FATIGUE AND BRAIN FOG ARE THE MOST COMMON SYMPTOMS, WHICH CAN DISRUPT KIDS’ SCHOOL AND SOCIAL LIVES.

If it wasn’t for the fact that a friend had tested positive for COVID-19, Lucas Denault wouldn’t have thought anything of his stuffy nose. But it was January 2021, when COVID was spreading rampantly, and Lucas had also caught it. He recovered and went back to school, back to track practice, back to student council meetings.

Months later, Lucas, then 15, started struggling to walk down the halls of his high school in Littlestown, Pa. His head ached all the time. He felt stabbing pains in his chest.

“It was just such a quick downfall,” said his mother, Karin Denault. He saw a pediatrician, then a cardiologist, who had Lucas do a stress test that left him vomiting after jogging for 10 minutes. Lucas knew he was sick, but, he said, “I had no idea what was wrong with me.”

Neither Lucas nor his mother had considered that his issues could be linked to his brief bout of COVID. But at the recommendation of a relative, he went for an evaluation at Kennedy Krieger, a pediatric care institute that had opened a long COVID clinic in Baltimore.

There, he was diagnosed with long COVID, an umbrella term that encompasses a wide range of symptoms that people may experience in the months after an acute COVID infection. While it appears to be less prevalent in children than adults, for young people like Lucas, long COVID can be a debilitating illness with few treatment options. Here’s what to know.

HOW MANY CHILDREN HAVE LONG COVID?

Experts debate how common long COVID is in children. A new paper published Wednesday in the journal Pediatrics estimates that 10 to 20 percent of children could have long COVID. Other analyses put the number closer to 1 percent of children.

Most parents should not be worried that their children will develop long COVID, said Dr. Stephen Freedman, a pediatric emergency medicine physician at Alberta Children’s Hospital. “I don’t get asked a lot, if at all, about ‘Is my child now at risk of developing long COVID?’ after we diagnose them with an acute infection,” he said. “And I think that’s appropriate.”

But many children do still develop persistent symptoms, and some who most likely have long COVID go undiagnosed, said Dr. Sindhu Mohandas, an infectious disease specialist at Children’s Hospital Los Angeles.

WHO IS MOST AT RISK?

Like adults, children who had more severe acute cases of COVID have a greater risk of lingering symptoms or new complications. One study of children who were in the intensive care unit for COVID found that about 1 in 4 still had problems several months later.

Many of those who were hospitalized had underlying health conditions, like asthma or obesity, that raised their risk for complications, said Dr. Adrienne Randolph, a critical care physician at Boston Children’s Hospital who led the study. However, she added, even some children who have had mild infections will develop long-term symptoms.

Clinicians say they tend to see long COVID more in older children, possibly because they’re better able to report symptoms than younger children.

WHAT ARE THE MOST COMMON SYMPTOMS?

Fatigue, brain fog, headaches and pain are reported most frequently. While these issues are often on the mild end of the spectrum, they can prevent children from participating fully in school or recreational activities. Anxiety and depression are also common with long COVID.

Symptoms can show up differently in children than adults, said Dr. Carlos Oliveira, a pediatric infectious disease specialist at Yale New Haven Children’s Hospital. Weight loss is more pronounced among school-age children, for example, and may be linked to changes in taste and smell.

“They used to love pizza and ate pizza every day, and after COVID, they can’t stand the smell,” he said.

Brain fog may show up as a decline in school performance. Young children may also act out, frustrated that they can’t easily do what they used to. It’s still not clear what the long-term impact of prolonged COVID symptoms may be on children’s development, said Dr. Laura Malone, director of the Pediatric Post-COVID-19 Rehabilitation Clinic at Kennedy Krieger, where Lucas was treated.

In severe cases, some children experience lingering respiratory and cardiovascular problems, including the heart condition myocarditis. Diabetes and other autoimmune disorders can also emerge on the heels of a COVID infection.

“It can really affect every organ system,” said Dr. Ziyad Al-Aly, chief of research and development at the V.A. St. Louis Health Care System and a clinical epidemiologist at Washington University in St. Louis, who researches long COVID. But, he added, diabetes and heart problems “tend to be much, much less prevalent in kids” than adults.

WHAT TREATMENTS ARE AVAILABLE?

If you think your child has long COVID, contact your pediatrician, Dr. Mohandas advised. They may refer your child to specialists to rule out other causes of their symptoms, or to pediatric long COVID clinics.

There are no drugs approved to treat long COVID, so doctors focus on managing symptoms and helping patients function. Some doctors will prescribe medications to address issues like headaches and muscle pains.

Dr. Mohandas said that much of the work she and other clinicians do revolves around validating the experiences of these young patients, many of whom “had previously been very healthy, so frequently, everyone tends to doubt their symptoms,” she said.

Dr. Malone said that schools should make accommodations for children who are struggling, including breaks during the day and extra time for tests.

Small changes helped Lucas, who was also diagnosed with postural orthostatic tachycardia syndrome, a cluster of symptoms that lead to extreme fatigue. He strained to get out of bed, so he started sleeping upright to make it easier. To improve his circulation, he sometimes dangled his feet off the bed and spelled his name with his toes. Six months after his first appointment, he was able to walk through his house without help.

Lucas is now a freshman at Princeton. When he first toured campus, his mother had to push him in a wheelchair. Last weekend, she came to watch him play club basketball.