Have any studies found differences in long-term blood clot or thrombosis risk after J&J vs mRNA vaccines?

1. The early signal: a rare, specific syndrome tied to adenoviral vaccines

In spring 2021 clinicians and regulators identified a distinct syndrome—thrombosis with thrombocytopenia (TTS, also called VITT)—characterized by unusual clot locations plus low platelets and anti‑PF4 antibodies; the signal was concentrated after first doses of adenoviral vector vaccines and led regulators to warn and restrict J&J use ^{[6] [7] [8]}. Safety reviews found the absolute numbers small but notable: analyses reported roughly 60 suspected TTS cases among more than 18 million J&J doses (with higher reporting in women 30–49 at roughly 1 per 100,000 in some analyses), prompting CDC preference for mRNA vaccines and updated fact sheets ^{[1] [2] [9]}.

2. Head‑to‑head data: cohort, registry and network studies

Large international observational studies compared adenoviral‑vector versus mRNA platforms and found higher risks of TTS/thromboembolic events after adenoviral vaccines than after mRNA vaccines. An international network cohort study using routinely collected health data across several European countries and the US set out specifically to quantify comparative risk and underpinned later policy shifts favoring mRNA vaccines ^[5]. Subsequent syntheses and cohort analyses reported that mRNA vaccines are associated with a markedly lower incidence of venous and arterial thromboembolic complications across age, sex and comorbidity subgroups ^[4].

3. mRNA vaccines: isolated reports, but no consistent long‑term signal

Case reports and some observational analyses documented venous thromboembolism, ischemic stroke or cerebral venous sinus thrombosis after mRNA doses in isolated instances; these prompted investigation but did not establish a broad safety pattern comparable to VITT after adenoviral vaccines ^{[10] [11] [6]}. Systematic reviews and pharmacovigilance work concluded that the pathophysiology of VITT—anti‑PF4 antibodies linked to adenoviral vectors—is not shared by mRNA platforms, and population‑level studies often found no association between mRNA vaccination and elevated venous thromboembolism rates ^{[12] [7] [11]}.

4. Relative risk vs absolute risk — the numbers matter

Authors and regulators emphasized that while relative increases (where found) can sound large, the absolute risk remains very small for vaccine‑associated thromboses compared with thrombotic risk from SARS‑CoV‑2 infection itself. For example, analyses report cerebral venous thrombosis was many‑fold more common after COVID‑19 infection than after mRNA vaccination, and regulatory bodies concluded vaccine benefits outweigh these rare risks ^{[11] [13] [9]}.

5. Limitations and sources of disagreement

Available studies vary in design (self‑controlled case series, cohort, passive surveillance), time windows (mostly within 4–28 days post‑vaccine), and outcome definitions; that heterogeneity affects direct comparability ^{[14] [5]}. Pharmacovigilance signals can overestimate risk because of reporting bias; conversely, observational datasets can undercount rare events or miss late‑onset cases beyond typical follow‑up ^{[4] [12]}. Some more recent analyses still flag small, dose‑specific signals for particular thrombotic outcomes after mRNA vaccines, but they do not approach the pattern or magnitude seen with adenoviral vectors ^{[10] [4]}.

6. Policy response: preferring mRNA and withdrawing J&J in the U.S. market

Regulators and advisory panels used these comparative data to favor mRNA vaccines: the US ACIP recommended mRNA over J&J and J&J became unavailable in the US as of May 2023 ^[9]. The CDC and FDA updated fact sheets and guidance to reflect the rare TTS risk tied to adenoviral vaccines ^{[2] [8]}.

7. What reporting and research still don’t settle

Available sources do not mention long‑term (>6–12 month) controlled randomized trial evidence directly comparing late thrombosis rates after J&J versus mRNA vaccination in modern booster contexts; most evidence derives from early‑pandemic dosing and observational registries ^{[5] [4]}. Ongoing pharmacovigilance and registry studies through 2023–2025 have improved granularity but cannot fully eliminate residual confounding or reporting biases inherent in observational surveillance ^{[4] [10]}.

Bottom line: multiple large observational and regulatory reviews agree TTS/VITT is a rare complication concentrated with adenoviral‑vector vaccines such as J&J, and comparative studies show mRNA vaccines carry a markedly lower incidence of serious thrombotic events; isolated thrombotic cases after mRNA doses were reported but do not establish a comparable platform‑wide long‑term risk ^{[1] [5] [4] [12]}.