This is a summary, written by members of the CITF Secretariat, of:

Buchan SA, Yon Seo C, Johnson C, Alley S, Kwong JC, Nasreen S, Calzavara A, Lu D, Harris TM, Yu K, Wilson SE. Epidemiology of myocarditis and pericarditis following mRNA vaccines in Ontario, Canada: by vaccine product, schedule and interval. medRxiv. 2021 Dec 05. doi:

The results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

A team that included CITF-funded researcher Dr. Jeffrey Kwong from ICES and Public Health Ontario has found that 70% of cases of myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the sac holding the heart) following COVID-19 vaccination in Ontario occurred after the second dose. From December 14, 2020, to September 4, 2021, during which 19.7 million doses of mRNA vaccine were administered, approximately 0.002% of vaccinated individuals, met the inclusion criteria for myocarditis or pericarditis. 77% of cases were in males, the median age was 24, and events commonly occurred three days after vaccination. Dose interval, vaccine product, and use of mix-and-match also played a role. The study, in preprint and therefore not yet peer reviewed, used Ontario’s passive vaccine safety surveillance system, COVID-19 vaccine registry, and health administrative databases.

Key findings:

  • This study examined events of myocarditis/pericarditis reported to Ontario’s passive vaccine safety surveillance system as an adverse event following immunization over the period from December 14, 2020, to September 4, 2021, during which 19.7 million doses of mRNA vaccine were administered. Over this period, 417 events of myocarditis/pericarditis were reported, of which 297 cases (approximately 0.002% of vaccinated individuals) met the inclusion criteria for myocarditis or pericarditis, based on individual case review and scoring using Brighton Collaboration case definitions. Of these 297 events:
    • 7% occurred after the second dose, 76.8% occurred in males and the median age of affected individuals was 24.
    • Most events (97.6%) involved an emergency room visit, with 70.7% requiring hospital admission.
    • The median time to onset was three days after receiving the vaccine. For events following the second dose, 86.9% of cases arose within seven days after vaccination and 97.1% occurred within a month after the second dose.
    • Rates were higher in individuals with shorter (30 days), compared to longer (56 days) intervals between doses.
    • As for mix-and-match regimens, the highest rate was observed in those receiving Comirnaty (Pfizer-BioNTech) as a first dose and Spikevax (Moderna) as a second dose, especially when administered 30 days apart.

Overall, the highest rate of myocarditis or pericarditis in Ontario was observed in males aged 18-24 who received the Moderna Spikevax vaccine as their second dose; this rate was five times higher than that observed in males of the same age receiving Pfizer-BioNTech Comirnaty vaccine as their second dose. The exact mechanism underlying myocarditis or pericarditis following immunization is not known at this time.

The Canadian Immunization Research Network (CIRN) Provincial Collaborative Network (PCN), headed by Dr. Kwong, strives to evaluate post-market vaccine safety by identifying associations with various adverse events of special interest. The CIRN-PCN team, in collaboration with passive vaccine safety surveillance partners, and other vaccine safety surveillance systems, funded in part by the CITF, continue to monitor vaccine safety across Canada.