Incidence of myocarditis/pericarditis after COVID-19 mRNA vaccination among children and young adults in the US

In a population-based study published in Annals of internal medicine, the authors found that myocarditis/pericarditis 0-7 days after mRNA vaccination in people aged 5 to 39 years occurred in approximately 1 in 200,000 doses after the first dose and in 1 in 30,000 doses after the second dose of the primary series, and in 1 in 50,000 doses after the first booster. Incidence varied markedly by age and sex, but a disproportionate number of cases occurred in males, particularly among adolescents after dose 2 and first boosters.

The authors observed that the incidence after the first boosters was generally higher than after dose 1, which is consistent with reports from Israel. However, in contrast to this previous report, they did not consistently observe a lower incidence after the first accelerator than after the second dose in the primary vaccination series. Rates of myocarditis/pericarditis observed in the VSD population were higher, particularly after first boosters, than those reported in the US Vaccine Adverse Event Reporting System (VAERS), but the patterns noted in subgroups by gender and age were are similar.

VAERS reporting rates may be lower due to the passive nature of VAERS reporting compared to VSD case detection using active surveillance. Both VSD and VAERS are found disease indicators during days 0 to 7 after vaccination, which were higher than the pre-pandemic background values ​​reported by Oster and colleagues; however, pre-pandemic rates cannot be directly compared to post-vaccination rates because underdiagnosis of myocarditis/pericarditis in this age range was more likely pre-pandemic than post-vaccination when follow-up was longer.