Amid the COVID-19 pandemic, a new U.S. study serves as a reminder of how severe the seasonal flu can be.
Researchers found that among 90,000 Americans hospitalized with the flu, 12% had a serious heart complication, including heart attack and sudden heart failure. Many ended up in the intensive care unit, and 7% died in the hospital.
The study shows that COVID-19 isn't alone among viruses that can up the odds for heart events.
The findings also underscore the importance of getting the flu shot — especially this year, experts said.
"The flu is not 'just a cold,'" said Dr. Kathryn Edwards, a professor at Vanderbilt University School of Medicine in Nashville. "People often don't appreciate how serious it can be."
A yearly flu shot is always recommended, she said, especially for people older than 65 and those with certain chronic conditions such as heart disease, diabetes and asthma. They are at heightened risk of flu complications.
This season, flu vaccination is particularly important because of the pandemic, Edwards said. That's in part to avoid having hospitals overrun by both flu and COVID patients.
Edwards, who was not involved in the study, is also a spokesperson for the Infectious Diseases Society of America.
While the flu usually causes temporary miseries like fever, cough and a sore throat, it can also lead to complications such as pneumonia and other secondary infections. The infection can also worsen certain chronic health conditions, including heart disease.
But that is not necessarily common knowledge, said Dr. Shikha Garg, the senior researcher on the new study.
"I don't know that these cardiac complications are on people's radar," said Garg, of the U.S. Centers for Disease Control and Prevention. "This study shows just how common they are."
For the study, Garg's team looked at 90,000 flu hospitalizations in 17 U.S. states between 2010 and 2018. Just under 12% of those patients suffered a heart complication.
Not surprisingly, people with existing heart conditions were at greater risk: 21% of those patients had a cardiac complication — as did 19% of patients with kidney disease and 15% of those with diabetes.
Edwards said the flu may directly trigger heart complications, by spurring body-wide inflammation. If, for example, a person has artery-clogging "plaques," that inflammation could rupture the plaque and lead to a heart attack.
Of study patients with heart complications, almost one-third landed in the ICU, and 7% died.
Despite the severity of the flu, Americans frequently skip the annual vaccine. During the 2018-2019 season, the CDC says, between 34% and 56% of adults got the flu shot, with variations by state.
"Everyone age 6 months and older should be vaccinated," Edwards said.
The shot is no guarantee against the flu, she added, but it is typically about 50% effective. And even if it does not prevent the infection entirely, Edwards said, it may reduce the severity of the illness.
"We don't know for sure what's going to happen in the fall," Garg said. But since both the new coronavirus and the flu are likely to be in circulation, she added, Americans should be prepared.
"Now more than ever," Garg said, people should get the flu shot.
It is possible, Edwards noted, that the efforts in place to battle COVID — from social distancing to hand-washing to mask-wearing — will take some of the bite out of flu season. But those things do not take the place of vaccination, she said.
Garg agreed. "The flu vaccine is safe, and it's our most important tool for prevention."
The findings were published Aug. 24 in the Annals of Internal Medicine.