Myocarditis rare after Covid vaccination, with differences between ages and vaccines

Development of myocarditis after SARS-CoV-2 vaccination is rare but appears to be more frequent among males aged 18-29 and especially after the second mRNA-1273 (Moderna) vaccine.

The study is published in CMAJ (Canadian Medical Association Journal) on Nov. 21, 2022.

“In this study, we found higher observed rates of myocarditis after receipt of mRNA vaccines than expected, but absolute rates were low,” the authors said. “We observed a higher rate of myocarditis among males aged 18–29 years after receipt of the second dose of mRNA-1273 (Moderna) vaccine compared with those who received BNT162b2 (Pfizer-BioNTech), though the rate was lower after the third dose.…..Although observed rates of myocarditis were higher than expected, the benefits of vaccination against SARSCoV-2 in reducing the severity of COVID-19, hospital admission and deaths far outweigh the risk of developing myocarditis,” they added.

Primary exposure was any dose of an mRNA vaccine against SARS-CoV-2. Primary outcome was incidence of development of myopericarditis within 7 and 21 days after vaccination.

The investigators analyzed retrospectively data from the BC (British Columbia, Canada) COVID-19 Cohort, which included more than 10.2 million doses of mRNA vaccines administered to people aged 12 years and older from Dec. 15, 2020, to Mar. 10, 2022.

About 7 million were BNT162b2 (Pfizer-BioNTech) doses and 3.2 million were mRNA-1273 (Moderna) doses.

The endpoint of the study was the rate of subjects per 100,000 who were admitted to hospital or who visited an emergency department because of myocarditis within 7 and 21 days of vaccination.

The rate of myocarditis 21 days after vaccination was 1.37 per 100,000 people compared with estimated 0.39 per 100,000 for nonvaccinated people.

Notably, the highest rates of myocarditis were in males (2.15 per 100,000), among those aged 18–29 (2.97 per 100,000), after the second dose (2.27/100,000 doses) and in people vaccinated with mRNA-1273 (Moderna) (1.75 per 100,000).

For males aged 18–29 receiving the mRNA-1273 (Moderna) vaccine, the rate was 22.9 per 100,000 doses.

Following the third dose, rates of myocarditis fell, including among subjects aged 18–29.

The authors concluded, “Although absolute rates of myocarditis were low, vaccine type, age and sex are important factors to consider when strategizing vaccine administration to reduce the risk of postvaccination myocarditis. Our findings support the preferential use of the BNT162b2 vaccine over the mRNA-1273 vaccine for people aged 18–29 years.”