US investigating handful of myocarditis cases following COVID-19 vaccination

a teen gets the Pfizer COVID-19 vaccine in Michigan. — A teenager gets the Pfizer COVID-19 vaccine in Michigan on April 6, 2021. (Image credit: Matthew Hatcher / Stringer/Getty)

A U.S. vaccine safety working group is looking into a small number of cases of heart inflammation in young people following COVID-19 vaccines, Reuters reported. Most cases reported so far have been mild.

The condition, called myocarditis, is also somewhat common, and the rate seen so far in vaccinated people does not differ from the baseline expected in the population, meaning there may be no link.

And even if a link can be established, "vaccines are going to unequivocally be much more beneficial outweighing this very low, if conclusively established, risk," Dr. Amesh Adalja, senior scholar at the Johns Hopkins Center for Health Security, told Reuters, referring to the immense benefit the vaccine provides in preventing a COVID-19 infection.

The cases seem to be occurring more often in adolescents and young adults, in males and about four days after receiving the second shot of mRNA vaccines made by PFizer-BioNTech and Moderna, according to the the U.S. Centers for Disease Control and Prevention (CDC) Advisory Committee on Immunization Practices.

To see whether there is a link, the researchers are reviewing records and asking clinicians to be on the alert to report such cases.

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Myocarditis is an inflammation of the heart muscle that often occurs after infection with a virus, such as influenza or the novel coronavirus that causes COVID-19. Symptoms can range from mild fatigue and chest pain that resolves on its own to heart rhythm irregularities, cardiac arrest and even death in rare, severe cases. Most cases resolve with no long-term effects, according to a 2020 review in the Journal of Cardiothoracic and Vascular Anesthesia. Males tend to be more commonly affected by myocarditis, according to that study.

About 10 to 22 people per 100,000 — or up to 70,000 people in the U.S. — are diagnosed with viral myocarditis every year, according to the study. However, many cases are so mild that they are never diagnosed, meaning true incidence is likely higher.

So far, more than 4 million people under age 18 have received mRNA COVID-19 vaccines, and tens of millions ages 24 and younger have received the vaccine, according to the CDC, so many cases would be expected by chance in this population. The CDC did not say how many cases of myocarditis they were investigating.

Other vaccines, such as the flu vaccine, can cause myocarditis, so it's important to evaluate any potential link between COVID-19 vaccines and the heart condition, Adalja told Reuters. Because of that, the CDC started monitoring for cases of this heart inflammation in those who have been vaccinated since COVID-19 vaccinations began.

Cases of the heart condition following the Pfizer-BioNTech vaccine have also been reported in Israel and Europe, and regulators there have so far found no link to the vaccines, Biospace reported.

Originally published on Live Science.

Tia is the editor-in-chief (premium) and was formerly managing editor and senior writer for Live Science. Her work has appeared in Scientific American, Wired.com, Science News and other outlets. She holds a master's degree in bioengineering from the University of Washington, a graduate certificate in science writing from UC Santa Cruz and a bachelor's degree in mechanical engineering from the University of Texas at Austin. Tia was part of a team at the Milwaukee Journal Sentinel that published the Empty Cradles series on preterm births, which won multiple awards, including the 2012 Casey Medal for Meritorious Journalism.