Researchers at the University of Leicester in England performed an analysis of nearly 200 studies of prior COVID patients, involving nearly 750,000 people in all. The patients—some of whom were hospitalized and some of whom weren’t—lived across the globe.

More than 45% of study participants had at least one lingering symptom four months out from their initial infection. A quarter of the patients reported fatigue, and a similar number said they felt pain or discomfort. Meanwhile, sleep issues, breathlessness, and problems participating in normal daily activities were reported in just under a quarter of patients, according to the study.

Often, no clinical abnormalities could be found to explain such symptoms. But some signs were reported in many patients who had been hospitalized with COVID, including changes in lung structure and function. An abnormal CT scan and/or X-rays were found in nearly half of previously hospitalized patients, in addition to a decreased capacity to diffuse carbon monoxide in nearly a third of patients.

“Changes in pulmonary function are similar to those observed following other viral infections including SARS and MERS,” the authors wrote.

When nonhospitalized COVID survivors were singled out, more than a third of them had lingering symptoms at four months, the study found.

“The reasons as to why so many patients are experiencing long COVID remains unknown,” the authors wrote, adding that possible causes include organ damage, inflammation, altered immune systems, and psychological effects.

While some studies have found a higher rate of long COVID in females, the study out of Leicester didn’t find that any particular age group or gender experienced higher rates of the disabling condition. Researchers weren’t able to reliably assess any potential association with race, as only a quarter of studies examined provided participants’ race or ethnicity.

Nearly 20% of American adults who’ve had COVID—an estimated 50 million—report having long COVID symptoms, according to data collected by the U.S. Census Bureau this summer.

Long COVID is roughly defined as symptoms that persist or appear long after the initial infection is gone, but a consensus definition has not yet been broadly accepted. Many experts contend that long COVID is best defined as a chronic-fatigue-syndrome–like condition that develops after COVID illness, similar to other post-viral syndromes that can occur after infection with herpes, Lyme disease, and even Ebola. Other post-COVID complications, like organ damage and post–intensive-care syndrome, should not be defined as long COVID, they say.