(HealthDay)—Acute myocarditis is rare in adults who receive at least one dose of the COVID-19 mRNA vaccine, with an incidence of 5.8 cases per 1 million individuals after the second dose, according to a research letter published online Oct. 4 in JAMA Internal Medicine.

Anthony Simone, M.D., from the Kaiser Permanente Los Angeles Medical Center, and colleagues calculated the incidence rates of myocarditis using vaccine administration as the denominator and compared it to the incidence among unexposed individuals between Dec. 14, 2020, and July 20, 2021, and to the incidence among vaccinated individuals during a 10-day period one year before vaccination.

Data were included for 2,392,924 Kaiser Permanente Southern California members aged 18 years or older who received at least one dose of the COVID-19 mRNA vaccine and 1,577,741 unexposed individuals. The researchers identified 15 cases of confirmed myocarditis in the vaccinated group (two and 13 after the first and second doses, respectively), for an observed incidence of 0.8 and 5.8 cases per 1 million after the first and second doses, respectively, during a 10-day observation window. All cases occurred among men, with a median age of 25 years. During the study period, there were 75 cases of myocarditis among unexposed individuals. The incidence rate ratio was 0.38 (95 percent confidence interval, 0.05 to 1.40) and 2.7 (95 percent confidence interval, 1.4 to 4.8) for the first and second doses, respectively.

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