Myocarditis Risk Still Low After COVID-19 Vaccine Booster

The BNT162b2 mRNA vaccine (Pfizer-BioNTech) reduces risk from SARS-CoV-2 but seems associated with an increased risk for myocarditis, particularly in younger males. A key question is whether the booster (the third vaccine dose) is also associated with a higher risk for myocarditis.

In July 2021, Israel approved the use of a third vaccine dose for the general population, and the Israel Defense Forces began vaccinating military personnel with BNT162b2 in August 2021. These people were followed through October 2021 for cases of myocarditis.

There were 126,029 military personnel vaccinated, most of whom were 18 to 24 years old. In follow-up, nine men aged 18 to 24 years developed myocarditis, including one case that developed post-COVID-19. There were no myocarditis cases among women. Among the eight men without COVID-19 infection, four had myocarditis symptoms within a week of vaccination and three at 8 to 10 days after vaccination (one person developed myocarditis more than 2 weeks after vaccination and was excluded from further analysis). These 7 episodes were mild and not associated with arrhythmias or heart failure. The incidence rate for men 18 to 24 years old was estimated to be 6.43 per 100,000 in the first week after receiving the third dose.