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Heart inflammation after a COVID-19 vaccine is rare. But new study shines light on why it happens

A COVID-19 vaccine is prepared for use at a clinic in New Haven on November 06, 2021.
Joe Amon
Connecticut Public
A COVID-19 vaccine is prepared for use at a clinic in New Haven on November 06, 2021.

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The link between COVID-19 vaccination and inflammation of the heart muscle – myocarditis – is exceedingly rare, but was observed primarily in adolescent boys during the pandemic.

No one really knew what caused this rare condition until now.

After the new COVID-19 vaccines rolled out earlier in the pandemic, scientists proposed a number of hypotheses on the causes of myocarditis after an mRNA vaccination: including the development of autoimmune diseases, or allergic responses.

But a new study by scientists at the Yale School of Medicine, published in the journal Science Immunology, found certain signatures in the immune cells indicating a larger than necessary immune response in the body. That led to rare cases of an inflammation of the heart that was generally mild and resolved within days.

An exaggerated immune response

Cytokines, secreted by cells – primarily “helper T cells” in the immune system – can rev up a bit too much in response to the mRNA COVID-19 vaccine, said Akiko Iwasaki, professor of immunobiology; and of molecular, cellular, and developmental biology at the Yale School of Medicine.

Other immune cells called “killer cells,” which kill viruses and tumors, can also have an exaggerated response.

The findings suggest a longer interval between the first and second doses.

“When you have the mRNA vaccine doses that were given in a relatively short time window," Iwasaki said, "this sort of revved up immune response to cause myocarditis is increased in the adolescent male recipients.”

A Canadian study found more than 56 days to be the optimal wait time, Iwasaki said.

Heart inflammation more frequent in the unvaccinated with COVID

The Yale researchers and infectious disease expert Dr. Ulyses Wu, who was not involved with the Yale study, cautioned that COVID-19 also has been shown to cause rare cases of myocarditis and a host of other cardiac complications.

But cases of heart inflammation – while rare – are still substantially higher in unvaccinated adolescent boys versus those who got vaccinated against COVID-19.

According to the Centers for Disease Control and Prevention, among males aged 12 to 17, about 22 to 36 per 100,000 experienced myocarditis within 21 days after receiving a second vaccine dose.

Among unvaccinated males in this age group, however, the rate of myocarditis was 50.1 to 64.9 cases per 100,000 after infection with the COVID-19 virus.

Sujata Srinivasan is Connecticut Public Radio’s senior health reporter. Prior to that, she was a senior producer for Where We Live, a newsroom editor, and from 2010-2014, a business reporter for the station.

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