Nearly 60% of patients with long COVID had organ impairment a year after catching the sometimes deadly disease, according to a new study released Tuesday by the Journal of the Royal Society of Medicine.

London-based researchers examined 536 long COVID patients, only 13% of whom were initially hospitalized with COVID. The majority—62%—were diagnosed with organ impairment six months after having COVID. The vast majority—nearly 60% of all long COVID patients studied—continued to experience organ impairment a year later. What’s more, more than a quarter saw impairment of two organs or more.

Long COVID symptoms seemed generally associated with being young and female, and having single-organ impairment, Amitava Banerjee, professor of clinical data science at the University College London Institute of Health Informatics, said in a news release on the findings.

“Several studies confirm persistence of symptoms in individuals with long COVID up to one year,” Banerjee said. “We now add that three in five people with long COVID have impairment in at least one organ, and one in four have impairment in two or more organs—in some cases without symptoms.”

The researchers were unable to determine exactly what about the COVID virus was causing long COVID symptoms, even after performing blood tests and MRIs. Currently, there are no official diagnostic criteria for long COVID. Even the definition of the condition varies depending on whom you talk to, though it’s generally considered to consist of new symptoms that start during a COVID infection or appear after one, and persist for weeks or months.

With more than 200 symptoms identified—from lingering cough and fatigue to ear numbness and a sensation of “brain on fire”—long COVID is undoubtedly not one but multiple conditions, experts say.

True long COVID, some contend, is best defined as a chronic-fatigue-syndrome-like condition that develops after a COVID infection, similar to other post-viral syndromes that can occur after an infection with herpes, Lyme disease, and Ebola, among others.

Other post-COVID complications like organ damage should not be defined as long COVID and better fit into the larger umbrella category of PASC, some experts say. Also known as post-acute sequelae of COVID-19, the term is used to encompass a wide variety of COVID consequences, from chronic-fatigue-like symptoms and subsequent heart disease to lasting lung damage and odd new symptoms like urinary incontinence, itching, and skin lesions.

As of Jan. 16, 15% of U.S. adults reported having long COVID symptoms at some point in the pandemic, and 6% reported lingering symptoms, according to a Jan. 26 report by the Kaiser Family Foundation, citing data from the U.S. Centers for Disease Control and Prevention.

The percent of Americans who’ve experienced COVID and still report long COVID symptoms dropped from 19% in June to 11% in January, according to the report.