Nov. 22, 2022 – The overall risk of myocarditis after a COVID-19 vaccination remains rare, according to a new study published in the Canadian Medical Association Journal.

At the same time, developing inflammation of the heart muscle appears to be more common in men ages 18-29 who receive the Moderna shot. The researchers recommended the Pfizer shot for this group.

“Although observed rates of myocarditis were higher than expected, the benefits of the vaccine in reducing the severity of COVID-19, hospital admission, and deaths far outweigh the risk of developing myocarditis,” says Naveed Janjua, MBBS, the senior study author and executive director of data and analytic services at the British Columbia Centre for Disease Control.

Still, the number of people who developed myocarditis after vaccination is “somewhere between three to six times less than what we see after COVID disease,” says C. Buddy Creech, MD, director of the Vanderbilt Vaccine Research Program. Creech, who wasn’t involved with this study, has led COVID-19 vaccine clinical trials throughout the pandemic.

Janjua and colleagues looked at data from the people in British Columbia who were vaccinated for COVID-19 from December 2020 to March 2022. They looked for hospital admissions or emergency department visits for myocarditis or myopericarditis (inflammation of the sac-like tissue layer that surrounds the heart) within 7-21 days after vaccination. The research team also compared the number of observed cases to the cases expected if there were no link between a COVID-19 vaccine and myocarditis.

Overall, more than 10.2 million doses of Pfizer or Moderna vaccines were given to people ages 12 and older in British Columbia during that time, including nearly 7 million Pfizer doses and 3.2 million Moderna doses. Nearly 4 million were first doses, about 3.9 million were second doses, and 2.3 million were third doses.

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

Also, they found 141 cases within 21 days, as compared with 20 expected cases. The rate of myocarditis was 1.37 cases per 100,000 vaccine doses, as compared to an expected rate of 0.39 per 100,000 population. The observed rate was about 7 times higher than expected.

Analyzing by age, myocarditis cases were highest among ages 12-17 and 18-29, and lowest among ages 70-79. By sex, myocarditis cases were higher for men than women. 

“The numbers are small [for Moderna versus Pfizer], and therefore may not be entirely precise, but this has been a common theme,” Creech says. “This may be due to the slightly higher amount of antigen in the Moderna vaccine compared to Pfizer.”

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

“At the end of the day, the absolute number of myocarditis cases after vaccination are very low, though higher than we would expect. Both Pfizer and Moderna, as well as NIH, CDC, and others, have launched large-scale studies in order to understand why this is happening,” he says. 

Finally, Creech says, the cases of myocarditis after vaccination have been mild. 

“This should provide parents a measure of confidence as they seek to protect their families from COVID disease, including the often not mild cases of myocarditis following COVID disease,” he says.

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