Since the end of 2020, public health experts and medical professionals have been warning people in the U.S. about the most common COVID vaccine side effects. That way, once they were rolled out widely, none of us would be alarmed about experiencing a fever, headache, fatigue, soreness or redness at the injection site, etc. But some side effects have emerged since then that have caused more concern. In mid-April, for example, use of the Johnson&Johnson vaccine was paused temporarily amid reports of rare blood clots, some of which were fatal. After the Centers for Disease Control and Prevention (CDC) and U.S. Food and Drug Administration (FDA) determined the benefits of the Johnson&Johnson vaccine outweighed the risks, the FDA added a warning to that vaccine about the potential for clotting complications. Now, the two other two vaccines approved for emergency use in the U.S., Pfizer and Moderna, have gotten new warnings, courtesy of the FDA. If you got either of those vaccines, the FDA now says there are three delayed side effects you should look out for.
On June 25, the FDA added new warnings for both vaccine provider and recipients to their fact sheets on Pfizer and Moderna over rare cases of heart inflammation called myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the lining outside the heart). The agency added the warnings just two days after the CDC's Advisory Committee on Immunization Practices (ACIP) met and confirmed the "likely association" between myocarditis and pericarditis and the Pfizer and Moderna vaccines, both of which use mRNA. However, it's rare, treatable, and usually mild, they say.
While the FDA's new warning notes the chance of experiencing heart inflammation is "very low," they urge recipients of Pfizer and Moderna to "seek medical attention right away if you have any of the following symptoms":
Shortness of breath
Feelings of having a fast-beating, fluttering, or pounding heart
The FDA's new warning, which is the same on the fact sheets for both Pfizer recipients and Moderna recipients, says symptoms of myocarditis and pericarditis don't typically crop up right away, but tend to begin "within a few days following receipt of the second dose"
The FDA also urges recipients who've experienced myocarditis or pericarditis in the past to tell their vaccination provider. For providers, the FDA notes there's an "increased risks of myocarditis and pericarditis, particularly following the second dose."
According to CDC data, as of mid-June, more than 1,200 cases of myocarditis or pericarditis have occurred in patients who were given the Pfizer or Moderna vaccine. Most of the cases have occurred among younger men between 16 and 39 years old after their second shot. The CDC estimates there will be about 16 cases of myocarditis or pericarditis for every million second doses among this demographic, which works out to 0.0016 percent, the American Heart Association (AHA) points out.
James de Lemos, MD, a professor of medicine at UT Southwestern Medical Center in Dallas, told the AHA that by contrast, the chances of a young person getting myocarditis after COVID-19 is "much higher"—between 1 percent and 3 percent. "It's not something that people, in my view, should be afraid of. Because the risks are low, and the benefits of vaccination are overwhelming," de Lemos said.
In a statement following the meeting, the ACIP said something similar. "The facts are clear: this is an extremely rare side effect, and only an exceedingly small number of people will experience it after vaccination," they said. "Importantly, for the young people who do, most cases are mild, and individuals recover often on their own or with minimal treatment. In addition, we know that myocarditis and pericarditis are much more common if you get COVID-19, and the risks to the heart from COVID-19 infection can be more severe."