Risk of myocarditis or pericarditis remains low in adolescents after Pfizer-BioNTech vaccination

A CITF-funded study, published in JAMA Pediatrics, reported that among adolescents the risk of myocarditis or pericarditis after receiving the monovalent Pfizer-BioNTech mRNA vaccine varies according to age and sex. There is some evidence to suggest an increased risk with shorter intervals between doses 1 and 2. However, the risk of these events remains very rare (<0.01% in ages 12-15 years and <0.1% in ages 16-17), with no deaths reported, and should be considered in relation to the benefits of vaccination. The study utilized adverse event following immunization (AEFI) data collected as part of routine vaccine safety surveillance in Ontario and administrative data from ICES and was a collaboration between Dr. Jeffrey Kwong (University of Toronto) and members of the vaccine safety surveillance team of Public Health Ontario.

Key findings:

• Approximately 1.65 million doses of the Pfizer-BioNTech vaccine were administered between December 14, 2020, and November 21, 2021 in Ontario, with 77 reported cases of myocarditis or pericarditis among those aged 12 to 17 years, which met the inclusion criteria during the study period.
• Of the 77 adolescents (mean age, 15 years; 63 males), 26 developed myocarditis or pericarditis after dose 1, and 51 developed myocarditis or pericarditis after dose 2.
• Among the 26 with myocarditis or pericarditis after dose 1, five had a previous PCR-confirmed SARS-CoV-2 infection, with a median time of 162 days between infection and vaccination. Of those with a reported event after dose 2, none had a previously confirmed SARS-CoV-2 infection. No individuals with a reported event after either dose had a previous history of myocarditis or pericarditis.
• Overall, 74 individuals (96.1%) with an event were assessed in the emergency department, and 34 (44.2%) were hospitalized (median length of stay: 1 day).
• On clinical presentation, symptoms included chest pain (74), shortness of breath (33), gastrointestinal symptoms (e.g., nausea, vomiting; 23), fever (14), and palpitations (13). Symptoms were similar across diagnoses of myocarditis, pericarditis, or myopericarditis/perimyocarditis.
• The majority (57) were treated with nonsteroidal anti-inflammatory drugs only, and 11 required no treatment. Two individuals were admitted to the intensive care unit.
• The highest reported incidence was observed among males aged 16 to 17 years after dose 2 (15.7 per 100 000 doses administered). The reporting rate was highest in those with a short (i.e, ≤30 days) dosing intervals (21.3 per 100 000 doses administered).

The results suggest that the reported incidence of myocarditis or pericarditis after the Pfizer vaccine was very rare (i.e., <0.01% based on standard pharmacovigilance definitions) in adolescents overall. Although the incidence of myocarditis or pericarditis was higher in those aged 16 to 17 years than in those aged 12 to 15 years, the outcome was still rare (i.e., <0.1%).