Moderna Reveals Slightly Higher Rates of Myocarditis in Young People Who Received Its COVID-19 Vaccine
Moderna’s chief medical officer said in a call with reporters on Thursday that its COVID-19 vaccine offers a high level of protection against severe disease, hospitalization, and death from COVID-19, but it comes with a higher—yet still relatively rare—risk of heart problems.
Paul Burton, M.D., Ph.D., shared that data has found that there have been 13.3 cases of myocarditis per 100,000 people who received the Moderna vaccine compared to 2.7 cases per 100,000 people who got the Pfizer-BioNTech vaccine, per CNBC. But it's important to note that data show that the risk of myocarditis is greater if you become ill with COVID-19— about 150 in 100,00 people—significantly higher than the risk with the COVID-19 vaccine.
The news comes just days after Moderna revealed that the Food and Drug Administration (FDA) is taking extra time to analyze information on reports of myocarditis before deciding whether to authorize the company’s mRNA COVID-19 vaccine for children between the ages of 12 and 17. “The FDA notified Moderna that this review may not be completed before January 2022,” the company said in a press release. “The safety of vaccine recipients is of paramount importance to Moderna. The company is fully committed to working closely with the FDA to support their review and is grateful to the FDA for their diligence.”
This is not the first time the mRNA COVID-19 vaccines have been linked to myocarditis. The Centers for Disease Control and Prevention (CDC) shared a report issued by the Advisory Committee on Immunization Practices (ACIP) in May, detailing the link. The ACIP said it received “relatively few” reports of myocarditis in some people who have received an mRNA COVID-19 vaccine (either the Pfizer-BioNTech or Moderna vaccine), but the report did not clarify exactly how many. The cases seem to occur mostly in teens and young adults, more often in males than females, more often after dose two than dose one, and usually within four days after vaccination.
“Most cases appear to be mild, and follow-up of cases is ongoing,” the report says. Rates of myocarditis after vaccination haven’t differed from “expected baseline rates” but the ACIP wanted to share the information with healthcare providers, so experts are ready to identify and treat the heart condition should it become an issue for a patient.
Since then, the link between myocarditis and the mRNA vaccines has grown. However, the CDC, other health organizations, and doctors point out that the risk of myocarditis if you develop COVID-19 is much worse.
Here’s what you need to know.
Back up: What is myocarditis?
Myocarditis is a heart condition that causes inflammation of the heart muscle (a.k.a myocardium), according to the National Institutes of Health (NIH). The inflammation enlarges the heart, forcing it to work harder, which can decrease its ability to pump blood normally. Myocarditis is considered a rare condition, and affects between 10 to 20 out of 100,000 people in the world each year.
What are the symptoms of myocarditis?
Some people with myocarditis have no obvious symptoms, the NIH says. But, when people do experience signs of the condition, they can include:
Chest pain
Abnormal heartbeat
Shortness of breath
Fatigue
Fever
Headache
Sore throat
Diarrhea
Leg swelling
What typically causes myocarditis?
It’s frequently linked to viral, bacterial, parasitic, and fungal infections, per the NIH—but viruses are the most common cause of injuring the heart in myocarditis cases. “This injury can be a direct result of the virus infecting and killing heart muscle cells, or it can be an indirect result due to the body’s immune response against the virus,” says Eliot Peyster, M.D., a cardiology physician at Penn Medicine.
Many cases of myocarditis happen due to cross-reactivity, where antibodies that are designed to target and kill a virus also bind to and injure heart muscle cells in the process, Dr. Peyster explains.
Myocarditis can also be caused by certain medications and drugs, including drugs used to treat cancer, antibiotics like penicillin, sulfonamide drugs, some anti-seizure medications, and some illegal drugs like cocaine, according to the Mayo Clinic. Exposure to chemicals like carbon monoxide or having an underlying inflammatory or autoimmune condition like lupus can also raise your risk of myocarditis.
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Why might the COVID-19 vaccine lead to myocarditis?
It’s important to point out that this is still being investigated, and experts have not confirmed a cause-and-effect relationship between the two.
It’s “difficult” to figure out a potential link between the COVID-19 vaccines and myocarditis because the mRNA vaccines don’t contain an actual virus, says infectious disease expert Amesh A. Adalja, M.D., senior scholar at the Johns Hopkins Center for Health Security.
