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COVID-19 has confused health and medical experts since the beginning of the novel coronavirus pandemic in late 2019 and early 2020. Caused by the virus SARS-CoV-2, the disease has killed at least 4.5 million people worldwide, and more than 600,000 people just in the US. Although most people who are infected will develop mild or moderate symptoms — or none at all — scientists are continuing to research symptoms of the coronavirus that linger with potentially damaging effects.

“Long COVID,” also referred to as “long-haul COVID” or “chronic COVID” is a condition where a person continues to experience symptoms of COVID-19 after their body has cleared the virus. Long COVID comes under the umbrella of Post-COVID conditions, which the CDC describes as “new, returning or ongoing health problems” caused by the disease more than a month after infection.

In a June study that reviewed the insurance records of nearly 2 million people who were diagnosed with COVID-19, researchers found that 23% of people of all ages developed a condition 30 days or more after infection. In the study, 19% of people who had asymptomatic COVID-19 infections developed a condition, as well as 27.5% of people who had symptoms but weren’t hospitalized and half of all patients that were hospitalized with COVID-19. The most common post-COVID conditions found in the study were pain, breathing problems, high cholesterol, high blood pressure, fatigue and malaise.

Another study by researchers at the Washington University School of Medicine and the Veterans Affairs St. Louis Health Care System found that kidney damage and disease might be a long-term symptom of COVID-19, even in people with mild or moderate cases of the coronavirus. The study, published in the Journal of the American Society of Nephrology, looked at medical records from the US Department of Veteran Affairs. It found that patients who had COVID-19 but weren’t hospitalized had a 15% increased risk of suffering a major kidney event, such as chronic kidney disease, were 30% more likely to have acute kidney injury and more than twice as likely to develop end-stage kidney disease as people who didn’t have COVID-19.

For those hospitalized with COVID-19, the risk of developing kidney damage or failure is even higher. For those admitted to the ICU with COVID-19, the risk was higher still: Patients were 13 times more likely to develop end-stage kidney disease, the Washington University School of Medicine in St. Louis reported.

As many as 9 in 10 adults with chronic kidney disease don’t know they got it, according to the Centers for Disease Control and Prevention. If kidneys can’t do their job and remove waste from our bodies, dialysis or kidney transplants are needed as treatment. In order to catch kidney dysfunction early, health care providers should monitor it in patients who’ve had COVID-19, according to the study’s lead author, Dr. Ziyad Al-Aly.

“People who were hospitalized for COVID-19 or needed ICU care are at the highest risk,” Al-Aly said. “But the risk is not zero for those who had milder cases. In fact, it’s significant. And we need to remember that we don’t yet know the health implications for long-haulers in the coming years.”

Not all people who get COVID-19 will develop kidney problems or other organ damage. But for many who test positive for the coronavirus and recover, their journey isn’t over. As put by a writer for The Atlantic: “Despite long-haulers’ fight for recognition, any discussion of the pandemic still largely revolves around two extremes — good health at one end, and hospitalization or death at the other.”

What are the symptoms of long COVID?

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Common symptoms include most of the symptoms you’d expect with actual COVID-19. The CDC reports them as:

Tiredness or fatigueDifficulty thinking or concentrating (sometimes referred to as “brain fog”)HeadacheLoss of smell or tasteDizziness on standingFast-beating or pounding heart (also known as heart palpitations)Chest pain or stomach pain Difficulty breathing or shortness of breathCoughJoint or muscle pain”Pins and needles” feelingDepression or anxietyFeverSymptoms that get worse after physical or mental activitiesChanges in menstrual cyclesRash

More severe symptoms of long COVID include damage to other organs including the brain, heart and lungs, as well as blood clots or blood vessel problems, according to the Mayo Clinic.

Dr. Nasia Safdar, medical director of infection control at the University of Wisconsin, told CNET in June that the key to discerning long COVID is by paying attention to new symptoms that develop or ones that never go away, after about 30 days post-infection.

“The most common ones that we’re seeing are those that are dealing with what’s called higher executive functions,” Safdar says. “Concentration, memory, being able to do your job the way you could before.”

A persistent feeling of brain fog might be hard to tie to COVID-19, especially after you’ve tested negative. But that’s one of the most commonly reported symptoms in a long-haul case, according to Safdar, along with shortness of breath and fatigue.

“Those kinds of symptoms are hard for people to describe,” she says, “but they’ve clearly noticed a change from the way they were before.”

You should seek medical attention “certainly any time there’s worsening of symptoms,” Safdar says. It is also important to get evaluated if you cleared the virus, but are still having a hard time functioning the way you used to.

How common is long COVID, and how long does it last?

Because COVID-19 is such a new disease, the lasting effects of the coronavirus are still being researched. Safdar says she’s seen studies that suggest up to 40% of all people with COVID-19 will have persisting symptoms, or as few as 10%, but that actual numbers likely vary depending on the population and who was in the study. The June study from FAIR Health found that about 23% of people will develop long COVID.

If you consider long COVID only in cases where people develop new symptoms they didn’t have before, for example, the percentage of people who have long COVID is smaller, she says. If you include cases with both new symptoms and persisting symptoms, which is the CDC definition, the number of people affected by long COVID is much higher.

How long does the condition last? Safdar says there isn’t enough information right now to say for certain, and that there will be a lot of “variation” in people’s recoveries.

“We do know that as long as six months out, people still have symptoms,” she says. “So it may take as much as a year — hopefully not longer than that.”

What causes long COVID?

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Like everything else involving the coronavirus, research is underway. Information reported by Healthline suggests that there may be long-term changes in gene expression, and that the spike protein made when someone is infected with COVID-19 can affect healthy cells, causing lasting symptoms.

Other theories include inflammation of the immune system, decreased function of the immune system and post-traumatic stress from illness. Lasting symptoms may also be due to organ damage caused by COVID-19, in addition to the organ damage itself. For example, the coronavirus weakens blood vessels and causes them to leak, which can cause long-term problems with the kidneys and liver, according to the Mayo Clinic.

If I have long COVID, does that mean I’m still contagious?

No. Once your body has cleared the virus and you test negative for COVID-19, you won’t spread it. Long COVID is also a different condition than a longer-than-normal COVID-19 infection, where the virus is still active in a person and making them sick.

However, note that it’s possible to get COVID-19 more than once, and testing negative doesn’t mean you’ll never test positive again.

Can long COVID be treated?

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Safdar says that practicing standard healthy behaviors like eating nutritious foods, getting rest and avoiding substances like tobacco can reduce COVID-19 severity. But given that even people who had mild cases of COVID-19 can be long-haulers, there isn’t a real answer right now.

One thing that might help a long COVID case, Safdar says, is getting the vaccine. She says there have been recorded cases of people improving after getting vaccinated, and that the vaccine might be changing people’s immune response and interfering with long COVID symptoms. But, she says, “the mechanism is not clear.”

What is clear is the importance of vaccines. “I think that would be an important message — vaccination serves two purposes,” Safdar says. “One, of course you want to get it before you have COVID so it protects you from it, but even in the people who have had the infection, anecdotally, it seems that vaccination helps with the symptoms of long COVID.”

The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.