Epidemiology of myocarditis and pericarditis following mRNA vaccines in Ontario, Canada: by vaccine product, schedule and interval

Importance Increased rates of myocarditis/pericarditis following COVID-19 mRNA vaccines have been observed. However, little data are available related to product-specific differences, which have important programmatic impacts. Objective The objective of this study was to estimate reporting rates of myocarditis/pericarditis following COVID-19 mRNA vaccine by product, age, sex, and dose number, as well inter-dose interval. Design We conducted a population-based cohort study using passive vaccine safety surveillance data. All individuals in Ontario, Canada who received at least one dose of COVID-19 mRNA vaccine between December 14, 2020 and September 4, 2021 were included. Setting This study was conducted in Ontario, Canada (population: 14.7 million) using the provincial COVID-19 vaccine registry and provincial adverse events following immunization database. Participants We included all individuals with a reported episode of myocarditis/pericarditis following COVID-19 vaccine in the study period. We obtained information on all doses administered in the province to calculate reporting rates. Exposure Receipt of COVID-19 mRNA vaccine (mRNA-1273 [Moderna Spikevax] or BNT162b2 [Pfizer-BioNTech Comirnaty]). Main Outcome(s) and Measure(s) Reported rate of myocarditis/pericarditis meeting level 1-3 of the Brighton Collaboration case definitions. Results There were 19,740,741 doses of mRNA vaccines administered and 297 reports of myocarditis/pericarditis meeting our inclusion criteria. Among these, 69.7% occurred following the second dose of COVID-19 mRNA vaccine and 76.8% occurred in males. The median age of individuals with a reported event was 24 years. The highest reporting rate of myocarditis/pericarditis was observed in males aged 18-24 years following mRNA-1273 as the second dose; the rate in this age group was 5.1 (95% CI 1.9-15.5) times higher than the rate following BNT162b2 as the second dose. Overall reporting rates were higher when the inter-dose interval was shorter (i.e., ≤30 days) for both vaccine products. Among individuals who received mRNA-1273 for the second dose, rates were higher for those who had a heterologous as opposed to homologous vaccine schedule. Conclusions and Relevance Our results suggest that vaccine product, inter-dose interval and vaccine schedule combinations may play a role in the risk of myocarditis/pericarditis, in addition to age and sex. Certain programmatic strategies could reduce the risk of myocarditis/pericarditis following mRNA vaccines.