How does risk of blood clots with Janssen compare to myocarditis risk with mRNA vaccines?
Executive summary
Janssen’s adenovirus vaccine was linked to thrombosis with thrombocytopenia syndrome (TTS) at roughly 4 cases per million doses overall and higher (9–10 per million) in women 30–49, with about a 15% mortality among reported U.S. TTS cases; mRNA vaccines carry a small but measurable myocarditis risk concentrated in young males, with most patients reported as recovering and no confirmed myocarditis deaths in reviewed VAERS reports as of late 2021 [1] [2] [3]. Public-health panels concluded that mRNA vaccines’ benefit–risk balance is more favorable than Janssen’s where mRNA options are available [4] [3].
1. The headline numbers: rare but different harms
The two risks are both rare but qualitatively different: TTS after Janssen occurred at about 4 cases per million doses overall and up to 9–10 cases per million in women aged 30–49, and among 54 U.S. TTS cases 15% died and another 17% required post-acute care, signaling substantial severity [1] [4]. By contrast, myocarditis after mRNA vaccines was identified most often in males under ~30, typically within a week of dose two, with preliminary follow-up showing over 90% fully or probably recovered by three months and no confirmed myocarditis deaths in fully reviewed VAERS reports as of the cited analyses [2] [3].
2. Severity and outcomes matter as much as incidence
Regulators emphasized not just how often events occurred but how severe they were: TTS cases after Janssen frequently required intensive care and carried a measurable fatality rate, whereas myocarditis cases after mRNA vaccines have generally been described as milder with good short‑term recovery in most followed patients [2] [3]. Multiple sources therefore judged the long‑term health impact of Janssen‑linked TTS and Guillain–Barré syndrome to be potentially more severe than the “apparently less severe” myocarditis outcomes after mRNA vaccination [2].
3. Who is at greatest risk — and how that shaped policy
Risk profiles differ by age and sex. TTS risk concentrated among younger adult women after the Janssen shot, while myocarditis was concentrated among younger males after mRNA doses, especially after the second dose [1] [2] [5]. These diverging patterns informed ACIP and CDC guidance that, when mRNA vaccines are available, their benefit–risk balance is more favorable and they should generally be preferred over Janssen [4] [3].
4. Absolute comparison: numbers on a shared scale
Direct head‑to‑head numbers in the sources put TTS at multiple cases per million for Janssen (4 per million overall; up to ~9–10 per million in some women) and myocarditis after mRNA vaccines at low single‑digit to low‑double‑digit cases per million depending on age/sex and dose — with myocarditis incidence highest in young men (sources show myocarditis incidence varying by study and subgroup) [1] [6] [7]. Exact comparative rates depend on the age/sex group you examine; ACIP’s benefit–risk tables stratified by age and sex reached the conclusion that mRNA vaccines generally prevent more hospitalizations while carrying different rare risks [2].
5. Context: infection itself raises both risks
All sources stress that SARS‑CoV‑2 infection carries a substantially higher risk of blood clots and cardiac complications than vaccines do, and vaccination reduces COVID‑19–related cardiac and clot risks for up to a year after infection in observational studies [8] [9] [10]. That context is central to public‑health recommendations weighing vaccine harms against the much larger harms of COVID‑19 itself [11].
6. Uncertainties, follow‑up and evolving guidance
Longer‑term follow‑up for myocarditis patients was still ongoing in the cited reports; preliminary results were reassuring but final long‑term data were pending [2] [1]. Surveillance systems and benefit–risk assessments evolved, and by 2023 ACIP and FDA actions (including preferring mRNA vaccines and updating Janssen fact sheets) reflected accumulating safety data [1] [12] [13].
7. Takeaway for individuals and clinicians
Where mRNA vaccines are available, U.S. advisory panels recommended them over Janssen because the overall benefit–risk profile favored mRNA platforms given higher effectiveness and the more severe nature of Janssen‑associated TTS and GBS; nonetheless Janssen retained value where mRNA vaccines were contraindicated or unavailable [4] [3] [1]. Personal risk calculus should consider age, sex, prior COVID history, and vaccine availability — and clinicians should counsel patients using the subgroup risks and the stronger harms associated with COVID‑19 infection itself [2] [8].
Limitations: available sources do not provide a single, definitive per‑age‑and‑sex comparison table covering every subgroup; numbers above derive from the cited CDC, ACIP and peer‑reviewed summaries and may vary by dataset and time period [2] [1] [6].