Originally Published: 12 AUG 22 00:01 ET

By Jen Christensen, CNN

(CNN) -- Nearly 30% of children who were hospitalized for Covid-19 or MIS-C, a rare but serious Covid-related condition, had symptoms more than a couple of months after their diagnosis, a new study found.

The study, published Friday in the journal Pediatrics, said the most common symptoms for children hospitalized with Covid-19 were fatigue, shortness of breath, cough, headache, muscle and body aches and fever.

MIS-C is a rare condition that affects children, often -- but not always -- after Covid-19 infection. Doctors don't know exactly what causes it. The condition seems to follow some weeks after infection and can cause certain parts of the body to be inflamed. It can affect major organs including the kidneys, brain, lungs and heart.

For kids with MIS-C, "activity impairment" -- trouble doing normal activities -- was the most common symptom in the study. Other serious symptoms included not being able to exercise or even walk as much as usual, sleeping more than usual and trouble concentrating at school.

As of last week, nearly 14.2 million children in the US have tested positive for Covid-19 since the start of the pandemic. Some studies have estimated that between 2% and 10% of all children diagnosed with Covid-19 -- not just the ones who were hospitalized -- develop long Covid. However, doctors think the issue is significantly underdiagnosed.

There have been 8,798 kids whose illness met the definition of MIS-C, according to the US Centers for Disease Control and Prevention, as of August 1.

For those with MIS-C, the new study found that a higher proportion needed intensive care-level support while they were in the hospital than in children with severe Covid. Kids with asthma and obesity who had MIS-C appeared more likely to have symptoms after they were discharged.

The researchers on the new study surveyed 358 children who were cared for at 25 pediatric hospitals between May 2020 to May 2021, before the surge in cases caused by the Delta or Omicron variants. It's unclear whether the children would have had different outcomes with those variants.

The study period was also before children were eligible to get vaccinated for Covid-19.

"Vaccine uptake for children is still not very high, but vaccines could prevent this," study co-author Dr. Adrienne Randolph, a senior associate in critical care medicine at Boston Children's Hospital and a professor of anesthesia at Harvard Medical School, told CNN. Randolph's earlier research has found that vaccination can help prevent MIS-C and serious illness in children.

The reassuring news, Randolph said, was that most of the kids hospitalized for MIS-C or Covid did not have symptoms lasting months after their initial infection.

There were also far fewer cases of long Covid among hospitalized kids than in hospitalized older adults, she said.

However, the risk of severe illness and lingering complications among children hospitalized for Covid-19 is higher than the risk of severe side effects from the vaccine, which are very rare.

Randolph thinks parents are underestimating that risk when they fail to get their children vaccinated.

"People will say 'kids don't get very sick; why worry about it,' but the truth is that some kids do get very sick. We've shown that even though survival is very high, some will have persistent symptoms, and some will have severe illness. Being hospitalized itself is also very disruptive. You don't want your kid getting in the hospital with severe pneumonia -- period. It can affect their future lung function. MIS-C can affect their heart," Randolph said. "Hopefully, this data will help people see exactly how important vaccination is."