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Crushing fatigue. Brain fog. Trouble breathing weeks after contracting COVID-19. Scientists call it post-acute sequelae of COVID-19. Most people just call it "long COVID."
For millions of people, these and other symptoms are keeping them from getting back to their lives months after their last positive COVID-19 test.
But what is long COVID, exactly? How common is it? Who gets it, and why?
As with so many things over the past two pandemic years, the answer to the most basic questions is, "We don't know yet."
Studies are starting to narrow things down. But a lot still is up in the air.
"I would take everything we have so far with a grain of salt," Dr. Nahid Bhadelia, founding director of the Boston University Center for Emerging Infectious Diseases Policy and Research, said on a press call organized by the Infectious Diseases Society of America.
The silver lining may be that with so many suffering the aftereffects of COVID-19, research may shed light on similar but poorly understood syndromes, such as chronic fatigue syndrome that have debilitated people long before COVID-19 showed up.
With time and support, "the majority - and I would almost say the vast majority - of people with long COVID will get better," added Dr. Kathleen Bell, chair of the Department of Physical Medicine and Rehabilitation at the University of Texas Southwestern Medical Center. "But I don't think, at this point, that anyone can say how long does long COVID last."
How common is it?
Estimates of how many people get long COVID are all over the map.
An analysis summing up 57 studies on the subject found that on average, more than half of COVID-19 patients still had symptoms six months after infection.
But the range was enormous. In some of the studies, less than a quarter of patients had long-term symptoms, while in others, three-quarters did.
One of the difficulties with pinning down long COVID is defining what it is and what it isn't.
"Currently, the bucket is very large," Bhadelia said. "It's anybody who has persistent symptoms four weeks or longer" after infection.
Fatigue is the most common symptom. Many complain of "brain fog" - memory problems and difficulty concentrating or processing information. Patients frequently have trouble breathing. Other common symptoms include headaches, muscle pain, rapid heartbeat, dizziness or ringing in the ears.
There's also a lot of anxiety, depression and insomnia, which may be partly reactions to the symptoms but also appear to be related to the virus itself, Bell said.
The challenge for both doctors and patients is that many other things can cause these symptoms besides long COVID, she noted.
Who gets it?
Vaccination cut the rate of long COVID symptoms in half in one study and down to baseline in another.
Diabetes and asthma raise the risk.
People who got seriously ill with COVID-19 are more likely to have prolonged symptoms, but even some people who had only mild to moderate cases are struggling months later.
"In general, you can say that people who have more severe infections will have a longer period of time of recovery. But that's not the whole story," Bell said.
Some recent studies are pointing to what may be causing long COVID, but nothing is conclusive yet.
One theory is that long COVID is an autoimmune condition in which the immune system mistakenly attacks the patient's own body.
In a new study, researchers found patients with long COVID had high levels of antibodies to components of the patient's own immune system, even though very few of them had a previously diagnosed autoimmune condition.
Viral reawakening?
The study also raised the possibility that COVID-19 wakes up latent infection of another common virus, called Epstein-Barr.
An estimated 90% of the world's population carries the Epstein-Barr virus, but usually the immune system keeps it under control.
The virus also causes mononucleosis, which "puts you flat on your back with fatigue for a month or more, which is not that different from some long COVID symptoms," study co-author James Heath, president of the University of Washington Institute for Systems Biology, noted in a YouTube video the institute posted.
Overactive inflammation may be another factor, perhaps involving tiny blood clots carrying inflammatory molecules throughout the body.
Whatever the cause, COVID-19 is not the only ailment to leave patients with lingering symptoms.
Scientists are studying persistent headaches, joint pain and vision problems in Ebola survivors in West Africa. Chikungunya can leave patients with arthritis lasting months. Other viral illnesses may leave patients with chronic fatigue syndrome.
"We just haven't understood many of these conditions," Bhadelia, at Boston University, said.
Now that there are millions of people suffering with long-term, debilitating symptoms, scientists may learn a lot more about what causes them and how to treat them.
"This is going to tell us a lot more about other viruses and other pathogens," Bhadelia said. "Everything that affects us from our environment, everything that triggers a change in our body, leaves a fingerprint on us."
Steve Baragona is an award-winning multimedia journalist covering science, environment and health.
He spent eight years in molecular biology and infectious disease research before deciding that writing about science was more fun than doing it. He graduated from the University of North Carolina at Chapel Hill with a master's degree in journalism in 2002.