The Link Between Myocarditis and COVID-19 mRNA Vaccines

[Originally published: June 1, 2021. Updated: June 24, 2021]

Note: Information in this article was accurate at the time of original publication. Because information about COVID-19 changes rapidly, we encourage you to visit the websites of the Centers for Disease Control & Prevention (CDC), World Health Organization (WHO), and your state and local government for the latest information.

Myocarditis, an inflammation of the heart muscle, is a medical condition you may not have heard much about—until recently. It became a trending topic this spring when the Centers for Disease Control and Prevention (CDC) reported that it is monitoring a small number of cases of heart inflammation that have arisen in young adults not long after mRNA COVID-19 vaccination. The side effect is considered important but uncommon—arising in about 12.6 cases per million second doses administered. And now the Food and Drug Administration (FDA) has announced it will place a warning on the mRNA vaccines. It’s important to note that the vaccination is still recommended for everyone who is eligible.  

The CDC’s Advisory Committee on Immunization Practices said that there’s a “likely association” between the Pfizer and Moderna vaccines and reported cases of heart inflammation. This inflammation may occur in the heart muscle (myocarditis) or in the outer lining of the heart (pericarditis).

The safety group reports that the majority of cases have occurred in people 30 years old and under, mostly in males, and more often than not, inflammation occurred after the second dose of the vaccine.  

In May, the Advisory Committee on Immunization Practices COVID-19 Vaccine Safety Technical (VaST) Work Group, a special panel that advises the CDC, reported that the cases were occurring about four days after vaccination.  

Myocarditis can be caused by a variety of common viruses, explains Jeremy Asnes, MD, chief of pediatric cardiology at Yale Medicine and co-director of the Yale New Haven Children’s Hospital Heart Center. “Examples include viruses that cause typical colds or gastrointestinal illnesses. Myocarditis can also be caused by certain medications, as well as by some autoimmune diseases,” he says. “And, though rare, myocarditis can be caused by an immune response to a vaccine such as the smallpox vaccine, which was the most successful vaccine in world history.”  

At this point, it’s too early to tell what may be causing the recently reported myocarditis cases. As with any medical intervention, physicians and agencies—including both the Food and Drug Administration (FDA) and CDC—continually monitor the use and side effects of vaccines, which is why they’re looking into these cases. 

Still, doctors are reassuring patients and families. “This is an incredibly rare event,” says Dr. Asnes. He adds that patients seem to get better quickly. “I think that the most important message at this point is that we are still recommending eligible people older than 12 follow the vaccine recommendations from the CDC,” he says. “While we are taking these reports of myocarditis very seriously, we—along with the CDC—continue to feel that the benefits of being vaccinated against COVID-19 far outweigh the very small risk of getting myocarditis related to the vaccine itself.” 

Quite simply, “Myocarditis is inflammation of the heart muscle,” says Erica Spatz, MD, MHS, a Yale Medicine cardiologist, “and there's a range of different symptoms and levels of severity of myocarditis. Most often the inflammation is mild, and we treat it with anti-inflammatory medications, such as ibuprofen or a prescription medication called colchicine,” she says. “Myocarditis, in most cases, gets better on its own without medical intervention.” In rare cases, it can cause arrhythmia or weakness of the heart.  

Myocarditis can occur in both children and adults.

With myocarditis, there is often no identifiable cause. Sometimes, however, the triggering event can be a common virus. “We know some viruses are more likely to cause myocarditis,” Dr. Spatz says, such as the flu, adenovirus, parvovirus (Fifths Disease) or Coxsackie (hand, foot, and mouth disease). But patients who didn’t have an obvious viral infection are also diagnosed with myocarditis. Some might report having had an upper respiratory infection in the previous few weeks, she says.

There have been some case reports of myocarditis after COVID-19 infection. Some were in college athletes, which initially raised concerns among doctors. “That number was originally thought to be really high,” says Yale Medicine cardiologist Rachel Lampert, MD. “But in subsequent publications, reporting on studies with tens of thousands of athletes, the actual incidence of myocarditis in college athletes with COVID is much lower than had been originally thought. In a recent study of over 19,000 athletes who underwent COVID testing, 3,018 had COVID, and of these, just 0.7% had cardiac involvement.”  

The fact that the condition itself has a range of known and unknown causes makes it especially difficult to understand right now. Doctors have observed that myocarditis sometimes follows COVID-19 infection. But “Whether the virus itself infects the heart, or whether it's the immune reaction to the virus that causes the myocarditis, is one of the things we don't know,” Dr. Lampert says.  

Another unknown is whether or not SARS-CoV-2, the virus that causes COVID-19, is more likely to cause myocarditis than other viruses. “Whether myocarditis was actually more common with COVID than with other viruses, we don't really know. We know that some people with COVID got myocarditis. But there was a lot of COVID at one time, and we may have been more tuned into these cases as compared with sporadic cases due to other viruses that are less prevalent,” says Dr. Lampert. “It’s one of the many questions we don't know the answer to.”

Chest pain is the most common symptom of myocarditis, and the discomfort is typically more noticeable and pronounced when lying down rather than when sitting up. The pain is caused by inflammation, which can be found in the heart muscle, as well as in the pericardium, which is the sac the heart sits in, explains Dr. Asnes.  

Adds Erin Faherty, MD, a pediatric cardiologist at Yale Medicine, “The pain is typically sharp and persistent versus brief pain that only lasts a couple of minutes. The patients we have seen all presented with persistent chest discomfort.”

If you or a loved one experiences chest or heart pain after vaccination (or at any other time), seek medical attention. It’s always a good idea to have it checked out by a doctor. The CDC also says to be on the lookout for shortness of breath and/or a rapid, fluttering, or pounding heartbeat following vaccination and to seek medical attention if you or your child experience these symptoms.

While physicians are waiting for more information and guidance from the CDC and FDA, it may put minds at ease to know that cardiologists find that “in general, if the heart has a temporary inflammation, it gets better,” says Dr. Lampert. 

[Visit Yale Medicine's Vaccine Content Center for more stories on COVID-19 vaccines.]

Note: Information provided in Yale Medicine articles is for general informational purposes only. No content in the articles should ever be used as a substitute for medical advice from your doctor or other qualified clinician. Always seek the individual advice of your health care provider with any questions you have regarding a medical condition.