Risks of Myocarditis and Pericarditis Following Vaccination with SARS-CoV-2 mRNA Vaccines in Japan: An Analysis of Spontaneous Reports of Suspected Adverse Events

OBJECTIVE: To identify the risks of myocarditis or pericarditis after vaccination with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) mRNA vaccines in Japan. METHODS: We conducted an observed-to-expected analysis (OE analysis) of spontaneous reports of suspected adverse events from pharmaceutical companies, calculating rate ratios with myocarditis and pericarditis after the vaccination of the mRNA vaccines Comirnaty (BNT162b2) and Spikevax (mRNA-1273) and expected rate of myocarditis and pericarditis in the population before the COVID-19 pandemic. These reports dated from 17/2/2021 to 14/11/2021 and from 22/5/2021 to 14/11/2021 for Comirnaty and Spikevax, respectively. The observed-to-expected ratios (OE ratios) for each vaccine were estimated by age groups and sex. RESULTS: We identified 281 and 195 cases of myocarditis or pericarditis for Comirnaty and Spikevax, respectively, which were administrated 163,059,502 and 31,768,352 doses for Comirnaty and Spikevax until the 14th of November 2021, respectively. The OE ratios were statistically significantly higher in adolescent and young adult males in their age of teens and twenties after the second dose in a two-dose series [Comirnaty in teens male: 6.15 (95% CI, 2.26–21.98), Comirnaty in twenties male: 2.86 (95% CI, 1.13–8.38), Spikevax in teens male: 41.59 (95% CI, 5.64–43,281.94), Spikevax in twenties male: 16.84 (95%CI, 6.77–57.49)]. CONCLUSIONS: Risks of myocarditis and pericarditis following SARS-CoV-2 mRNA vaccines in Japan seems to be significantly elevated for adolescent and young adult males. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s43441-022-00466-1.