Evidence continues to mount that Covid isn’t the only viral illness that can lead to persistent and sometimes debilitating symptoms. 

Research published Thursday in The Lancet Infectious Diseases finds that the flu virus may also have long-lasting effects on health. 

With the arrival of the pandemic and the resulting rash of long Covid cases, doctors had to rethink their ideas about viral infections, said senior study author Dr. Ziyad Al-Aly, chief of research and development at the VA St. Louis Health Care System and a clinical epidemiologist at Washington University in St. Louis. 

“Our conception of these illnesses as acute events that you deal with and then put behind you has changed,” he said. “The acute phase is like the tip of an iceberg. People who get these infections may need attention beyond the acute phase. We need to ask if they have fully recovered, if they are able to go to the gym like before, if they have the same mental acuity.”

This view of viral illnesses and their potential long-term impacts may lead to research that will also help people with other little understood conditions, such as chronic fatigue syndrome and fibromyalgia, which experts believe might be triggered by viral infections, Al-Aly said.

To take a closer look at the possible after-effects of Covid and the flu, Al-Aly and his colleagues turned to data from the U.S. Department of Veterans Affairs. They focused on records from 81,280 veterans  who were hospitalized with Covid from March 2020 through June 2022, and 10,985 who were hospitalized with the flu from October 2015 through February 2019. Nearly all the patients in the study were men, and the average age was around 71 years old. 

In the 18 months after infection, both groups were at increased risk of death, hospital readmission, and health problems that involved a number of organs, including the heart, kidneys, brain and the digestive system. 

Covid patients were more likely to experience long-lasting problems with multiple organ systems at the same time, the study found. Flu patients also experienced this, but were more likely to have lasting symptoms related specifically to the lungs.

Lung problems in people who had recovered from the flu could range from coughs that could persist for months to severe shortness of breath caused by inflammation and scarring deep in the lungs, Al-Aly said.

Overall, the researchers found that illness caused by Covid was more severe, with higher rates of death, hospital readmission and adverse effects on multiple organ systems, than what was associated with the flu.

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One thing the researchers don’t know is how broadly applicable their findings are to the general population — for the most part, older men made up the patients in the study and all were sick enough to be hospitalized. 

“I think they would be applicable to people hospitalized with these conditions,” Al-Aly said. “What we don’t know is whether and to what extent this will apply to people with mild Covid and mild flu who do not require hospitalization.”

Long Covid spotlighted the concept that viral illnesses could spark a long-lasting syndrome of symptoms, said Dr. Todd Rice, a professor of medicine in the division of allergy, pulmonary and critical care medicine and director of the medical intensive care unit at the Vanderbilt University Medical Center.

The new study underscores that “other viruses can have this long tail effect,” he said. “It shows that the flu can lead to a lot of pulmonary symptoms, such as post-viral cough that can last for months. In contrast, Covid can lead to a lot of nonpulmonary symptoms, including fatigue, muscle weakness and brain fog.”

Doctors are seeing similar syndromes in patients infected with other respiratory viruses, Rice said. “Many have symptoms if they push themselves,” he added. “If they used to be able to run 5 miles, now they become short of breath and can only run 3.”

Another study, published in October, found that symptoms can linger after a variety of common respiratory viruses, including common cold viruses, as well as influenza.

Dr. Anita Gupta, an adjunct assistant professor of anesthesiology, critical care and pain medicine at the Johns Hopkins University School of Medicine, praised the new research. 

“I think it’s a very high-impact study that highlights the gravity of these conditions and the importance of preventing infection with these viruses,” she said.

Currently there is no cure for these post-viral syndromes. That’s because “we really don’t understand how the viruses are triggering severe health problems and why they persist,” Gupta said. ”There’s a lot of research going on. I think we will figure it out.”