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Here's what we know about long COVID now, from symptoms to treatment

After years of research and dozens of studies, here's what we know about long COVID.

Some people who get COVID-19 can experience lingering symptoms or long-term effects for months or even years after infection, also known as long COVID or post-COVID conditions, according to the U.S. Centers for Disease Control and Prevention.

Long COVID is a medical mystery that has perplexed doctors and researchers for the last two years. It's not even entirely clear how common it is: The CDC estimates that 13.3% of people with COVID experienced post-COVID conditions at one month or longer after infection and about 2.5% at three months or longer. More than 30% at 6 months among patients who were hospitalized.

Long COVID is also the subject of numerous studies and a recent congressional inquiry. At a House Select Subcommittee hearing on long COVID last week, experts testified that the poorly understood condition is disabling millions of Americans and putting stress on the health care system and economy, TODAY previously reported.

Although there are still more questions than answers on long COVID, NBC News medical contributor Dr. Natalie Azar joined the 3rd hour of TODAY to share what we do and don't know so far.

What is long Covid?

While there is no globally accepted definition of long COVID, "according to the CDC, it is symptoms that last for one month or longer after the onset of illness," Azar explained, and the symptoms cannot be explained by another condition. "(There's) no alternative diagnosis. ... You have to rule other things out," she said.

Not all long COVID is the same, Azar said, but common symptoms include extreme fatigue, persistent shortness of breath, heart rate and blood pressure dysregulation, "brain fog," headaches, pain, and gastrointestinal problems, said Azar.

Some people may experience symptoms that persist after infection or go away and later come back, Azar explained: “A lot of people don’t have some of the symptoms early on and then five months later develop things."

The experience of long COVID varies greatly. "It's mixed bag, which makes it harder to study," Azar said. There is no laboratory test for long COVID, so it's a clinical diagnosis, she added.

Who is most likely to develop long COVID?

It's important to note that long COVID can happen to anyone, including previously healthy individuals. So it's hard to pinpoint risk factors or recognize who will go on to experience long COVID, Azar explained.

"But what we do know is that people who initially have severe symptoms, maybe they're hospitalized or in the ICU ... or have underlying health conditions, which means they have more severe illness ... tend to have more long symptoms and take longer to recover, maybe more long term impairment," said Azar.

Vaccination status could also play a role, "because vaccination reduces viral loads ... if you have less viral load, you have less of that ... from the virus," she added.

While the cause of long COVID is unknown, per Azar, research is underway, and experts are trying to figure out if it has to do with immune response.

"Is there a persistent virus there? Is there a persistent triggering of the immune system? Is there organ damage? There's a whole immune milieu happening that researchers are studying, but we don't have one unifying explanation," said Azar.

Long COVID and Chronic Fatigue Syndrome

Azar noted that long COVID in some patients bares a resemblance to chronic fatigue syndrome, a disabling and complex illness with no known cause, where patients experience overwhelming fatigue that is not improved by rest and interferes with daily activities, according to the CDC.

So while this is not the first time millions of people have experienced this kind of situation, said Azar, it is the first time the global research community and funding has been focused on what "CFS patients have been longing for for decades," said Azar.

While there are no specific treatments for long COVID, doctors can treat specific symptoms, Azar said. “Patients who have persistent shortness of breath may do pulmonary rehab. ... There’s treatment for pain. ... Right now, it’s very much a holistic approach,” Azar explained.

In the meantime, there are clinical trials taking place and researchers are working to find answers.

“It’s frustrating for the patients who are suffering because it doesn’t mean an immediate intervention that’s going to help them today, but we're getting there,” said Azar.