Myocarditis after the COVID vaccine is still rare, but the risks are still there

Nov. 22, 2022 — The overall risk of myocarditis remains rare after vaccination against COVID-19. new study appeared in Canadian Medical Association Journal.

However, inflammation of the heart muscle appears to be more common in men aged 18 to 29 who received the Moderna injection. The researchers recommended the Pfizer injection to this group.

“Although the observed rate of myocarditis was higher than expected, the benefits of the vaccine in reducing the severity of COVID-19, hospitalizations and deaths far outweigh the risk of developing myocarditis,” says Naveed Janjua, MBBS, lead author of the study. is executive director of data and analysis services at the British Columbia Center for Disease Control.

Still, the number of people who develop myocarditis after the vaccine is “somewhere between three and six times less than what we see after COVID disease,” says C. Buddy Creech, MD, director of the Vanderbilt Vaccine Research Program. Creech, who was not involved in this study, led clinical trials of the COVID-19 vaccine during the pandemic.

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Janjua and colleagues looked at data from people vaccinated against COVID-19 between December 2020 and March 2022 in British Columbia. Hospital admissions or emergency department visits for myocarditis or myopericarditis were sought (inflammation of the sac-like tissue the layer surrounding the heart) within 7-21 days after vaccination. The research team also compared the number of observed cases with the expected cases in the absence of a link between the COVID-19 vaccine and myocarditis.

In all, more than 10.2 million doses of Pfizer or Moderna vaccine were administered to people 12 years of age and older in British Columbia during that time, including nearly 7 million Pfizer and 3.2 million Moderna doses. Nearly 4 million were the first batch, about 3.9 million were the second batch, and 2.3 million were the third batch.

The researchers found 99 cases of myocarditis within 7 days of vaccination, compared to the seven expected cases. The rate of myocarditis was 0.97 per 100,000 vaccine doses, compared with 0.23 per 100,000 population. The observed rate was approximately 15 times higher than expected.

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Additionally, 141 cases were found within 21 days, compared to the 20 expected. The rate of myocarditis was 1.37 cases per 100,000 vaccine doses, compared with the expected rate of 0.39 per 100,000 population. The observed rate was about seven times higher than expected.

Analyzing by age, myocarditis was highest among 12-17 and 18-29-year-olds, and lowest among 70-79-year-olds. According to gender, myocarditis cases were higher in men than in women.

“The numbers are small [for Moderna versus Pfizer], and so it may not be completely accurate, but it’s a common theme,” says Creech. “This is because the Moderna vaccine has a slightly higher amount of antigen than the Pfizer vaccine.”

The study confirmed what other researchers are seeing in the United States and around the world, Creech says.

“At the end of the day, the absolute number of cases of post-vaccination myocarditis is very low, although it is higher than we would expect. Both Pfizer and Moderna, along with the NIH, CDC and others, have launched large-scale studies to understand why this is happening,” he says.

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Finally, Creech says that cases of post-vaccination myocarditis are mild.

“This should give parents some confidence when trying to protect their family from COVID disease, including the often mild myocarditis that follows COVID disease,” he says.