How dangerous is myocarditis? The truth about the scary-sounding condition.

Over the past two weeks, the Food and Drug Administration and Centers for Disease Control and Prevention weighed approval for Pfizer’s COVID-19 vaccine for kids aged 5 to 11 years old. The FDA issued an emergency-use authorization for the pediatric vaccine, which was quickly followed by an endorsement from the CDC. 

During the days-long meetings, questions of heart inflammation following the vaccine loomed large. And by most accounts, reporting on those discussions has led to concern among parents about side-effects. But just how dangerous this vaccine side effect is still isn’t completely clear, and according to some experts, it might not be a cause for concern at all. 

Currently, the FDA has paused its approval of Moderna’s vaccine for adolescents while it reviews data from Europe on the risk of inflammation in or around the heart following mRNA vaccination. That inflammation is called myocarditis and pericarditis, depending on where it takes place. It’s rare: Even in the highest-risk group, 16 to 18 year olds, it occurs in about 70 out of a million vaccinations, CDC data shows. But according to the EU, Moderna’s shot is more likely to cause this inflammation in adolescents. Several Scandinavian countries no longer give Moderna to people under 30.

CDC data from millions of vaccinations show that the inflammation, when it occurs, goes away within a matter of days without any serious consequences. And infectious disease experts and pediatricians widely agree that the risks of COVID—including a full-body inflammatory syndrome called MIS-C—are much more substantial.

Sarah Long, a pediatric infectious disease expert at Drexel University College of Medicine, and a member of the CDC’s vaccine advisory panel, thinks that the post-vaccine heart inflammation might be different from the myocarditis that’s seen during an infection with COVID-19 or from some other cause. It’s so different, and less severe, than typical myocarditis, she says, that giving it the same name has created unnecessary worry over the condition.

Myocarditis isn’t a disease in its own right, like the flu—it’s more like pneumonia, which can be caused by viruses or bacteria.

Long says that vaccine-related events are completely different, and much less dire, than typical myocarditis. She says that the typical disease can be so severe that she’d rather treat many people with vaccine-associated myopericarditis than a single case of viral myocarditis. She says that the typical disease can be so severe that she’d rather treat many people with vaccine-associated myopericarditis than a single case of viral myocarditis. “Give me a patient with vaccine-associated myopericarditis every day,” she says, “for one who has true viral myocarditis.”

Long thinks of myocarditis in three different categories. The first type consists of “typical myocarditis,” as it existed before COVID, which affects just under 1 in 100,000 children, and is most common in 15 to 18 year old boys. It can be caused by a variety of factors, including an overactive immune response to a virus, and sometimes turns serious, she says. “It is a typical syndrome that is not good to have. They take a long time to get better. Some of them never get better. There is mortality, and some of them get to heart transplant.”

Then there’s myocarditis that comes from COVID itself. It’s usually found as part of a larger inflammatory condition called MIS-C, which occurs weeks after an acute COVID infection. It appears to be related to an overactive immune response, and causes serious inflammation throughout the body, including the heart. Adults with heart conditions have also experienced severe complications from COVID-19.

Then there’s inflammation from the vaccine. Long says that in her experience seeing patients and reviewing cases as a journal editor, kids with vaccine-related symptoms present very differently: If MIS-C and myocarditis are apples and oranges, she says that the vaccine condition is an avocado. “The typical thing is they have the acute onset of chest pain. That’s bizarre.”

Those unusual symptoms appear to be because vaccine-related inflammation isn’t concentrated in the muscle of the heart itself: it’s in the lining around the heart, the pericardium. The lining, unlike the muscle of the heart, has pain sensors. But while inflammation of the heart lining sounds concerning, Long points out that surgeons sometimes remove it altogether during heart procedures, and patients do fine. It’s just not as essential to a healthy heartbeat.

[Related: Why are kids’ immune systems different from adults?]

So a kid with “typical” myocarditis might come to the hospital with no chest pain, but would be breathing quickly because of a malfunctioning heart. A kid with the vaccine symptoms would instead have chest pain, but “their echocardiograms, which are measuring function, are almost always normal.”

Long thinks we should be calling the vaccine side-effect pericarditis or perimyocarditis, to emphasize the fact that it’s taking place in the heart lining. I remember some of the early discussions of this, I kept saying to people, ‘please let’s not call this myocarditis.’”

“If parents were to look up myocarditis, they would be quite upset about this,” she says. But the chest pain associated with pericarditis is much less serious—the CDC hasn’t confirmed a single death from the condition.

That’s not to say that everyone thinks the two are completely different conditions. In an overview of vaccine-related heart inflammation published this summer, about 60 percent showed some change in heart activity, indicating that the heart muscle itself was affected.

But data presented earlier this week by CDC scientist Matthew Oster backs up real-world differences between the conditions. In cases of typical myocarditis, about 30 percent of patients didn’t get normal heart function back after a month. In cases of MIS-C myocarditis, the majority of patients got back to normal within ten days. In vaccine-related inflammation, everyone was up and running in five days.

Children experiencing chest pain is certainly nothing to be brushed off lightly, says Long. “But we now have had time to see that these are different entities, not with this same prognosis. Nobody has died of myopericarditis, and children are dying of coronavirus. So of course it’s a benefit-risk ratio that comes out in the direction of vaccination.”