Most of the 45 children included in the study were clear of their heart-related symptoms caused by a rare complication called multi-system inflammatory syndrome within four months, the data showed.
Before their recovery, most were critically ill and required intensive care. Nearly 80% had some type of heart dysfunction.
In addition, nearly half of the children had moderate to severe heart abnormalities, including decreased ability of the heart to pump properly, coronary artery dilation and leaking heart valves.
Two-thirds of the children experienced a temporary decrease in white blood cell levels during their hospital stays, as well.
"It was both challenging and scary because we didn't know how these patients would do long term," study co-author Dr. Kanwal M. Farooqi said in a press release.
"It is a relief that this study shows that most of the severe heart and immunologic manifestations we saw in kids with MIS-C resolved rapidly," said Farooqi, a pediatric cardiologist at NewYork-Presbyterian Morgan Stanley Children's Hospital in New York City.
Although children have been considered to be at lower risk for infection and serious illness due to COVID-19, that may be changing because a vast number remain unvaccinated as new variants of the virus emerge, experts have said.
MIS-C, which has been compared with another rare disorder, Kawasaki disease, has been reported in a small minority of infected children, but it causes severe inflammation of the heart, according to Farooqi and her colleagues.
This inflammation, in turns, affects how the heart muscle functions, the researchers said.
Not all of the children who develop MIS-C have severe COVID-19. Many of the children in this study were asymptomatic or had mild COVID-19 symptoms, at least initially.
However, as their illnesses progressed, they developed a variety of non-respiratory symptoms including abdominal pain, skin rashes, heart abnormalities and, in some cases, extremely low blood pressure.
To ease the inflammatory response, physicians treated the children with steroids, and most received intravenous drugs to regulate the immune system.
One-third received respiratory support, though none required the use of an external life-support machine, and more than half received therapies to increase and maintain blood pressure.
Heart-related abnormalities returned to normal within a few weeks after the children left the hospital and nearly all abnormalities were gone within four months, researchers said
"We are recommending that children who had more than mild dysfunction on cardiac ultrasound ... get a cardiac MRI at six months and see a pediatrician before being cleared for competitive sports," said Farooqi, an assistant professor of pediatrics at Columbia University.
"These kids were quite sick, but at our hospital, where we began using steroids and other treatments routinely for MIS-C, most of the patients responded rapidly and were discharged by about five days," she said.