How do myocarditis rates compare between mRNA and adenovirus COVID vaccines?
Executive summary
Studies and pharmacovigilance consistently show myocarditis is a very rare adverse event after COVID-19 vaccination but disproportionately linked to mRNA vaccines, especially in younger males and after second doses; for example, CDC estimated ≈12.6 myocarditis/pericarditis cases per million second mRNA doses in people 12–39 [1], while some surveillance and cohort studies still detect small but measurable risk after adenovirus vaccines (ChAdOx1) though generally lower than for mRNA in most analyses [2] [3].
1. The headline comparison: mRNA shows higher reported rates
Multiple reviews and large surveillance studies concluded myocarditis reports were concentrated after mRNA vaccines (Pfizer-BioNTech BNT162b2 and Moderna mRNA‑1273), and authors note mRNA platforms have the highest incidence in comparative signal detection [2] [4]. The American Heart Association and CDC summarized that myocarditis is a rare complication after mRNA vaccination and cited an approximate rate of 12.6 cases per million second mRNA doses for ages 12–39 [1]. Meta-analyses and pharmacovigilance systems therefore place mRNA vaccines as the principal signal for vaccine-associated myocarditis [4] [5].
2. Adenovirus vaccines: not zero, but generally smaller signals
Adenovirus-vector vaccines (for example ChAdOx1/AstraZeneca) were initially thought not to be linked to myocarditis, but later population studies detected a small increased risk after first doses in some analyses [5] [6]. Large English self-controlled case series found elevated myocarditis risk after mRNA priming and boosters but did not find the same elevation after a priming course with the adenovirus ChAdOx1-S vaccine — indicating differing risk profiles by platform and dose sequence [3]. Observational papers and reviews caution adenovirus-associated myocarditis events are rare and usually numerically smaller than the mRNA-associated signals [2] [5].
3. Magnitude varies by vaccine brand, dose and recipient age/sex
Within mRNA products, researchers reported higher myocarditis incidence with Moderna (mRNA‑1273) than Pfizer (BNT162b2) in some studies; suggested explanations include Moderna’s higher mRNA dose (100 µg vs 30 µg) or formulation differences [7]. One large population study reported incidence rate ratios for myocarditis 1–7 days after dose 1 of AstraZeneca, Pfizer and Moderna as 1.76, 1.45 and 8.38 respectively, with an IRR of 23.1 after Moderna dose 2 — underscoring especially large relative signal after second Moderna doses in that analysis [6]. Risk concentrates in males under 40 and after the second mRNA dose [8] [1].
4. Infection still poses a larger myocarditis risk than vaccination
Authors who examined both vaccine-associated and infection-associated myocarditis consistently found that SARS‑CoV‑2 infection confers a greater myocarditis risk than vaccination overall. A self‑controlled study in England concluded overall myocarditis risk is greater after infection than after vaccination, though the relative difference narrows for certain subgroups (young men after second mRNA‑1273) [8] [9]. Reviews and professional societies emphasize the comparison when weighing vaccine risks and benefits [9] [8].
5. Surveillance systems and methodology shape reported rates
Apparent differences between vaccine platforms are affected by surveillance method (spontaneous reports vs active surveillance), population vaccination patterns (mRNA often used in younger cohorts in some countries), and case definitions/confirmation rates. Systematic reviews of spontaneous reporting note that rollout patterns — e.g., younger people receiving more mRNA vaccines — can bias comparisons and complicate attributing risk to platform alone [10]. Authors warn that some reported percentages from trials or VAERS-style counts should be interpreted with caution [5] [2].
6. Remaining uncertainties and alternative interpretations
While many sources conclude mRNA vaccines have the larger signal, several papers and experts point out adenovirus vaccines have produced rare myocarditis reports in later studies, undermining a simple “only mRNA” story [6] [5]. Some investigators suggest mechanisms could involve spike protein immune responses common to platforms rather than mRNA per se; others highlight dose/formulation differences between mRNA products as explanatory [7]. Available sources do not mention long‑term cardiac sequelae frequency beyond the time windows analyzed here.
7. What this means for decision‑making
Public health groups weigh the rare myocarditis risk against the larger myocarditis and other cardiovascular risks from SARS‑CoV‑2 infection; that calculus has driven recommendations favoring vaccination while tailoring products, intervals and dosing for younger males in some jurisdictions [8] [2]. Clinicians and regulators continue active surveillance and brand‑specific comparisons to inform booster formulations and dosing strategies [11] [12].
Limitations: this summary uses the supplied sources only; numerical estimates vary by study design, age group, vaccine brand and dose interval, and some later data (e.g., updated vaccine formulations and 2024–25 trials) suggest myocarditis signals may have changed for newer boosters [11].