The incidence of myocarditis and pericarditis after Covid vaccination is low and most patients make a full recovery, a large international study from Nationwide Children's Hospital found.
Most of the cases occurred in male teens and young adults and usually after the second dose of a primary series of either the Pfizer-BioNTech or Moderna vaccine, according to the study published Monday in JAMA Pediatrics.
Although a majority of the people recovered quickly, 93% of the cases required hospitalization and 23% of the cases were serious enough to require admission to the intensive care unit. No deaths were observed.
While the study's findings are "reassuring," the vaccine-associated heart problems "should not be viewed as always mild," said Dr. Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center in Boston. He was not involved in the research.
The findings confirm earlier studies that found myocarditis, an inflammation of the heart muscle, and pericarditis, an inflammation of the outer lining of the heart, following Covid vaccination are rare.
But many of those studies, the authors wrote, relied on self-reporting to safety surveillance systems and contained populations from certain countries, where people may have similar risk factors.
Researchers at Nationwide Children's Hospital reviewed 23 peer-reviewed studies through August across multiple locations, including the U.S., Hong Kong, South Korea and countries in Europe.
The review included more than 800 teens and young adults who developed myocarditis and pericarditis following Covid vaccination.
The heart-related conditions usually occurred within four days after the second dose of a Covid vaccine, the study found.
What are myocarditis symptoms?
The most common symptoms were:
Shortness of breath
Levels of troponin, specific proteins that may indicate issues with the heart, including heart injury, were elevated in about 84% of the patients, according to the study.
Most patients recovered with rest and medication. However, 87% had abnormalities on their cardiac MRI, raising the question of whether there may be long-term consequences to the condition, Barouch said.
Pfizer and Moderna are both conducting long-term clinical trials to track health issues - if any - in the years following a diagnosis of vaccine-associated heart problems in teens and young adults.
The research has limitations, including that some of the studies reviewed were observational, meaning that they can't prove cause and effect. The researchers were also unable to determine whether the timing between doses influenced the incidence of the condition. (Studies have found that a wider gap between doses may reduce the risk of the inflammation condition.)
It also doesn't address what makes young men more susceptible to the heart-related health conditions, although some experts expect the virus's spike protein, once produced in the cell after vaccination, may generate a reaction in the body that can cause inflammation in the heart.
Daniel Salmon, director of the Institute for Vaccine Safety at Johns Hopkins University, said he would still like to see estimates of myocarditis risk by age, gender, vaccine type and spacing between doses.
The study's authors said that the overall incidence of myocarditis following Covid vaccination was consistent with estimates from the U.S. and Israel, which found the occurrence of the condition to be low at 0.3 to 5 cases per 100,000 vaccinated people.
It's important to note that Covid vaccines are not the only vaccinations that have been linked to myocarditis, said Dr. Paul Offit, a vaccine expert at Children's Hospital of Philadelphia.
It's also been seen after smallpox vaccination, he said, though the number of cases may appear higher after Covid vaccines because "we're paying so much more attention."
The authors also noted that the risk of developing myocarditis after Covid infection is significantly higher than after vaccination.
This article was originally published on NBCNews.com