Simone A, Herald J, Chen A, et al. - In an integrated health care system in the US, the occurrence of acute myocarditis in adults following COVID-19 mRNA vaccination was rare, at an incidence of 5.8 cases per 1 million individuals after the second dose (1 case per 172 414 fully vaccinated individuals). The signal of increased myocarditis in young men warrants further research.

• In this population-based cohort study, members from Kaiser Permanente Southern California who were aged 18 years or older and received at least 1 dose of the BNT162b2 (Pfizer) or mRNA-1273 (Moderna) mRNA vaccine were included (n=2 392 924; 50.2% received mRNA-1273 and 50.0% BNT162b2).

• This cohort comprised 54.0% women, 31.2% White, 6.7% Black, 37.8% Hispanic, and 14.3% Asian individuals.

• In the vaccinated group, 15 cases of confirmed myocarditis were documented (2 after the first dose and 13 after the second), for an observed incidence of 0.8 cases per 1 million first doses and 5.8 cases per 1 million second doses over a 10-day observation window.

• All were males, with a median (IQR) age of 25 (20-32) years.

• In the unexposed group, 75 cases of myocarditis were found, with 39 (52%) males and median (IQR) age of 52 (32-59) years.

• For myocarditis, the estimated incidence rate ratio was 0.38 and 2.7 for the first dose and second dose, respectively.

• Of those who developed myocarditis postvaccination, all tested negative for SARS-CoV-2 by polymerase chain reaction on admission and 14 (93%) complained of chest pain between 1 to 5 days post-vaccination.

• Conservative management resolved symptoms in all cases; no patients needed intensive care unit admission or readmission following discharge.