Soon after the first COVID-19 vaccines appeared in 2021, reports of rare cases of heart inflammation, or myocarditis, began to surface.

In most instances, the myocarditis has been mild and responded well to treatment, though up to four potentially mRNA vaccine-related deaths from myocarditis in adults have been reported worldwide. No known verified deaths of children have been reported based upon publicly available data. The exact number remains a topic of very heated debate because of variability in the reporting of possible myocarditis-related deaths.

Studies have largely confirmed that the overall myocarditis risk is significantly higher after an actual COVID-19 infection compared with vaccination, and that the prognosis following myocarditis due to the vaccine is better than from infection. The specific myocarditis risk varies by age and has been debated because of differing views among a small group of physicians related to risk tolerance and support for or against COVID-19 immunization for specific age groups.

As pediatric cardiologists, we specialize in heart issues relevant to kids of all ages. We believe it is important to weigh the risk of myocarditis caused by COVID-19 immunization against not only viral myocarditis from COVID-19, but also all the other complications that COVID-19 can lead to.

Comparing risks of myocarditis from severe disease versus COVID-19 vaccination or infection is difficult to do well, and debate continues over which of those outcomes poses a higher risk.

Myocarditis explained

Myocarditis is any condition that causes heart inflammation. A closely related condition called pericarditis refers to inflammation of the outside lining of the heart. For the purpose of this article, we focus primarily on myocarditis, since it has the potential for being a more severe condition. Most cases of myocarditis are caused by infections, particularly viral ones.

Myocarditis can be confirmed by a combination of an electrocardiogram, an ultrasound heart picture called an echocardiogram and some blood testing. When it is available, cardiac magnetic resonance imaging, or MRI, is the most accurate method to diagnose myocarditis that doesn’t involve an invasive procedure.

A mistaken assumption is that all myocarditis is severe, since it implies damage to the heart. However, mild cases in which there is very little swelling and only temporary damage to the heart are more common than severe cases that require a machine to support heart function.

Vaccination versus infection risk

The challenge of parsing risks of myocarditis from viral infection compared with COVID-19 vaccination is due in part to the difficulty of establishing a diagnosis of myocarditis and its population rates accurately.

The United States Vaccine Adverse Event Reporting System, or VAERS – which is an initial reporting system for vaccine side effects – is by itself inadequate to determine the rate of any vaccine-associated side effect. This is because any side effect can be reported, and verification of a reported event only takes place afterward by the Centers for Disease Control and Prevention.

That vetted data is then reported in more robust databases like the Vaccine Safety Datalink. A very small number of the myocarditis events following COVID-19 vaccination have resulted in significant long-term consequences like heart rhythm troubles. However, such cases do not reflect the majority.

Thankfully, severe myocarditis after mRNA vaccination for COVID-19 is extremely rare. A 2021 study from Nordic scholars, which looked at comparative risks of myocarditis and heart arrhythmia in patients who experienced myocarditis after COVID-19 infection versus immunization found that the risks vary significantly by age group.

This has been touted as a reason not to vaccinate healthy young men against COVID-19. The follow-up study, however, found that the comparative risks of negative outcomes were worse from myocarditis from COVID-19 infection and other viral myocarditis than from vaccination in all patients older than 12 years of age.

And it’s worth noting that, as of mid-March 2023, the U.S. still leads the world in COVID-19 hospitalizations.

There have also been rare myocarditis cases reported with the newer non-mRNA Novovax vaccine, though we researchers do not yet know population-level rates.

Myocarditis risk by age and gender

A survey of all currently available research reveals that the risk of myocarditis after COVID-19 vaccination is highest in young men between the ages of 18 and 39 and older teen boys in the age range of 12 to 17, with the highest risk after the second dose of vaccine. The cause appears to be related to how the immune system processes the mRNA and sometimes generates an excessive immune response.

Myocarditis risk related to COVID-19 immunization is markedly lower in children younger than 12 years of age and much lower in adult males older than 50. The risk of severe disease from COVID-19, particularly in those older than 50 years, has been far higher throughout the pandemic than the risk of myocarditis from COVID-19 vaccination. The risk of vaccination myocarditis is uniformly lower in girls than in boys.

Infants younger than 6 months can get immunity only from their mother’s antibodies unless they are exposed to COVID-19 themselves, as vaccines for this age group are not available.

How to parse the risks

While the risks of myocarditis have been highest in teen boys and young men regardless of cause, the severity and outcome of myocarditis was much worse at the 90-day mark when it stemmed from COVID-19 infection or other viral diseases. This mirrors our team’s research on this same topic.

This discussion also doesn’t take into account the clot and heart attack risks from COVID-19 itself. Because COVID-19 damages blood vessels in all parts of the body, some organ damage such as kidney failure, blood clots, heart attacks and strokes can occur.

We recognize a need for more research into how people fare over the medium and long terms following a case of immunization-related myocarditis. This is why research is ongoing, and researchers like us are committed to following the data for years to come.

COVID-19 risks in children

While there have been far fewer deaths from COVID-19 in children than adults, COVID-19 is still one of the leading causes of childhood death in the U.S., based on an early 2023 study. But COVID-19 deaths are not the only relevant measure of its effect in kids. COVID-19 has also killed more children in a shorter time period than several other vaccine-preventable diseases, such as hepatitis A and meningitis before the availability of their vaccines.

The argument that some have made that fewer children than adults die from COVID-19, or that it is often mild in children, has never been an acceptable justification to not do everything possible to protect children from it. For instance, doctors don’t stop treating pediatric cancer patients purely because there are fewer of them than adult cancer patients. And we don’t retire the measles vaccines only because most kids who get measles get only a mild case.

The primary risk that COVID-19 presents now to children is long COVID, followed by the risk of severe disease. The estimated percentage of children acquiring long COVID is still being debated, but the symptoms from long COVID can be extraordinarily debilitating. These include severe fatigue, brain fog, sleep disturbance, dizziness, nerve pain and more.

Weighing the decision to vaccinate

We believe that the decision of whether to vaccinate against COVID-19 should be based upon the patient’s age, other health problems, relative risk from vaccines, how much and what type of COVID-19 is in your community, and the patient’s and family’s preference.

Two ways that have been suggested by the CDC and the Public Health Agency of Canada to decrease the risk of COVID-19 vaccine myocarditis are to opt for Pfizer and to space your doses out by at least eight weeks. This is because Pfizer has slightly lower rates of myocarditis than Moderna.

Adults who are immunocompromised or have other medical problems known to worsen COVID-19 disease severity still carry the highest risk of severe disease. They should therefore follow the CDC COVID-19 vaccination schedule with additional boosters, if advised by their physician.

While COVID-19 immunizations are not as efficient at preventing viral transmission now as they were with the earliest variant, they remain highly effective at reducing severe illness and hospitalization, even in kids, and particularly in the high-risk state of pregnancy.

Thankfully kids have fared far better from COVID-19 infection than adults. The primary risks of severe COVID-19 for children are among babies and infants, as well as children with health problems that put them at high risk, children with the most significant types of congenital heart disease or those with other medically complex conditions. Children in those groups derive the most benefit from the primary COVID-19 vaccine series; therefore, the decision to vaccinate in their case should be easier.

Informed consent that comes with vaccination should involve discussion of infection risks. The risk of immunization will never be zero because of variability in immune system responses; therefore, making the decision should always involve considering the most-up-to date information available.

I am a professional member of the Society of Cardiovascular Magnetic Resonance and have participated in peer reviewed research (not as lead author) now published in the Journal of Pediatrics. No funding was used in the creation of this article.

Jennifer H. Huang does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.