What are the confirmed rates of myocarditis after mRNA COVID-19 vaccination by age and sex in peer‑reviewed studies?

1. Who gets it and when: the consistent signal in the data

Multiple peer‑reviewed reviews and national surveillance systems identify the same pattern: myocarditis after mRNA vaccines is uncommon overall but occurs most frequently in adolescent and young adult males, typically within about four to seven days after the second mRNA dose ^{[7] [1] [2]}.

2. Absolute rates reported in peer‑reviewed population studies

Population‑based analyses show wide numeric variation, but some peer‑reviewed estimates offer concrete per‑million figures: a Canadian study reported rates after second doses highest in men aged 18–24 years, with about 59.2 myocarditis cases per million second doses for Pfizer‑BioNTech and 299.5 per million for Moderna in that age group under certain conditions ^[3]. Other large surveillance and cohort studies summarized in systematic reviews report smaller or larger point estimates depending on case definitions, age bands and follow‑up windows, reflecting heterogeneity across Taiwan, France, the U.S. and Canada ^{[2] [4]}.

3. Relative risk measures and outliers in the literature

Some peer‑reviewed cohort studies report large relative increases in myocarditis risk after mRNA vaccination compared with historical baselines—one South Korean study described a very large relative increase (reported as 620% higher risk) though that is a relative measure rather than an absolute rate and has been presented outside the primary surveillance context ^[8]. Context matters: several analyses and manufacturer summaries underscore that, in absolute terms, vaccination‑associated cases remain rare and that myocarditis following SARS‑CoV‑2 infection occurs at substantially higher rates (for example, CDC data cited ~1,500 cases per million COVID‑19 patients) ^{[5] [6]}.

4. Which vaccine, which dose, and what can change the numbers

Peer‑reviewed work shows risk varies by vaccine product and dosing schedule: Moderna (mRNA‑1273) has often been associated with higher myocarditis rates in younger males than Pfizer (BNT162b2) in multiple population studies, and the second dose of the primary series carries the largest risk signal ^{[2] [4]}. Studies also indicate that longer inter‑dose intervals reduce observed myocarditis rates, with Ontario and other analyses reporting lower rates when the gap between dose 1 and dose 2 was extended ^{[4] [3]}.

5. Clinical course, outcome and why raw rates don’t tell the whole story

Regulators and manufacturers point out that most post‑vaccine myocarditis cases have been clinically mild, resolve with supportive care, and show fewer longer‑term cardiovascular complications compared with conventional myocarditis in some cohorts ^{[1] [6]}, but long‑term outcome data are still accumulating and vary by study design.

6. Caveats: heterogeneity, surveillance bias and interpretation

Peer‑reviewed estimates differ because studies use different case definitions, surveillance systems (passive reports versus active follow‑up), age groupings, inter‑dose intervals, vaccine mixes and background infection rates; ethnicity and local health‑care reporting also change measured incidence, so no single “confirmed” rate applies universally ^{[2] [7]}. Relative‑risk headlines (e.g., "620% higher") can overstate public perception without accompanying absolute rates and confidence intervals ^[8].

7. Bottom line for readers weighing the evidence

Peer‑reviewed studies converge on clear qualitative conclusions—myocarditis after mRNA vaccination is rare, concentrated in young males after the second dose, varies substantially by vaccine product and dosing interval, and tends to be milder than myocarditis following COVID‑19 infection—while absolute risk estimates for the highest‑risk group range from a few dozen to several hundred cases per million second doses depending on the study and context ^{[3] [4] [1] [5]}.