Myocarditis after COVID vaccine still rare, but risk still exists

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

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

“Although the incidence 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 and director of data and analytical services at the British Columbia Center for Disease Control.

Still, the number of people who developed myocarditis after vaccination is “somewhere between three to six times less than we see from COVID disease,” 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 a COVID-19 vaccine 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 room visits for myocarditis or myocarditis (swelling of a sac-like tissue layer surrounding the heart) within 7-21 days after vaccination. The research team also compared the number of cases that occurred to the number of cases that would be expected if there were no link between the COVID-19 vaccine and myocarditis.

Overall, more than 10.2 million doses of Pfizer or Moderna vaccines were given to people 12 years of age 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 the expected seven cases. The incidence of myocarditis was 0.97 cases per 100,000 vaccine doses, compared with 0.23 per 100,000 population. The ratio was about 15 times higher than expected.

They also found 141 cases within 21 days, compared to the expected 20 cases. The incidence of myocarditis was 1.37 per 100,000 vaccine doses, compared with 0.39 per 100,000 population. That percentage was about 7 times higher than expected.

Analysis by age, myocarditis cases were highest in the age group of 12-17 and 18-29 years, and lowest among 70-79 years. By gender, the incidence of myocarditis was higher in men than in women.

“The numbers are small [for Moderna versus Pfizer], and so may not be entirely accurate, but this has been 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 are seeing in the United States and around the world, Creech says.

“At the end of the day, the absolute number of heart attacks after vaccination is very low, although higher than we would have expected. Both Pfizer and Moderna, as well as the NIH, CDC and others, have launched extensive research to understand why this is happening,” he says.

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

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

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