Among the many confounding aspects of the coronavirus is the spectrum of possible symptoms, as well as their severity and duration. Some people develop mild illness and recover quickly, with no lasting effects. But studies estimate that 10 to 30 percent of people report persistent or new medical issues months after their initial coronavirus infections — a constellation of symptoms known as long Covid. People who experience mild or moderate illness, as well as those without any underlying medical conditions, can nonetheless experience some debilitating long-term symptoms, including fatigue, shortness of breath, an erratic heart rate, headaches, dizziness, depression and problems with memory and concentration.

Such lingering medical issues are so varied that one study by a patient-led research group evaluated 203 symptoms that may fluctuate or even appear out of the blue after people seem to have recovered.

As Dr. Ziyad Al-Aly, the chief of research and development at the VA St. Louis Healthcare System and a clinical epidemiologist at Washington University in St. Louis, said, “If you’ve seen one patient with long Covid, you’ve seen one patient with long Covid.”

How doctors currently diagnose long Covid

There is little consensus on the exact definition of long Covid, also known by the medical term PASC, or post-acute sequelae of Covid-19. While the World Health Organization says long Covid starts three months after the original bout of illness or positive test result, the Centers for Disease Control and Prevention sets the timeline at just after one month.

Some researchers and health care providers use other time frames, making efforts to study and quantify the condition more difficult, said Dr. Al-Aly, who has conducted many studies on long-term post-Covid issues.

When patients experiencing persistent symptoms go to their doctors, tests like electrocardiograms, chest X-rays, CT scans and blood work don’t always identify physiological problems, Dr. Al-Aly said. Researchers are working to pinpoint certain biological factors, called biomarkers, that correlate with persistent Covid symptoms. These could include signs of inflammation or certain molecules produced by the immune system that might be measured by blood tests, for example.

For now, doctors must rely on their patients’ descriptions of symptoms and rule out alternative explanations or causes. Some post-Covid clinics have multidisciplinary teams of specialists evaluate patients to figure out the best treatment options.

What causes long Covid?

It’s unclear what exactly drives long Covid, but research has begun to offer some clues. Some experts theorize that an immune response that goes into overdrive when you first get sick may lead to inflammation and damage throughout the body, eventually resulting in long Covid symptoms, said Dr. Michael Peluso, an infectious disease physician at the University of California, San Francisco.

“We know that during acute Covid-19, some people have a really revved-up immune response and some people have a reduced immune response, and that response can determine the trajectory of how well somebody does,” he said.

Another explanation, experts say, could be that your immune system never fully shuts down after the initial infection.

Who is at risk?

Research offers some hints about which patients might face a greater risk of long-term symptoms. In a study of 209 patients published in January, researchers found four factors that could be identified early in a person’s coronavirus infection that appeared to correlate with an increased risk of having ongoing symptoms two to three months later.

One factor was the level of coronavirus RNA in the blood early in the infection, an indicator of viral load. Another was the presence of autoantibodies — antibodies that mistakenly attack tissues in the body as they do in conditions like lupus and rheumatoid arthritis. A third factor was the reactivation of Epstein-Barr virus, which can cause mononucleosis and infects most people, often when they are young, and then usually becomes dormant.

The fourth factor was having Type 2 diabetes, although experts say that in studies involving larger numbers of patients, diabetes might be only one of several medical conditions that increase the risk of long Covid.

Studies from post-Covid clinics have also found other pre-existing medical conditions that may put people at risk for long Covid. In a report on the first 100 patients treated for neurological and cognitive symptoms at a post-Covid clinic at Northwestern Memorial Hospital in Chicago, 42 percent reported previously having depression or anxiety, though such patients might simply be more comfortable seeking neurological treatment, doctors said. Other pre-existing conditions included autoimmune diseases and headaches.

