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. 2025 Sep:284:114642.
doi: 10.1016/j.jpeds.2025.114642. Epub 2025 May 13.

Active Surveillance for Myocarditis and Pericarditis in Canadian Children from 2021 to 2022: A Canadian Immunization Monitoring Program ACTive Study

Karina A Top  1 , Julie A Bettinger  2 , Joanne E Embree  3 , Taj Jadavji  4 , Rupeena Purewal  5 , Laura Sauvé  2 , Jesse Papenburg  6 , Shelley L Deeks  7 , Sarah E Wilson  8 , Nagib Dahdah  9 , Manish Sadarangani  2 , Scott A Halperin  10 , Fatima Kakkar  11 , Shaun K Morris  12 ; IMPACT Investigators
Collaborators, Affiliations

Active Surveillance for Myocarditis and Pericarditis in Canadian Children from 2021 to 2022: A Canadian Immunization Monitoring Program ACTive Study

Karina A Top et al. J Pediatr. 2025 Sep.

Abstract

Objective: To compare characteristics of children assessed at Canadian Immunization Monitoring Program Active (IMPACT) centers for myocarditis, myopericarditis, and pericarditis according to COVID-19 vaccination status, and to evaluate the association with vaccination.

Study design: IMPACT conducted active surveillance for myocarditis, myopericarditis, and pericarditis by monitoring admission lists and diagnostic codes for children ≤16 years of age assessed in emergency departments or hospitalized with these conditions (June 2021 through December 2022). Clinical details and immunization histories were entered in an electronic database. Characteristics of cases meeting Brighton Collaboration case definitions for myocarditis and/or pericarditis were compared between COVID-19 mRNA vaccine-proximate (vaccination 0-21 days prior to presentation), remotely vaccinated (vaccinated >21 days prior), and unvaccinated cases. Relative incidence of myocarditis/myopericarditis/pericarditis 0-21 days after vaccination was estimated via a self-controlled case series analysis adjusted for pre-Omicron vs Omicron periods. Cell sizes <5 were suppressed.

Results: Overall, 168 cases were included: 73 (43%) vaccine-proximate cases, 47 (28%) remotely vaccinated, 39 (23%) unvaccinated, and 9 (5%) with unknown vaccination status. Among vaccine-proximate cases, 88% (64/73) were male, and 95%-99% (69-72/73) were 12-16 years of age. Among unvaccinated cases, 56% (22/39) were male, and 36% (14/39) were 12-16 years of age. Unvaccinated cases appeared more likely to require intensive care unit care (28% vs <7% of vaccine-proximate cases). The adjusted relative incidence of myocarditis/myopericarditis/pericarditis 0-21 days postvaccination was 7.1 (95% CI 4.5-11.1).

Conclusions: Although myocarditis, myopericarditis, and pericarditis were associated with COVID-19 mRNA vaccination, children with postvaccine myocarditis, myopericarditis, and pericarditis appeared to have a less severe initial course than unvaccinated children.

Keywords: COVID-19 mRNA vaccine; SARS-CoV-2; adverse event following immunization; vaccination.

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Conflict of interest statement

Declaration of Competing Interest This study was funded by the Public Health Agency of Canada via the Sero-surveillance Research Program (COVID-19 Immunity Task Force). The Canadian Immunization Monitoring Program Active (IMPACT) is a national surveillance initiative managed by the Canadian Paediatric Society. M.S. is supported via salary awards from the BC Children's Hospital Foundation and Michael Smith Health Research BC. The funders had no role in the study design, conduct, analysis, or in the decision to submit for publication. KAT has received grants from the Coalition for Epidemic Preparedness Innovations for vaccine safety studies outside the submitted work. MS has been an investigator on projects funded by GlaxoSmithKline, Merck, Moderna, Pfizer, and Sanofi-Pasteur; all funds have been paid to his institute, and he has not received any personal payments. SAH has received grants and consulting fees from Pfizer and GlaxoSmithKline outside the submitted work. SKM reports speaker fees from and has served on ad hoc advisory boards for GlaxoSmithKline Canada, Sanofi-Pasteur, and Pfizer, all outside of the submitted work. JP reports research funding from Merck and MedImmune to his institution and personal fees from Enanta, all outside of the submitted work. RP reports an honorarium from Biomerieux outside of the submitted work. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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