Research has found that children are at an elevated risk of developing myocarditis, an inflammation of the heart muscle, after receiving an mRNA Covid-19 vaccine. But experts say that risk is small and the condition is temporary—and the risk of Covid-19 damaging the heart is much bigger.

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The risk of myocarditis from Covid-19 vaccines

Myocarditis usually comes from an infection from either a virus or bacteria and causes symptoms like shortness of breath, rapid or irregular heartbeat, or chest pain, the New York Times reports. Each year, about 10 to 20 people out of every 100,000 develop the condition. Research suggests a that myocarditis can be a rare side effect of vaccinations against Covid-19. Current trends suggest the chances of developing myocarditis are highest after the second dose of an mRNA vaccine in male patients ages 16 to 29, as there are roughly 11 cases for every 100,000 vaccinated males in this age group, the Times reports.

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One study from Kaiser Permanente Northern California found that participants ages 12 to 39 who were vaccinated with the Pfizer-BioNTech vaccine had a 19-times higher risk of myocarditis than the general population. Participants ages 18 to 39 who were vaccinated with the Moderna vaccine had a 37-times higher risk of the condition.

"The rate after Moderna is objectively higher than the rate after Pfizer," said Nicola Klein, director of the Kaiser Permanente Vaccine Study Center. "It's consistent, regardless of which cases we use, or whether it's males only or both sexes."

While those numbers may sound high, said James de Lemos, a cardiologist at the University of Texas Southwestern Medical Center, it's important to remember the absolute numbers are small. "Thirty times a small number is still a small number," he said. "The math still favors vaccination in adolescents and children."

De Lemos also noted that myocarditis linked to mRNA vaccines is significantly less common and severe than the condition occurring in patient who contract Covid-19.

Myocarditis can occur in Covid-19 patients when the coronavirus infects the cardiac muscle and the lining of blood vessels, which can cause long-term damage to the heart and other organs, the Times reports. It's also possible for the virus to weaken the heart to the point that a patient needs a heart transplant.

By comparison, myocarditis observed post-vaccination is typically mild and transient. "It's unsettling, but rarely life-threatening," de Lemos said.

Since the start of the pandemic, 657 children have died from Covid-19, and at least 5,200 children have developed multisystem inflammatory syndrome following their coronavirus infection, the Times reports.

While myocarditis risk post-vaccination is real, "those are real numbers that are bigger," said Brian Feingold, an expert on heart inflammation in children at UPMC Children's Hospital of Pittsburgh.

"If you look at an isolated risk, you could really get yourself very worked up and scared," Feingold added. But the risk that Covid-19 causes permanent heart damage is "way more likely," he said.

Still, Moderna announced that it is reviewing data on the risk of myocarditis post-vaccination in children ages 12 to 17, and may not be ready to submit data to FDA until January 2022. (Mandavilli, New York Times, 11/1; American Hospital Association, 11/1; Mandavilli, New York Times, 11/2)