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Acute Myocarditis Following COVID-19 mRNA Vaccination in Adults Aged 18 Years or Older

Our take —

This observational study compared myocarditis incidence rates among US adults (aged 18 years and older) between those who received a SARS-CoV-2 mRNA vaccine December 2020 to July 2021 and two control groups: 1) these same individuals one year prior to vaccination; and 2) unvaccinated adults temporally matched to vaccinated adults. The study used data from approximately 3.9 million adults in the Kaiser Permanente Southern California health care system. Though myocarditis incidence 10 days after vaccination was extremely low (5.8 cases per 1 million vaccinated individuals) and primarily occurred among young males, myocarditis rates following the second mRNA vaccine dose were elevated relative to unvaccinated individuals (2.7 times higher) and to themselves in the previous year (3.3 times higher) in an unadjusted analysis. Similar to recent studies, all 15 cases of myocarditis (among 2.4 million vaccinated individuals) after vaccination in this large and ethnically diverse sample in the US were mild and self-limited. These results add to existing evidence that while risk of myocarditis appears elevated following vaccination, is remains rare and generally mild.

Study design

Prospective Cohort

Study population and setting

This observational study assessed myocarditis rates following SARS-CoV-2 vaccination with mRNA vaccines among Kaiser Permanente Southern California (KPSC) members aged 18 years or older between December 14, 2020 and July 20, 2021. Two cardiologists adjudicated all hospitalizations with myocarditis listed as the discharge diagnosis within 10 days of vaccine administration to identify cases. Clinical characteristics associated with each case were presented. The study compared incidence rates during the 10-day period after each individual’s first and second mRNA vaccine dose to myocarditis rates among unvaccinated individuals between December 14, 2020 and July 20, 2021 and to myocarditis rates among vaccinated individuals during the 10-day period 1 year prior to vaccination.

Summary of Main Findings

Pfizer and Moderna vaccines were administered in equal proportions among the 2.4 million KPSC members with at least 1 dose of the vaccine (93.5% received 2 doses). More than half (54%) of vaccinated members were female, 37.8% identified as Hispanic, 31.2% as White, 14.3% as Asian, and 6.7% as Black. Vaccinated members had a median age of 49 years (interquartile range [IQR] 34, 64 years) Of the 1.6 million unvaccinated individuals, 49.1% were female, 39.2% identified as Hispanic, 29.7% as White, 8.8% as Black, and 6.6% as Asian. Unvaccinated individuals had a median age of 39 years (IQR 28-53 years). The authors identified 15 cases of myocarditis (2 after dose 1, 13 after dose 2 among vaccinated individuals and 75 cases of myocarditis among unvaccinated individuals. Vaccinated individuals were all male and had a median age of 25 years (IQR 20, 32 years) compared to unvaccinated individuals, who were 52% male and had a median age of 52 years (IQR 32, 59 years). Vaccinated individuals with myocarditis did not have prior heart disease and only two of 15 had evidence of cardiac dysfunction on echocardiogram (1 of whom recovered after discharge).

Unvaccinated individuals had 2.2 cases of myocarditis over a 10-day observation period per 1 million individuals. After the first mRNA vaccination dose, there were 0.8 cases over 10 days per 1 million individuals (incidence rate ratio [IRR] 0.38, 95% confidence interval [CI] 0.05, 1.40 compared to unvaccinated individuals and IRR 1.0, 95% CI 0.1, 13.8 compared to the same vaccinated individuals a year before vaccination). After the second mRNA vaccination dose, there was an incidence of 5.8 cases over 10 days per 1 million individuals (incidence rate ratio [IRR] 2.7, 95% confidence interval [CI] 1.4, 4.8 compared to unvaccinated individuals and IRR 3.3, 95% CI 1.0, 13.7 compared to vaccinated individuals a year before vaccination).

Study Strengths

This study included careful adjudication of myocarditis cases in a large, diverse cohort of individuals who did and did not receive mRNA-based SARS-CoV-2 vaccinations. It was also able to compare myocarditis incidence among vaccinated individuals to themselves the year before the vaccine became available, given that vaccinated and unvaccinated individuals had different demographic characteristics.

Limitations

Due to the rare incidence of myocarditis, there were not enough events to assess myocarditis incidence by age or sex. The study also excluded adolescents under 18 years old. This is particularly important because other studies have found higher rates of myocarditis in young males relative to other age/sex subgroups. The small number of events also limited confounder adjustment, increasing the likelihood of confounding biasing the incidence rate ratio estimates. Additionally, choosing a 10-day observation period may have undercounted myocarditis cases, underestimating the reported incidence. Relying on myocarditis being listed as a discharge diagnosis or reported by clinicians may have undercounted cases of myocarditis, although this is unlikely to be differential across groups.

Value added

This study adds to the evidence that myocarditis after mRNA-based SARS-CoV-2 vaccines, while extremely rare, is more common in young males. It also adds to the evidence that post-vaccination myocarditis cases, when they happen, are likely to be mild and self-limited in adults.

This review was posted on: 26 October 2021