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What is the incidence rate of myocarditis following Janssen COVID-19 vaccination?
Executive summary
Available sources show that myocarditis and pericarditis have been reported after the Janssen (Ad26.COV2.S) COVID‑19 vaccine and that some safety reviews found an elevated reporting ratio within 7 days after vaccination (an observed-to-expected reporting ratio ~10.45 for a 7‑day window), but other studies found no clear signal or lacked power to detect an effect for Janssen specifically [1] [2] [3] [4]. Reporting and observational studies point to a substantially larger and better‑defined myocarditis risk following mRNA vaccines, especially in young males, than to a consistent myocarditis incidence estimate for Janssen [5] [3] [6].
1. What the official fact sheet and FDA review say: a detectable signal in surveillance
The Janssen fact sheet for providers warns that “reports of adverse events … suggest increased risks of myocarditis and pericarditis, particularly within the period 0 through 7 days following vaccination,” reflecting post‑authorization surveillance signals [1]. An FDA post‑authorization safety review reported 232 VAERS myocarditis/pericarditis reports through February 28, 2023, and calculated an observed‑to‑expected (O/E) reporting rate ratio of about 10.45 (95% CI 7.39–14.34) within 7 days after Janssen vaccination — a statistical signal in that specific short risk window [2].
2. Why that O/E ratio is not a direct “incidence rate” you can apply universally
The FDA’s O/E reporting ratio describes how many reports appeared in VAERS compared with expected background reports in a defined window; it is not the same as a population incidence rate per million doses and is influenced by reporting behavior and case ascertainment [2]. The Janssen fact sheet and FDA documents caution that adverse event rates from post‑authorization surveillance cannot be directly compared to clinical trial rates or used as precise incidence numerators without careful epidemiologic adjustment [1] [2].
3. Independent studies give mixed findings — some no signal, some limited signal
Pharmaco‑epidemiologic and VAERS‑based analyses differ. A VAERS‑based signal‑detection study found myocarditis enrichment was stronger for Pfizer (mRNA) than for Janssen, with Janssen not clearly associated with myocarditis in that analysis [7] [3]. A larger multi‑country electronic‑health‑record meta‑analysis likewise reported that Janssen was not associated with myocarditis incidence, but the authors noted limited statistical power because Janssen was less widely used in their data sources [4]. Those studies therefore present competing interpretations: some surveillance metrics indicate a short‑term signal [2], while other analytic studies did not find a robust association or could not exclude a small effect due to low uptake [3] [4].
4. How Janssen compares with mRNA vaccines on myocarditis risk
Regulatory summaries and population studies consistently show the myocarditis risk is clearest and largest after mRNA vaccines, concentrated in males aged ~12–24 and often after the second dose; regulators have quantified mRNA risks (for example, roughly 8 cases per million doses overall and ≈27 per million in males 12–24 in a cited mRNA data set) [5]. In contrast, the Janssen signal, when present, has been weaker, less consistent across studies, and often limited by small numbers [2] [3] [4]. The CDC and ACIP weighed these differences when comparing vaccine benefit‑risk profiles [6].
5. Limitations, uncertainties, and what “incidence” would require
Available sources show reporting counts (232 VAERS myocarditis/pericarditis reports after Janssen as of Feb 28, 2023 with roughly 19 million Janssen doses given by March 2023) but do not provide a single, consensus incidence per million doses that accounts for underreporting, case verification, age/sex stratification, or background rates corrected for healthcare‑seeking behavior [2]. Several studies explicitly say either they found no signal or had insufficient power to detect an effect for Janssen [3] [4]. Therefore, a reliable, stratified incidence (e.g., cases per million doses by age and sex) for Janssen is not consistently provided in the available reporting corpus.
6. Practical takeaways for readers and clinicians
Clinicians and patients should know myocarditis/pericarditis has been reported after Janssen vaccination and that short‑term surveillance measures identified a statistical signal in a 0–7 day risk window per FDA review [1] [2]. However, the magnitude and clinical pattern of risk appear much better defined for mRNA vaccines, especially in young males, than for Janssen; independent studies either found no association for Janssen or were underpowered to rule one out [3] [4] [6]. For specific risk estimates by age and sex or for case confirmation, available sources do not provide a single definitive incidence figure and further epidemiologic analyses would be required (not found in current reporting).
If you want, I can: (a) compute a crude reporting rate using the FDA counts and doses cited (simple VAERS reports per million doses) and explain its caveats; or (b) pull from other epidemiologic papers to produce stratified incidence estimates — but that would require additional sources beyond those you provided.