Studies also suggest that the risk of developing long Covid peaks in middle age, Dr. Peluso said. The average age of patients in the Northwestern study was 43. An analysis of 78,252 private health insurance claims across the United States found that people between the ages of 36 and 64 made up about two-thirds of the long Covid patients. (But that study did not include most Medicare recipients, so it involved relatively few older patients.)

Women may be disproportionately affected, with some studies finding that about 60 percent of patients are female. A similar pattern has emerged in other long-term conditions like ME/CFS (myalgic encephalomyelitis/chronic fatigue syndrome), which has several symptoms similar to those of long Covid.

Because the pandemic has had a significant impact on Black and Latino communities in the United States, and those groups have more limited access to medical care, they may have high numbers of long Covid cases as well, Dr. Peluso said.

Can vaccines protect against long Covid?

The picture is still coming into focus, but several studies suggest that getting a Covid vaccine can reduce — but not eliminate — the risk of longer-term symptoms.

The United Kingdom’s Health Security Agency conducted an analysis of eight studies that had looked at vaccines and long Covid before mid-January. Six found that vaccinated people who then became infected with the coronavirus were less likely than unvaccinated patients to develop symptoms of long Covid. The remaining two studies found that vaccination did not appear to conclusively reduce the chances of developing long Covid.

In that analysis, one study, which has not been peer-reviewed, of about 240,000 U.S. patients found that those who had received even one dose of a Covid vaccine before their infections were seven to 10 times less likely than unvaccinated patients to report symptoms of long Covid 12 to 20 weeks later. But another large study of electronic patient records at the U.S. Veterans Health Administration, also not yet peer reviewed, found that those who were vaccinated had only a 13 percent lower risk than unvaccinated patients of having symptoms six months later. Vaccinated patients mostly benefited by being less likely to develop lung problems and blood-clotting difficulties, said Dr. Al-Aly, one of the study’s authors.

“Reliance on vaccination as a sole mitigation strategy is wholly inadequate,” Dr. Al-Aly said. “It is like going to battle with a shield that only partially works.”

Seeking medical care

If you are concerned about any lingering symptoms after a confirmed or suspected coronavirus infection, don’t be afraid to ask for help. Checking in with your primary care provider is a good first step. More doctors are becoming aware of long Covid symptoms and can recommend tests that might at least rule out other causes of your symptoms.

“Even though we say that long Covid is when symptoms last for a month or three months after infection, you don’t have to wait that long to get help,” Dr. Al-Aly said. “People should really honor their symptoms.”

If you’re not getting help from a primary care doctor, you may want to seek out a post-Covid clinic, though Dr. Al-Aly acknowledged that “it’s easier said than done.” Access to post-Covid clinics can be difficult for those without adequate medical insurance. And, in some states, people may have to travel hundreds of miles to get to the nearest one. You can look up post-Covid clinics near you on the Survivor Corps database.

Bring your medical records if you’re visiting a new provider and make a list of all your symptoms, especially if you’re experiencing cognitive issues and are likely to forget some health concerns when your appointment comes around.

Some long Covid issues can be managed with existing medications or treatments for symptoms like headaches or gastrointestinal problems. Physical therapy and “cognitive rehab,” including approaches often used for patients who have experienced strokes or brain injuries, can also be helpful over time. Some people benefit from tailored physical and mental health rehabilitation services and breathing exercises, which can help them slowly build back strength and endurance for physical activities.

Other possible tools against long Covid, including antiviral treatments, are only beginning to be studied. The National Institutes of Health is devoting more than $1 billion to a major research effort called the Recover Initiative, but progress has been slow so far. Lawmakers are pushing for better funding for long Covid research and medical care.

Several groups, such as Body Politic, Long Covid Alliance and Survivor Corps, provide emotional support, as well as resources for seeking treatment, disability benefits and patient advocacy.

People with long Covid may also want to consider joining a research trial, Dr. Peluso said. You may be able to find continuing clinical studies at universities and academic centers near you, or sign up to be part of the Recover Initiative.

“Participating in research can be very empowering,” Dr. Peluso said.