(CNN) November 10 is a day Kim Ford remembers too well. It was the day last year when her 9-year-old son, Jack, was scheduled to get his Covid-19 vaccine at the school clinic. They were excited that he'd finally have some protection, but on November 9, he had the sniffles.
"When he woke up [November 10] and he was feeling even worse, I said, 'You know what, let's test you before you go in, because I don't want you to get the Covid vaccine if you actually have Covid,' " the Michigan mom said.
Jack tested positive for Covid-19 that day and he's lived with the symptoms ever since.
It has kept him from staying at school all day. He has to limit how much he plays baseball with the other neighborhood kids. Even playing Fortnite for too long can leave him feeling sick the next day.
He's one of potentially millions of kids with long Covid.
"My stomach hurts. It's kind of hard to breathe. You have a stuffy nose. It's just an absurd amount of things that you can feel," Jack Ford said. "It's really annoying at times. It's not like a cold, you know, it feels like Covid.
"People may think you're feeling faking it, but you're not faking it. You feel like you have Covid," he added.
It's not clear how many children go on to develop long Covid, because there's not enough research on it in this age group, some experts say.
Almost 13 million children have tested positive for Covid-19 since the start of the pandemic, according to the American Academy of Pediatrics. Studies suggest that between 2% and 10% of those children will develop long Covid, but the number may be larger. Many parents may not know their child has long Covid, or the child's pediatrician hasn't recognized it as such.
In adults, some research puts the number around 30% of cases.
"I personally believe that this is a very much an undiagnosed issue," said Dr. Sara Kristen Sexson Tejtel, who helps lead a long Covid pediatric clinic at Texas Children's Hospital in Houston.
Many doctors treating children at long Covid clinics across the country say they have long waits for appointments. Some are booked through September.
There are no specific tests for long Covid. It's not clear which children will have it, as it can happen even when a child has a mild case of Covid-19.
"It's startling how many of these children present and have a range of symptoms that we haven't fully appreciated. Some are coming in with heart failure after asymptomatic Covid infections," said Dr. Jeffrey Kahn, chief of the Division of Pediatric Infectious Disease at UT Southwestern Medical Center in Dallas. "What's striking to me is that it usually occurs about four weeks after infection, and infection can be really asymptomatic, which is really startling."
Even when kids with long Covid are tested for ailments that might cause these symptoms, it's possible nothing will show up.
"The tested me, and it looked like nothing was wrong with me, but they tried their best to find something," Jack Ford said.
His pulmonary function test and EKG came back normal. "The Covid clinic said this is very common in kids with long Covid. Sometimes, all the tests come back normal," Kim Ford said.
Dr. Amy Edwards, who runs the pediatric long Covid clinic at UH Rainbow Babies & Children's Hospital in Cleveland, agreed that it happens a lot.
"We also scoped them, and their GI tracts are normal. I do a big immune workup, and their immune system appears normal. Everything 'looks normal,' but the kids aren't functioning like normal," Edwards said. "I tell the families, 'you have to remember, there are limits to what medical science understands and can test for.' Sometimes, we're just not smart enough to know where to look for it."
Adults' problems tend to be more obvious, Edwards said, because they are more likely to have organ dysfunction that shows up on tests.
Doctors are still trying to understand why long Covid happens this way in children. They are also figuring out what symptoms define long Covid in children. Some studies in adults show a range of 200 symptoms, but there is no universal clinical case definition.
At Sexson Tejte's clinic in Texas, children tend to fall into a few categories. Some have fatigue, brain fog and severe headaches, "to the point where the some kids aren't able to go to school, grades are failing, those types of issues," she said.
Another group has cardiac issues like heart palpitations, chest pains and dizziness, especially when they go back to their regular activities.
Another group has stomach problems. A lot of these kids also have a change in their sense of taste and smell.
Sexson Tejte said it isn't totally different from the symptoms adults have, "but it's not the mixed bag of different organ system involvement with adults."
One of Jack Ford's symptoms affects the amount of energy he has for typical activities.
"Long Covid patients have post-exertional malaise, which is Jack's biggest issue," Kim Ford said. "So if he overdoes it -- and it doesn't even have to be physically overdoing it. It could be he was really upset about something the day before, or he could be really mentally engaged with something like watching TV or playing video games sitting in his chair -- will knock him out."
Energy has become such a problem that Jack can't go to school for a full day. His parents started him back with one to two hours a day and have gradually increased it to about 5½ hours a day.
"We've been trying to bump him up to six, but it hasn't worked so far," Kim Ford said. "He's woken up pretty miserable the next day."
Edwards, who runs the long Covid clinic in Cleveland, says she has to talk to parents about carefully balancing how much energy their children expend. Most healthy people can push through if they're tired, but those with long Covid can't. "It's like they have one bucket of energy, and it has to be used carefully for school, for play, to watch TV. Every single thing they do takes energy, and once that bucket is empty, that's it," Edwards said.
Some of her teen patients are exhausted just dealing with typical drama at school.
"Long-haulers have to think about every single aspect of their day and when they can expend that energy. They have to have that balance. Otherwise, they run out."
Many also have anxiety. Some of that may stem from the ailment itself or from the doubt they've heard from doctors or adults when they say they don't feel well.
Experts across the country say they've heard from patients whose complaints are ignored, even after a stark change in their health. They've been told that they are being dramatic or seeking attention, or that the symptoms are all in their head.
"I don't want to be too critical, but there are some doctors out there who just dismiss it outright," said Dr. Alexandra Yonts, director of the post-Covid clinic at Children's National in Washington. "The kids then just struggle. They get passed around from place to place."
Yonts thinks there needs to be better acknowledgment among doctors that long Covid can be a real problem.
"I've got two kids in wheelchairs after having had Covid who were never in wheelchairs before. There's one kid on crutches. I've got a kid who lost the use of her hands," Edward said. "These kids should be believed."
There's no specific treatment for long Covid, but most of these clinics are multidisciplinary.
At Edwards' clinic, which opened last year, experts can address pulmonary issues, digestive problems, physical rehabilitation, sleep issues, mental health problems and others. There's a nutritionist on staff, as well as an acupuncturist and a pediatrician who is licensed in Chinese herbal medicine.
In addition to working up a child's schedule so they can determine where to spend their energy and when to take breaks, Edwards' clinic teaches kids to meditate. They do massage therapy and mind-body exercises.
"Children need multiple elements of help. They get significantly better, really they do, if we're aggressive and they get intensive wraparound support and therapy," Edwards said.
But not all children are able to get into a clinic.
"I've talked to so many people working with pediatric Covid recovery, and they all say the same thing: 'We are worried about the kids who aren't getting the help, who don't have the parents who can advocate for them or navigate the medical system.' It keeps me up at night," Edwards said.
A lot of what her clinic does is to encourage kids to get enough sleep and to eat healthy food, but not all families can afford healthy food.
"It terrifies me for those families in particular, because they're already starting behind. And now they have kids with Covid long-haul," Edwards said. "You just have to hope more people will become aware of the problem and try to help."