How do myocarditis rates after Pfizer and Moderna vary by dose number and recipient demographics?
Executive summary
Myocarditis after mRNA COVID-19 vaccination is rare overall but concentrated in younger males and is most commonly reported after the second dose; several large studies and regulators show higher rates with Moderna (mRNA-1273) than Pfizer (BNT162b2) in those groups and dose-specific risk peaks within a week of vaccination [1] [2] [3] [4]. Reported incidence estimates vary by dataset and method — examples include ~27 cases per million doses for males 12–24 after 2023–24 formula vaccines (FDA labeling analyses) and much higher per-person estimates like ~36 per 100,000 after dose 2 in adolescent males in earlier academic reports — illustrating important differences between “per dose” and “per person” metrics and between surveillance systems [3] [5] [2].
1. Young men and dose number: where the risk concentrates
Multiple public-health reviews and population studies identify the highest myocarditis risk in adolescent and young adult males and show the second dose as the usual peak in risk; WHO’s vaccine-safety subcommittee noted myocarditis occurred more often in younger men and after the second dose [2], and the Canadian/British Columbia cohort found higher myocarditis per 100,000 doses in ages 12–17 and 18–29 and higher rates after dose 2 than dose 3 [4]. Clinical guidance from the CDC likewise states that myocarditis/pericarditis cases are rare but more frequent in younger males and supports the causal association with mRNA vaccines [1].
2. Moderna vs Pfizer: consistent signal of a higher rate with Moderna in younger people
Multiple reviews and surveillance datasets reported a greater incidence of myocarditis after Moderna (Spikevax, mRNA-1273) than after Pfizer (Comirnaty, BNT162b2) in younger males. WHO’s advisory subcommittee explicitly noted that some, though not all, data suggested higher incidence after Moderna in young males [2]. Population analyses cited by CMAJ found higher observed rates for mRNA-1273 than BNT162b2 and stated the highest ratios in younger age groups [4]. Moderna’s global safety analyses and journal reports also documented myocarditis reports concentrated in young males after dose 2 [6] [7].
3. Numbers differ by denominator: per-dose vs per-person and time windows matter
Regulatory labeling and academic papers use different denominators and windows. FDA-required label updates reported an unadjusted incidence of about 8 myocarditis/pericarditis cases per million doses in people 6 months–64 years and ~27 per million doses in males 12–24 during days 1–7 after the 2023–24 formula vaccines [3]. By contrast, an academic Q&A cited an estimate of 35.9 myocarditis cases per 100,000 persons after dose 2 in males 12–17 from earlier analyses — that larger figure reflects per-person risk in a high-risk subgroup and differing study timeframes and methodology [5]. These methodological differences explain why “rare” can mean a few per million doses in one dataset and several dozen per 100,000 people in another [3] [5].
4. Absolute risk remains low; context of COVID-19 infection and outcomes
Authors and public agencies emphasize that myocarditis after vaccination is uncommon and that myocarditis risk from SARS‑CoV‑2 infection is higher than from vaccination [8] [2]. CDC clinical considerations describe myocarditis/pericarditis after vaccination as rare and recommend standard cardiac evaluation when suspected [1]. Longitudinal follow-up studies described by Pfizer’s update and other work reported reassuring mid‑term outcomes for many cases, with low rates of cardiac death or long-term severe outcomes in studied cohorts [9].
5. Data limitations, surveillance bias and evolving formulas
Available sources show variation across countries, time periods and analytic methods. Passive surveillance (VAERS-style) and insurer-claims analyses yield different estimates than active, population-based cohort studies; some national regulators initially paused or preferred Pfizer for younger people based on early signals [2] [10]. FDA labeling uses recent commercial claims for the 2023–24 formula, which may not generalize to earlier formulations or other populations [3]. Pfizer’s company-posted analyses and Moderna’s safety database reports add manufacturer perspectives but are shaped by their datasets and case-classification approaches [9] [6] [7].
6. Competing narratives and political framing
Reporting includes clear scientific caution but also politicized claims: media stories and some public figures have accused authorities of hiding data (example search result) while regulators and journals continue to publish risk estimates and required label updates; these sources present competing emphases between “very rare” absolute rates and subgroup signals that motivated policy changes in some countries [11] [3] [2]. Readers should note institutional agendas: manufacturers publish safety summaries to show transparency [9], regulators update warnings to reflect evolving evidence [3], and independent academic studies aim to quantify risk by age, sex and dose [8] [4].
7. Bottom line for clinicians, patients and policymakers
For most people the chance of myocarditis after an mRNA vaccine is very small; risk concentrates in males 12–29 and is highest after the second dose, with Moderna often showing higher rates than Pfizer in that subgroup [4] [2] [3]. Decision‑making should weigh these rare, mostly short‑term risks against the larger risks of myocarditis and severe outcomes from COVID‑19 infection itself and use the precise metric (per dose vs per person) relevant to the question at hand; available sources do not mention long‑term catastrophic outcomes as common after vaccine-associated myocarditis [9] [1].