Background There have been international reports, including from the United States and Israel, of myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the lining around the heart) associated with COVID-19 mRNA vaccines (Pfizer-BioNTech and Moderna vaccines).2,3 Information to date indicates that these events occur more commonly after the second dose, within the week following vaccination (typically within 4-5 days), mainly in adolescents/young adults and more often in males than females.3,4 Early evidence from the Vaccine Safety Datalink (VSD), a collaborative project between the Centers for Disease Control and Prevention (CDC) and nine health care organizations in the United States, suggested that there may be mRNA vaccine product-specific differences in the rates of myocarditis and/or pericarditis in individuals aged 12-39 years, although the differences were not statistically significant.4 In early June 2021, Public Health Ontario (PHO) issued an Enhanced Surveillance Directive (ESD), requiring Public Health Units (PHU) to prioritize follow-up for any reports of myocarditis or pericarditis following any COVID-19 mRNA vaccine and notify PHO on the same day of PHU notification of the report. In addition to the start of enhanced surveillance, there were many other COVID-19 vaccine program changes that began in June 2021 including: an acceleration of second dose administration that was facilitated by increased Moderna vaccine supply, the increased use of mixed product schedules, an increase in the number of young adults and adolescents immunized, and a transition from a longer interval between first and second dose to shorter intervals such as those outlined in the vaccine product monographs (i.e., 21 or 28 days). These are important contextual factors for the interpretation of the data presented in this summary.
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