“It’s just a piece of genetic material,” he says. “The hypothesis is that the vaccine sets off an immune response that may have some collateral damage in select individuals who are predisposed to myocarditis for some reason.” That exact reason, he adds, “hasn’t been established at this point.”
As for why the Moderna vaccine has a greater risk of myocarditis than the Pfizer-BioNTech vaccine, it’s also unclear at this point, says Thomas Russo, M.D., professor and chief of infectious disease at the University at Buffalo in New York. However, he says, it may have to do with dosing: The Pfizer-BioNTech vaccine dosage is 0.3 milliliters while the Moderna vaccine is 0.5 milliliters.
Dr. Adalja agrees. “The higher rate could be related to the fact that the Moderna vaccine uses a higher dose of mRNA,” he says. “This may provoke a stronger immune response, as evidence by less waning of immunity in Moderna vaccine recipients. This stronger immune response may lead to higher rates of myocarditis.”
Has myocarditis been linked to other vaccines?
There is an established link between the smallpox vaccine and myocarditis, although the risk is small. One JAMA study found 18 cases of possible myocarditis in military service members after 230,734 were vaccinated against smallpox. “We actually counsel patients about the risk of myocarditis with the smallpox vaccine,” Dr. Adalja says.
There have also been case reports of myocarditis linked to the flu and tetanus vaccines, says Jennifer Wong, M.D., cardiologist and medical director of Non-Invasive Cardiology at MemorialCare Heart and Vascular Institute at Orange Coast Medical Center in Fountain Valley, CA.
But these vaccines are different from the COVID-19 mRNA vaccines. “It should be noted that the smallpox and flu vaccines use live or inactivated viruses to activate the immune system, which is quite different from the two most popular COVID-19 vaccines in the U.S.,” Dr. Peyster says.
How is myocarditis treated?
Most of the time, myocarditis gets better on its own or with standard treatment, Dr. Wong says, but the severity of the condition has a wide range. It can affect a small part of the heart and cause no issues whatsoever or progress to full-blown heart failure. That’s why, if you develop symptoms of myocarditis, Dr. Wong recommends going to the emergency room, just to be safe.
If you present with abnormal heart rhythms or severe heart failure, your doctor may give you medication to help reduce your risk of blood clots, including beta-blockers and angiotensin-converting enzyme (ACE) inhibitors, the Mayo Clinic says. “In most cases, the goal is to support the patient during the early stage of the illness to give the inflammation a chance to subside and the heart a chance to recover on its own,” Dr. Peyster says.
However, the latest report shouldn’t discourage you from getting vaccinated. “This is a very small percentage of [a specific group of] people who are reported to have this,” Dr. Wong says. It’s also important to take the risk-benefit ratio into account. The benefits of getting vaccinated so far outweigh the possibility of this rare condition, she says, if a cause-and-effect relationship is confirmed at all.
So, which vaccine should you get?
Dr. Russo stresses that rare cases of myocarditis that can develop as a result of the vaccine “tend to be very mild and transient—it seems to resolve in a few days.” And, he adds, it “seems to be more severe and prolonged with COVID-19. It’s hard to compare the two.”
While the overall risk of developing myocarditis with any mRNA COVID vaccine is low, Dr. Russo says he’s been advising people in the highest risk group—16- to 30-year-old males—to get the Pfizer-BioNTech vaccine over the Moderna vaccine. “At the end of the day, the risk is still small, but whatever edge you can get in life, go for it,” he says. And, if a person has a prior history of myocarditis, Dr. Adalja suggests using the Johnson & Johnson vaccine, which has not been linked to the heart condition.
Overall, experts stress the importance of getting the COVID-19 vaccine.
“It is always better to get the vaccine rather than COVID,” says William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine. “COVID not only cases a much higher rate of myocarditis but it can do so many other bad things to you—and so can long COVID.”
This article is accurate as of press time. However, as the COVID-19 pandemic rapidly evolves and the scientific community’s understanding of the novel coronavirus develops, some of the information may have changed since it was last updated. While we aim to keep all of our stories up to date, please visit online resources provided by the CDC, WHO, and your local public health department to stay informed on the latest news. Always talk to your doctor for professional medical advice.
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