'Long flu' is real, and we've likely 'ignored it for a long time'

People hospitalized for either COVID-19 or the flu face a heightened risk of medical issues, hospital admission and death in the 18 months after they're discharged.

That's according to a new study, published Thursday (Dec. 14) in the journal The Lancet Infectious Diseases, that compared the long-term outcomes for patients sickened with either viral infection. 

Awareness around long COVID — long-term symptoms that persist or newly emerge months to years after an initial COVID-19 infection — has steadily grown since the coronavirus SARS-CoV-2 first appeared. But the new study highlights another significant public health burden: what the study authors informally call "long flu."

Historically, people have thought of viral illnesses like influenza as mostly transient, or "acute," sicknesses. But that idea doesn't account for the impacts that sometimes linger in the "post-acute" phase, said senior study author Dr. Ziyad Al-Aly, a clinical epidemiologist at Washington University in St. Louis and chief of the research and education service at the Veterans Affairs (VA) Saint Louis Health Care System. 

"I think we've ignored this for a long time, and the COVID-19 pandemic kind of put it in the spotlight," Al-Aly told Live Science. As seen with COVID-19, "flu can lead to long-term problems," he said.

Related: When should you get a flu shot? What to know for the 2023-2024 flu season

However, when you compare COVID-19 and the flu head-to-head, the long-term risks tied to a COVID-19 hospitalization are more numerous and more severe than those tied to flu, Al-Aly emphasized, although the flu has more profound effects on the respiratory system, specifically.

"What we saw is that people with COVID-19 had a higher risk of death, hospitalization and sequelae [knock-on effects of disease] than people with the flu," he said. Therefore, "COVID remains a much more serious problem than seasonal influenza." 

For the new study, Al-Aly and colleagues drew information from VA databases to identify people who'd been hospitalized for either COVID-19 or the flu. They included more than 81,000 people who'd been hospitalized for COVID-19 between March 2020 and June 2022, as well as nearly 11,000 people who'd been hospitalized for seasonal flu between October 2015 and February 2019. (The rate of flu hospitalizations was very low between 2020 and 2022, due to precautions taken to reduce COVID-19 spread, so those years weren't included.)

Previous studies have compared the lingering effects of COVID-19 and flu out to about six months post-hospitalization — but the researchers wanted to look further, out to 18 months. 

They looked at the rates of death, hospital admission and intensive care unit (ICU) admission among the study participants, as well as the rates of 94 specific health outcomes related to 10 organ systems. For example, chest pain caused by poor blood flow to the heart (angina) was related to the cardiovascular system, while shortness of breath was related to the respiratory system.

The analysis revealed that the rates of death, medical care and poor health outcomes were "high in both those admitted to hospital for COVID-19 and those admitted to hospital for seasonal influenza," the authors reported. But overall, the risks were substantially higher for those hospitalized with COVID-19.

For every 100 people, the COVID-19 group saw about eight to nine additional deaths within the follow-up period compared with the flu group. Similarly, they experienced about 20 more hospital admissions and eight more ICU admissions per 100 people, compared with the flu group. 

Both COVID-19 and flu came with a risk of various health problems, but COVID-19 affected far more organ systems. COVID-19 was linked to a risk of 64 of the 94 studied symptoms and health conditions, hitting the cardiovascular, neurological and gastrointestinal systems, and more. Influenza was tied to just six: angina, fast heart rate (tachycardia), type 1 diabetes, cough, low blood oxygen and shortness of breath. 

The latter three symptoms tied to flu are related to the respiratory system, and in fact, the flu carried a higher risk of respiratory effects than COVID-19 through the entire follow-up period. This highlights that "flu is more of a respiratory virus, but COVID is more of a multisystemic illness," Al-Aly said.

For all organ systems except gastrointestinal, more than 50% of the health effects occurred more than one month after hospital discharge. This was true for both flu and COVID-19 and highlights the lasting impact both infections can have in the post-acute phase.  

The new study has several limitations. A major one is that it focused only on people hospitalized for either disease. "This is really a major limitation, that we studied it in this context," so more work is needed to understand how the long-term impacts of milder cases of flu and COVID-19 compare to one another, Al-Aly said. In addition, the VA data were heavily skewed toward older male patients, so there are remaining questions as to how these health burdens affect people of other ages and sexes.

With the data at hand, Al-Aly said he hopes the work raises both patients' and health care providers' awareness of the lasting impacts of these diseases.

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