Risk of Heart Inflammation Remains Elevated After COVID-19 Booster: Study

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The risk of heart inflammation following receipt of a COVID-19 booster shot remains elevated, according to a new study.

The primary series of the messenger RNA-based vaccines, made by Pfizer and Moderna, have already been linked to heart inflammation, but not as much research has been done on whether booster doses also present heart-related risks.

In the study, French researchers analyzed 4,890 myocarditis cases admitted to hospitals in the country between Dec. 27, 2020, and Jan. 31, 2022. They used 48,900 controls from the general population, matching for gender, age, and area of residence.

Receipt of a COVID-19 vaccine booster was found to result in an elevated risk of myocarditis, a type of heart inflammation, particularly receipt of a Moderna booster.

The adjusted risk after a Pfizer booster was three times higher among the boosted, and four times higher after a Moderna booster.

For people aged 12 to 29, the risk was 4.9 times higher for Pfizer booster recipients. Data for young Moderna booster recipients was not available because France suspended use of the vaccine in October 2021, and only made it available again the following month for those over 30 years old.

For those aged 30 and above, a Pfizer booster was associated with a 2.4 times higher risk of heart inflammation, while Moderna’s booster brought a 4.1 times higher risk.

The risks were more elevated if a person received the third dose within 170 days of the first dose. A booster is the third dose because a primary series is comprised of two doses. Similarly, the risk of myocarditis after the second dose is higher in cases where a person receives the dose within 27 days of the first dose.

U.S. authorities have advised young people to get their second shot up to eight weeks after the first, primarily because of the heart inflammation risk. The earlier guidance was three weeks or one month after the first.

“We found that the risk of myocarditis remained elevated after the booster dose and that longer intervals between each consecutive dose (including booster doses) may decrease the occurrence of vaccine-associated myocarditis,” the authors of the new study wrote.

The paper was posted ahead of peer review on the medRxiv preprint server. The authors work for the French government and several universities.

Jump in Myocarditis Hospitalization

In a separate study also conducted by French researchers, authors examined the risk of hospitalization for a number of diseases in 2021, the first full year of the pandemic, versus 2019, the last full year before the pandemic, and 2020.

They found a decreased risk for nearly all diseases when comparing data from 2021 to 2019, except for a 28 percent increased risk of myocarditis and a 10 percent increased risk of pulmonary embolisms.

They also found an increase in myocarditis in 2021 compared to 2020 that “coincided with the vaccination campaign in young individuals,” particularly among young men aged 10 to 29, a group at elevated risk from heart inflammation following COVID-19 vaccination. COVID-19 itself can also lead to heart issues.

Researchers, sponsored by Nimes University Hospital, said that adverse vaccine events likely explain at least a part of the jump in myocarditis cases leading to hospitalization. Another part could stem from the difficulty in accessing hospital care in the early part of the pandemic, when many countries imposed strict measures to try to combat the illness.

Zachary Stieber

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Zachary Stieber covers U.S. and world news. He is based in Maryland.

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