Background: Myocarditis is considered as a potential safety concern after COVID-19 mRNA vaccination; however, limited studies have quantified its risk in the United States population. We aimed to evaluate the risk of myocarditis after BNT162b2 (Pfizer-BioNTech) or mRNA-1273 (Moderna) vaccination.
Methods: We performed a self-controlled case series analysis of myocarditis reports to the US Vaccine Adverse Event Reporting System (VAERS) that occurred after the COVID-19 mRNA vaccinations from December 24, 2020 to January 28, 2022. We reported the relative incidence (RI) of myocarditis for various periods after COVID-19 mRNA vaccinations as well as the ratio of the RI after the second dose to the RI after the first dose.
Results: 1091 VAERS presumptive myocarditis reports after COVID-19 mRNA vaccinations (median age, 23 years; IQR, 17-36 years; 718 individuals [65.8%] < 30 years; 831 men [76.2%]) were identified. The median time to myocarditis onset was 3 days (IQR, 1-6 days). 824 (75.5%) reports were serious, with 11 (1%) death. We found an increased risk during the 1- to 3-day period after the second dose for both vaccines. The RI was 5.38 (95% CI, 2.20-13.26) for the Pfizer-BioNTech vaccine and 6.12 (95% CI, 2.04-18.97) for the Moderna vaccine. No significant increase was found in other risk periods. This observation was verified by dividing the RI after the second dose by the RI after the first dose. For the Pfizer-BioNTech vaccine, the RI for the period 1 through 3 days after the second dose was 6.33 (95% CI, 3.18-12.61) times as high as that for the same period after the first dose; for the Moderna vaccine, the ratio of RIs was 4.24 (95% CI, 1.94-9.25) for the same risk period. No significant RI differences in other risk periods were identified. A similar increased risk was also detected in people aged below 30 or in males.
Conclusions: We found an increased risk of myocarditis for period 1 through 3 days after the second dose of COVID-19 mRNA vaccines, especially in male and young adults. Our findings should be interpreted with the limitations of VAERS with considerations with the benefits of receiving COVID-19 mRNA vaccination.