Long-COVID, a condition in which the symptoms last for months after SARS-CoV-2 infection, affects approximately 10–20% of people. Researchers have now found a similar condition, called "long flu," wherein people hospitalized with seasonal influenza suffer long-term negative health effects.

Researchers from Washington University School of Medicine in St. Louis and the Veterans Affairs St. Louis Health Care System compared various factors, including risks of death, hospital admissions and adverse health outcomes involving major organ systems, associated with the viruses causing flu and COVID-19.

In both cases, patients hospitalized had an increased risk of death, hospital readmission and issues to multiple organs in the 18 months following the infection. The team also found that the risk was highest 30 days or later after the initial infection. The findings were published in the journal The Lancet Infectious Disease.

"A review of past studies on COVID-19 versus the flu focused on a short-term and narrow set of health outcomes. Our novel approach compared the long-term health effects of a vast array of conditions. Five years ago, it wouldn't have occurred to me to examine the possibility of a 'long flu.' A major lesson we learned from SARS-CoV-2 is that an infection that initially was thought to only cause brief illness also can lead to chronic disease. This revelation motivated us to look at long-term outcomes of COVID-19 versus flu," senior author Dr. Ziyad Al-Aly, from Washington University, said in a news release.

Researchers evaluated 81,280 patients hospitalized for COVID-19 between March 2020 and June 2022, and 10,985 hospitalized for seasonal influenza between October 2015 and February 2019.

The researchers noted that a patient's vaccination status did not affect the results. Patients who had COVID-19 faced a 50% higher risk of death than those who had seasonal influenza during the 18 months of follow-up.

COVID-19 patients also had a higher rate of overall risk, hospital admissions and health issues to organ systems compared to those affected by seasonal influenza. However, the flu posed a higher risk to the pulmonary system than COVID-19.

"This tells us the flu is truly more of a respiratory virus like we've all thought for the past 100 years. By comparison, COVID-19 is more aggressive and indiscriminate in that it can attack the pulmonary system, but it can also strike any organ system and is more likely to cause fatal or severe conditions involving the heart, brain, kidneys, and other organs," Al-Aly said.

In both infections, the risk of death and disability occurred in the months after infection, not in the acute phase or the first 30 days.

"The study illustrates the high toll of death and loss of health following hospitalization with either COVID-19 or seasonal influenza. It's critical to note that the health risks were higher after the first 30 days of infection. Many people think they're over COVID-19 or the flu after being discharged from the hospital. That may be true for some people. But our research shows that both viruses can cause long-haul illness," Al-Aly said.