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

Naveed Z, Li J, Spencer M, Wilton J, Naus M, García HAV, Otterstatter M, Janjua NZ. Observed versus expected rates of myocarditis after SARS-CoV-2 vaccination: a population-based cohort study. CMAJ. 2022;194(45):E1529-E1536. DOI:

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

In a study published in the Canadian Medical Association Journal (CMAJ), Dr.  Zaeema Naveed (British Columbia Centre for Disease Control) and colleagues from the CITF-funded Canadian Immunization Research Network (CIRN) found that the rates of myocarditis Myocarditis is an inflammation of the heart muscle, that can reduce the heart’s ability to pump blood. following a COVID-19 vaccine have been low, and that age, sex and the type of vaccine administered are important factors to consider to further reduce the risk of post-vaccination myocarditis.

Key findings:

  • From December 2020 to March 2022, 99 cases of myocarditis occurred in BC within seven days of a COVID-19 vaccination (or 0.97 cases per 100,000 vaccine doses). The rate rose to 1.37 cases per 100,000 vaccine doses (or 141 cases in total) when surveillance was extended to 21 days post-vaccination.
  • Researchers estimated expected rates based on the history of myocarditis cases in BC in 2019 before the start of the COVID-19 pandemic. They found that the incidence of myocarditis in 2020-2022 was 14.8 times higher (within 7 days) and 7 times higher (within 21 days) than what would have otherwise been expected.
  • More cases of myocarditis occurred within seven days post-vaccination in people aged 12-17 years (observed incidence rate: 2.64) and 18-29 years (2.63) than in older age groups.
  • Males were more likely to get myocarditis seven days after vaccination than females (1.90 versus 0.76).
  • The incidence of myocarditis within seven days post-vaccination was higher for individuals receiving their second dose than for those who received their third dose (1.90 versus 0.76). There was a higher incidence of myocarditis in those who received the Moderna vaccine when compared to the Pfizer vaccine (1.44 versus 0.74).
  • In males aged 18-29 years who received their second dose of Moderna, the incidence of myocarditis seven days post-vaccination was 148 times higher than what was expected, which is the highest increase observed across all sub-groups.

Findings from this study can be used to support vaccination strategies. In particular, the Pfizer vaccine might be advisable for younger males.

This study relies on health records from the BC COVID-19 Cohort, a surveillance platform that integrates both COVID-19-related and more general administrative data, from December 2020 to March 2022 across British Columbia. All individuals aged over 12 years with a record of an mRNA COVID-19 vaccination were included in the study, except those with a history of myocarditis or myopericarditis within a year before a COVID-19 vaccine dose. The researchers looked at the incidence of hospital admissions or emergency department visits for myocarditis or myopericarditis within seven and 21 days after vaccination, and calculated rates of myocarditis per 100 000 mRNA vaccine doses.