Ontario study examines Pfizer COVID-19 vaccine myocarditis rates in adolescents

A new study has characterized the development of myocarditis or pericarditis following the administration of the Pfizer (BNT162b2) COVID-19 vaccine to Ontario adolescents 12 to 17 years of age. The study examined characteristics and presenting symptoms of myocarditis or pericarditis, and clinical investigations (i.e., laboratory and diagnostic imaging) undertaken. It also compared the incidence of myocarditis or pericarditis in those aged 12 to 15 years to those 16 to 17 years, overall and by vaccine dosing interval. When the adolescent vaccine program started in Ontario in May 2021, an eight-week interval between dose 1 and dose 2 was used due to vaccine supply constraints. As supply increased, this interval was reduced to four weeks by late summer 2021. Following recommendations from Canada’s National Advisory Committee on Immunization in October 2021, the recommended dosing interval returned to eight weeks. The population-based cohort study used passive vaccine safety surveillance data linked to the provincial COVID-19 vaccine registry. The study included all adolescents in Ontario aged 12 to 17 years, who received one or more doses of the Pfizer vaccine between Dec. 14, 2020 and Nov. 21, 2021, and had an episode of myocarditis or pericarditis. Results are highlighted below: Approximately 1.65 million doses of Pfizer COVID-19 vaccine were administered to the 12 to 17 years age group. During the study period, 77 cases of myocarditis or pericarditis occurred in the 12 to 17 years age group (mean age 15.0 years; 81.8% male). 26/77 (33.8%) adolescents developed myocarditis or pericarditis after dose 1 of the Pfizer vaccine; 51 (66.2%) developed myocarditis or pericarditis after dose 2. Most events occurred within seven days of vaccine administration. The median time to symptom onset after dose 2 was two days. The majority of adolescents (96.1%) presented with chest pain, followed by shortness of breath (42.9%), gastrointestinal symptoms (e.g., nausea, vomiting [29.9%]), fever (18.2%) and palpitations (16.9%). Overall, 74/77 individuals (96.1%) were assessed in the emergency department, and 34 (44.2%) were hospitalized (median length of stay was 1 day). Two individuals (2.6%) were admitted to the intensive care unit. No deaths occurred. Most adolescents (74.0%) with myocarditis or pericarditis were treated with OTC nonsteroidal anti-inflammatory drugs only; 14.3% required no treatment. The highest reported incidence of myocarditis or pericarditis was seen among male adolescents aged 16 to 17 years after dose 2 (15.7 per 100,000; 95%CI, 9.7–23.9). Among those aged 16 to 17 years, the rate of myocarditis or pericarditis was highest in those with a short (≤ 30 days) vaccine dosing interval (21.3 per 100,000; 95%CI, 11.0–37.2). The results of this cohort study suggest that older adolescents (age 16 to 17) have a higher incidence of myocarditis or pericarditis compared to younger adolescents (12 to 15 years) after receiving the Pfizer COVID-19 vaccine. The risk of these events after vaccination is very rare (< 0.01%) in adolescents overall and rare (< 0.1%) in adolescents 16–17 years of age. These rare risks need to be considered in the context of the reduced risk of COVID-19 infection and associated outcomes as a result of protection from vaccination. The authors note that additional long-term follow-up of individuals who develop myocarditis or pericarditis following vaccination is needed. Reference 1. Buchan SA, Alley S, Seo CY, et al. Myocarditis or pericarditis events after BNT162b2 vaccination in individuals aged 12 to 17 years in Ontario, Canada. JAMA Pediatrics 2023; published online February 27, 2023. (accessed February 27, 2023).

Ontario study examines Pfizer COVID-19 vaccine myocarditis rates in adolescents

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