You’ve Heard of Long COVID. Long Flu Is a Health Risk, Too

Statistically, there’s a good chance you know somebody who has experienced Long COVID, the name for chronic symptoms including fatigue, brain fog, and pain following a case of COVID-19. About 14% of U.S. adults report having had Long COVID at some point, according to federal data.

But many people don’t realize that other viruses, even very common ones, can trigger similarly long-lasting and debilitating symptoms. A study published Dec. 14 in The Lancet Infectious Diseases focuses on the risk of developing "Long flu" after a severe case of influenza.

“We learned from COVID-19 that infections that are initially thought to cause only acute illnesses can cause chronic disease,” says co-author Dr. Ziyad Al-Aly, chief of research and development at the Veterans Affairs St. Louis Health Care System and a clinical epidemiologist at Washington University in St. Louis. That's true of influenza as well, the new research shows.

Al-Aly and his colleagues used records from the U.S. Department of Veterans Affairs to compare the long-term health outcomes of about 11,000 people hospitalized with influenza from 2015 to 2019 with those of about 81,000 people hospitalized with COVID-19 from 2020 to 2022. The researchers tracked how many people went on to develop any of 94 health risks associated with the two viruses in the year and a half after they were hospitalized.

Relative to influenza survivors, people who'd had COVID-19 were at increased risk of 64 of the identified complications, including fatigue, mental-health problems, and pulmonary, gastrointestinal, and heart issue. They were also more likely to die during the study period, in keeping with other research comparing the long-term outcomes of the two illnesses.

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Influenza survivors, meanwhile, were at increased risk of only six health problems, most of them related to the respiratory and cardiovascular systems.

“COVID is still more serious than the flu,” Al-Aly says, noting that it affects numerous organ systems while influenza is mostly a respiratory illness. But long-term health problems were common in both groups during the 18 months of follow-up. The researchers recorded about 615 health issues per 100 people in the COVID-19 group, compared to about 537 per 100 people in the influenza group. Those numbers reflect the fact that some people experience numerous chronic symptoms after an infection, Al-Aly says.

It’s important to note that the people included in the study were all hospitalized, meaning they were very sick during the acute phases of their illnesses. The study population was also overwhelmingly male and older, with an average participant age of 70. Older adults are known to be vulnerable to the worst effects of both COVID-19 and the flu. Therefore, the results may not translate to the entire population.

That said, previous research shows that even milder illnesses can lead to long-lasting health issues. Influenza, as well as other common viruses like Epstein-Barr (which causes mononucleosis), have long been thought to trigger myalgic encephalomyelitis/chronic fatigue syndrome, a post-infectious illness that shares many symptoms with Long COVID.

Newer research also underscores the potential effects of even routine illnesses. In a study published in October, researchers tracked people who had gotten sick with one of a variety of respiratory diseases, including COVID-19, the flu, and the common cold, and found that lingering symptoms popped up in each group. Similarly, in one 2022 study of people with various respiratory diseases, most of whom were not hospitalized, about half of people sick with something other than COVID-19 reported ongoing issues three months later. A 2021 study looking at both hospitalized and non-hospitalized patients also found that about 40% of people with influenza experienced at least one symptom commonly associated with Long COVID in the six months after their illnesses.

Taken together, Al-Aly says, these findings suggest that we need to start thinking of viruses differently. “Before the pandemic, I trivialized infections. [I thought] you get sick for a day or two or three, and then you bounce back, and it’s all over,” Al-Aly says. But as a growing number of studies show that’s not always the case, he has concluded that “infections deserve respect.”

That means doing whatever possible to avoid catching and spreading them, he says, including by wearing a mask during periods of heavy transmission, getting all recommended vaccines, and staying home when you feel unwell.