According to findings from a study published in JAMA Internal Medicine, acute myocarditis was rare in individuals who received at least one dose of the Pfizer-BioNTech or Moderna COVID-19 vaccine. The researchers conducted a population-based cohort study of 2,392,924 adult Kaiser Permanente Southern California members who received at least one mRNA vaccine dose between December 14, 2020, and July 20, 2021. The participants were aged 18 years or older.

The researchers identified cases of myocarditis after vaccination using reports from clinicians in the Kaiser Permanente network's Regional Immunization Practice Committee and hospitalization data within 10 days of vaccination. All candidates considered in this study were reevaluated by at least two cardiologists. The study authors calculated the incidence ratio of myocarditis postvaccination and compared the results with those of the unexposed group within the study period.

Among the study cohort, about half of patients received at least one dose of the mRNA-1273 CoV-2 vaccine, and the other half received at least one dose of the BNT162b2 vaccine. Fifty-four percent of participants were women, and 37.8% identified as Hispanic, 31.2% identified as white, 14.3% identified as Asian, and 6.7% identified as black. The median age was 49 years; 35.7% of participants were younger than age 40 years. In the study cohort, 35.7% of candidates were younger than age 40 years and 3.5% of candidates were completely vaccinated, i.e., had received two doses of an mRNA COVID-19 vaccine.

The researchers noted that 1,577,741 participants belonged to the unexposed group, whose median age was 39 years. In this group, 53.7% of individuals were younger than age 40 years. This group comprised 49.1% women, and 29.7% were of white ethnicity, 8.8% were black, 39.2% were Hispanic, and 6.6% were Asian.

The researchers identified 15 cases of confirmed myocarditis among vaccinated men—two after the first vaccine dose and 13 after the second vaccine dose. Specifically, nine white men, four Hispanic men, one Asian man, and one man of unknown ethnicity had myocarditis. This resulted in an observed incidence rate of 0.8 cases of myocarditis per 1 million first doses and 5.8 cases per 1 million second doses within 10 days of vaccination. According to the researchers, the men who had confirmed myocarditis after vaccination had no history of a prior cardiac disease, and eight received the BNT162b2 vaccine and seven received the mRNA-1273 vaccine.

All were hospitalized, and all tested negative for SARS-CoV-2. Fourteen (93%) reported chest pain between 1 and 5 days after vaccination. Symptoms resolved with conservative management in all cases; no patients required ICU admission or readmission after discharge.

The authors noted that the core strength of this study was the demographic diversity of the study cohort, but the study also had some limitations, including its observational design, lack of myocardial biopsy for definitive diagnosis, non-uniform testing across all cases, limited follow-up periods, possible underdiagnosis of subclinical cases, etc. They also indicated that due to the observational nature of the study, the correlation between COVID-19 mRNA vaccination and post-vaccination myocarditis could not be established.

The authors also concluded that the prevalence of acute myocarditis among individuals who received a single dose or two doses of the COVID-19 mRNA vaccine is a rare occurrence. However, the current study indicated that young men are more susceptible to myocarditis postvaccination, which necessitates further examination of this issue.

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