What are the rates of vaccine-induced thrombotic events after Janssen (J&J) COVID-19 vaccine by age and sex?
Executive summary
Available reporting shows vaccine-induced thrombotic thrombocytopenia (VITT/TTS) after the Janssen (Ad26.COV2.S) vaccine is very rare: U.S. case series and surveillance found on the order of a few dozen cases among millions vaccinated (for example, ~60 TTS reports among >18 million J&J doses as of March 2021) [1]. Most sources link the syndrome to adenoviral‑vector vaccines and report that cases clustered in younger adults and women, particularly women under 60, though absolute rates by narrow age‑and‑sex strata vary across studies and countries and are not consistently reported in the sources provided [2] [3] [4].
1. What the surveillance numbers actually show — tiny absolute risk but signal detected
Regulators and clinicians concluded a safety signal existed for adenoviral‑vector COVID vaccines, including Janssen’s Ad26.COV2.S, based on passive and active surveillance that detected dozens to low hundreds of unusual thromboses with thrombocytopenia against a background of millions of doses — for instance, a U.S. review documented 15 TTS cases early in the U.S. Janssen rollout and other summaries cite ~60 reports among >18 million recipients by mid‑March 2021 [5] [1]. Systematic reviews and pharmacovigilance analyses describe overall event rates on the order of fractions of a case per 100,000 to a few cases per million doses, with country‑to‑country variation [6] [7].
2. Who was affected — consistent pattern: more often younger women
Multiple clinical and review sources report that the atypical thrombotic events with thrombocytopenia occurred predominantly in women and in younger age groups (commonly cited as under 50–60 years). Case series for adenoviral vaccines reported more than 80% women among some cohorts and many patients younger than 55, prompting guidance that women aged 18–49 be informed about the increased risk with the Janssen vaccine [5] [2] [4]. Sources emphasize this is a pattern in reported cases, not proof of causation for every subgroup [3] [8].
3. How big the age‑ and sex differences are — available sources do not give consistent, precise rates
No single provided source gives a complete table of age‑and‑sex‑specific incidence rates for Janssen TTS across the full U.S. rollout. Some reports give counts (for example, 15 U.S. cases in an early CDC/FDA review; ~60 reports among >18 million recipients in VAERS by mid‑March 2021) and reviews compare crude rates across vaccines or countries, but precise per‑age‑per‑sex rates for J&J are not consistently published in the supplied documents [5] [1] [6]. Therefore: available sources do not mention a definitive, consistent set of Janssen TTS rates broken down by each age and sex stratum.
4. How regulators reacted — risk communication and preferential recommendations
Regulatory agencies and professional societies repeatedly weighed the rare risk against COVID‑19 risks. The CDC/FDA reviewed cases and allowed Janssen use to continue for adults while later recommending mRNA vaccines be preferred when possible; professional hematology guidance describes TTS/VITT as a rare but serious syndrome and notes vaccine choice considerations for some populations [8] [9]. Several countries restricted or recommended age‑based preferences for adenoviral vaccines because the risk‑benefit balance changes with age and local epidemic severity [2] [10].
5. Clinical features and case‑finding window — when to suspect it
Clinical reviews uniformly describe VITT/TTS as occurring typically 4–30 days after vaccination with thrombosis at unusual sites (notably cerebral venous sinus thrombosis or splanchnic thrombosis) together with thrombocytopenia and high D‑dimer; detection of anti‑PF4 antibodies supports the diagnosis [11] [12] [13]. This consistency in presentation was critical to identifying the safety signal and guiding testing/treatment.
6. Limitations, uncertainties and competing interpretations
Studies differ in methodology (passive reporting vs. active surveillance), case definitions, and denominators, producing variable rate estimates [6] [7]. Some authors note observed rates for certain thrombotic events after Ad26.COV2.S fall within historical background ranges for cerebral venous thrombosis, complicating causal inference [14]. Others point to epidemiologic clustering by age/sex and anti‑PF4 biology to argue for a vaccine‑associated syndrome [2] [3]. Both perspectives appear in the literature above.
7. What to do if you need precise age/sex rates
If you require exact incidence numbers by age and sex for Janssen/TTS, current reporting in these sources is insufficient: you should consult the original CDC/FDA safety analyses and country regulatory updates or peer‑reviewed pharmacoepidemiology studies that provide stratified denominators and adjusted incidence estimates, because the sources supplied here either report counts or broader summaries rather than a complete age‑by‑sex incidence table [5] [1] [6].
Sources cited above: [14], [5], [2], [8], [7], [6], [13], [12], [11], [9], [3], [4], [10], [1].