match” to circulating strains, Dr. Rochelle Walensky, director of the CDC, said in a briefing last week. 

Even after having a diagnosed case of the flu, getting the vaccine is still important, because it provides broader protection against different strains of influenza virus. It’s possible to catch flu more than once a season, experts say. 

The current vaccine protects against four strains: two influenza A strains and two influenza B strains.

“It’s not too late to get the flu shot,” Chin-Hong said. “We don’t know when flu season is going to end, and we saw a very long tail into the spring last year.”