Injecting Vaccine.
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The risk of myocarditis from COVID-19 vaccination was higher in males and increased with the number of vaccinations and boosters that people received, according to a new study in Annals of Internal Medicine that was led by researchers at Kaiser Permanente Vaccine Study Center.

There have been worldwide reports of cases of myocarditis/pericarditis after mRNA-based COVID-19 vaccines (Pfizer-BioNTech and Moderna), especially among younger male persons zero to seven days after they received dose two. Data is just starting to be gathered about the incidence of myocarditis/pericarditis after booster doses.

This team set out to estimate the incidence of myocarditis/pericarditis during days zero to seven after mRNA vaccination by age, sex, dose number, and product. The data was drawn from Vaccine Safety Datalink (VSD), a collaborative of eight integrated health care delivery systems with comprehensive medical records that has conducted active, population-based surveillance of prespecified outcomes after COVID-19 vaccination since December 2020.

In this population-based surveillance, the authors found that myocarditis/pericarditis zero to seven days after mRNA vaccination in persons aged five to 39 years occurred in approximately one in 200,000 doses after the first dose and one in 30,000 doses after second dose of the primary series, and one in 50,000 doses after the first booster. The incidence varied markedly by age and sex, however, with a disproportionate number of cases occurring in male persons, notably among adolescents after dose two and first boosters.

The authors observed incidence after first boosters was generally higher than after dose one, consistent with reporting from Israel. However, in contrast to this earlier report, they did not consistently observe a lower incidence after the first booster than after the second dose in the primary vaccination series.

Incidence rates of myocarditis/pericarditis observed in the VSD population were higher, particularly after first boosters, than those reported to the U.S. Vaccine Adverse Event Reporting System (VAERS), but patterns noted by sex and age subgroups were similar. Rates in the VAERS report may be lower because of the passive nature of VAERS reporting versus VSD’s identification of cases using active surveillance.

Both VSD and VAERS found incidence rates during days zero to seven after vaccination that were higher than the prepandemic background rates noted by Oster and colleagues; however, prepandemic rates may not be directly comparable with post vaccination rates because underdiagnosis of myocarditis/pericarditis in this age range was more likely prepandemic than post vaccination when surveillance was greater.

An earlier study, published in JAMA, this year also found that rates of myocarditis were highest in young men and after the second dose of vaccine. Males comprised 82% of the myocarditis cases for whom sex was reported.

That study used data from the Vaccine Adverse Event Reporting System (VAERS) that occurred after mRNA-based COVID-19 vaccine administration between December 2020 and August 2021 in 192 405 448 individuals older than 12 years of age in the U.S.

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