Diaz GA, Parsons GT, Gering SK, et al. - Following COVID-19 vaccination, two distinct self-limited syndromes, myocarditis and pericarditis, were found. In younger patients, myocarditis developed quickly, usually after the second vaccine. Pericarditis developed later in elderly patients, either after the first or second dose.

• Among 2,000,287 people receiving at least 1 COVID-19 vaccination, 58.9% were women, the median age was 57 years (interquartile range [IQR], 40-70 years), 76.5% received more than 1 dose, 52.6% received the BNT162b2 vaccine (Pfizer/BioNTech), 44.1% received the mRNA-1273 vaccine (Moderna), and 3.1% received the Ad26.COV2.S vaccine (Janssen/Johnson & Johnson).

• Myocarditis developed 3.5 days (IQR, 3.0-10.8 days) following vaccination (mRNA-1273 vaccine, 11 cases [55%]; BNT162b2 vaccine, 9 cases [45%]).

• In 15 cases, pericarditis developed after the first vaccine, and in 22 cases, it developed after the second immunization.

• After the most recent vaccination, the median onset was 20 days.

• During the prevaccine period, the mean monthly number of cases of myocarditis or myopericarditis was 16.9 compared with 27.3 during the vaccine period.

• During the same time periods, the mean number of pericarditis cases was 49.1 and 78.8, respectively.