The study, published this week to journal preprint server medRxiv, found that only one Omicron patient out of 18 interviewed had long-term symptoms, versus 10 out of 18 in a group of similar patients who had other COVID variants.

Symptoms were similar among those who experienced Long COVID, regardless of variant, wrote the authors, who are affiliated with the National Center for Global Health and Medicine Hospital in Tokyo.

The study defined Long COVID, which it refers to as “post COVID-19 condition,” as at least one symptom that lasts for at least two months, with an onset within three months of COVID infection. Symptoms seen in patients included fatigue, difficulty breathing, cough, hair loss, depression, brain dog, difficulty concentrating, and memory issues. Researchers were unable to rule out alternate diagnoses that could cause these symptoms, the report stated.

The study represents the first time epidemiological data on Long COVID in Omicron patients has been examined. But more research is needed to see if findings are applicable to Omicron patients as a whole, and to determine the long-term impact of the variant “on health-related quality of life and social productivity,” the paper stated.

Long COVID may already affect between 7 million and 23 million Americans who previously had the virus, or up to 7% of the U.S. population, according to the U.S. Government Accountability Office.

Different estimates of how many people are affected with long COVID vary widely, from 10% to 80% of COVID survivors. More than half of COVID survivors report symptoms that persist after six months, Penn State College of Medicine researchers reported last year.

Long COVID is a poorly understood condition that could potentially impact over a billion worldwide in just a few years, says Arijit Chakravarty, a COVID researcher and CEO of Fractal Therapeutics, a drug development firm. Experts say that it’s quickly growing into a major public health concern already overwhelming primary-care physicians. 

Just what is Long COVID? It depends on whom you talk to.

The World Health Organization defines long COVID as a condition that occurs in someone who had COVID, with symptoms that cannot be explained by another diagnosis, that last for two months or more. The symptoms can persist following the initial onset, or come and go over time, the organization says, adding that a diagnosis of long COVID usually wouldn’t be made until three months after acute illness.

The Mayo Clinic defines long COVID as a set of symptoms stemming from COVID that persist for more than four weeks after diagnosis.

In reality, long COVID is likely an umbrella term for a combination of issues and conditions: people who have long-term COVID infections who are able to continue to spread the disease; people whose COVID after-effects clear up after a few weeks; and people with long COVID itself.

What’s more, COVID patients whose disease was severe enough to require ICU admission may suffer post-ICU complications like muscle weakness, shortness of breath, cognitive issues, anxiety, and depression—symptoms that look a lot like long COVID, but are not, further muddying the waters. Those issues might occur due to extended periods of immobility and ventilator use, and other traumatic medical events.

It seems like most any ailment—from ear numbness, a sensation of “brain on fire,” and hallucinations—could be symptoms of Long COVID, according to a landmark July study published in British medical journal The Lancet.

The study identified more than 200 potential Long-COVID symptoms in 10 organ systems, with 66 symptoms typically lasting more than seven months. Researchers surveyed nearly 4,000 sufferers with confirmed or suspected COVID from nearly 60 countries, with illness of a month or longer.