Myocarditis still rare after COVID vaccination, but risks remain

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

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

“Although the observed rates of myocarditis were higher than expected, the benefits of the vaccine in reducing COVID-19 severity, hospitalizations and deaths far outweigh the risk of developing myocarditis,” said Naveed Janjua, MBBS, the lead study author and Executive Director for Data and Analysis Services at the British Columbia Center for Disease Control.

Still, the number of people who developed myocarditis after vaccination is “somewhat three to six times lower than what we’re seeing post-COVID illness,” says C. Buddy Creech, MD, director of the Vanderbilt Vaccine Research Program. Creech, who was not involved in this study, has led clinical trials of COVID-19 vaccines throughout the pandemic.

Janjua and colleagues examined data from people in British Columbia who were vaccinated against COVID-19 from December 2020 to March 2022. They searched for hospital admissions or emergency department visits for myocarditis or myopericarditis (Inflammation of the sac-like tissue layer surrounding 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.

In all, more than 10.2 million doses of Pfizer or Moderna vaccines were given to people 12 years and older in British Columbia during that time, including nearly 7 million Pfizer doses and 3.2 million Moderna doses. Almost 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 of vaccination, compared to seven expected cases. The rate of myocarditis was 0.97 cases per 100,000 vaccine doses, compared to an expected rate of 0.23 per 100,000 population. The observed rate was about 15 times higher than expected.

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

When analyzed by age, cases of myocarditis were highest in 12-17 and 18-29 year olds and lowest in 70-79 year olds. By gender, myocarditis cases were higher in males than females.

“The numbers are small [for Moderna versus Pfizer], and so maybe not quite accurate, but that was a common theme,” says Creech. “This may be due to the slightly higher amount of antigen in the Moderna vaccine compared to Pfizer.”

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

“Finally, the absolute number of post-vaccination myocarditis cases is very low, albeit higher than expected. Both Pfizer and Moderna, as well as the NIH, CDC and others have started large-scale studies to understand why this is happening,” he says.

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

“This should give parents some level of confidence as they try to protect their families from COVID-19, including the often non-mild cases of post-COVID myocarditis,” he says.


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Myocarditis still rare after COVID vaccination, but risks remain
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