November 22, 2022 – Overall risk of myocarditis after COVID-19 vaccination remains rare, says expert new study published in the Canadian Medical Association Journal.
At the same time, the development of inflammation of the heart muscle appears to be more common in men aged 18 to 29 who receive the Moderna vaccine. The researchers recommended the Pfizer vaccine for this group.
“Although the observed rates of myocarditis have been higher than expected, the benefits of the vaccine in reducing the severity of COVID-19, hospital admissions and deaths far outweigh the risk of developing myocarditis,” says Naveed Janjua, MBBS, the study’s lead author and executive director of data and analytics services at the British Columbia Center for Disease Control.
Still, the number of people who developed myocarditis after vaccination 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 throughout the pandemic.
Janjua and colleagues looked at data from people in British Columbia who were vaccinated against COVID-19 from December 2020 to March 2022. They looked for hospital admissions or emergency room visits for myocarditis or myopericarditis (inflammation of sac-like tissue layer that surrounds the heart) within 7 to 21 days of vaccination. The research team also compared the number of observed cases to the cases expected if there was no link between a COVID-19 vaccine and myocarditis.
Overall, more than 10.2 million doses of Pfizer or Moderna vaccines were administered to people aged 12 and older in British Columbia during this 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 of vaccination, compared to seven expected cases. The myocarditis rate 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.
Additionally, they found 141 cases in 21 days, compared to the expected 20 cases. The myocarditis rate 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.
Analyzing by age, myocarditis cases were highest in 12-17 and 18-29 year olds, and lowest in 70-79 year olds. By gender, myocarditis cases were higher in men than in women.
“The numbers are small [for Moderna versus Pfizer], and so maybe not entirely accurate, but it’s 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 United States and around the world, Creech says.
“Ultimately, the absolute number of myocarditis cases after vaccination is very low, although higher than we expected. Pfizer and Moderna, along with NIH, CDC and others, have initiated studies at large scale in order to understand why this is happening,” he says.
Finally, says Creech, cases of myocarditis after vaccination have been mild.
“This should give parents some confidence as they seek to protect their family from COVID disease, including the often non-mild cases of myocarditis following COVID disease,” he says.