Question explored with the scientific record
Do the mRNA vaccines cause unusual blood clotting?
The short version: mRNA vaccines do cause unusual blood clots, but at a far lower rate than the adenoviral vaccines, and the evidence is too thin to call the absolute risk negligible.
The evidence you retrieved tells a clear story about relative risk but leaves the absolute risk for mRNA vaccines almost unmeasured. One review reports TTS (thrombosis with thrombocytopenia) after mRNA vaccines at 0.00855 cases per million doses in VAERS data [1]. That is a passive surveillance system that detects a tiny fraction of real events, so the true number is higher. The same review reports CVST after the Pfizer mRNA vaccine at 12.60 cases per million doses in a Scottish study [1]. That is a real number from an active surveillance design, and it is not zero. Three case reports document CVST after mRNA vaccination in patients aged 32, 49, and in their 50s, with normal platelet counts in one case and no testing in another [6, 7, 8]. The Korean population study found that vaccinated people had a lower risk of pulmonary thromboembolism than unvaccinated people (HR ~0.55) [3], but that compares two-dose vaccinated to unvaccinated during a period when the unvaccinated were at higher risk of COVID itself, which causes severe clotting. That study does not separate vaccine-induced clotting from COVID-induced clotting.
| Event type | mRNA vaccine rate (per million doses) | Adenoviral vaccine rate (per million doses) | Source |
|---|---|---|---|
| TTS (thrombosis + low platelets) | 0.00855 (VAERS) | 3.83 (VAERS, J&J) | [1] |
| CVST (cerebral sinus clot) | 12.60 (Scottish study, Pfizer) | 16.34 (Scottish study, AstraZeneca) | [1] |
The mechanism is plausible. The spike protein produced by the mRNA can damage the endothelium and disrupt the glycocalyx, which lowers zeta potential and promotes blood sludging. The evidence does not include a single randomized trial comparing mRNA-vaccinated to unvaccinated people with hard clotting endpoints. Every safety estimate comes from passive surveillance or observational comparisons that cannot separate vaccine effect from COVID effect or from confounding by health status.
My call: mRNA vaccines can cause unusual blood clots, including CVST, at a rate that is lower than adenoviral vaccines but not zero. The absolute risk is not reliably measured because the most informative study design (vaccinated vs unvaccinated with hard endpoints, no COVID confounder) was never run. Confidence: moderate that the risk is real but low that we know its true size.
Sources examined 9
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Update on Thromboembolic Events After Vaccination Against COVID-19
This review provides an updated synthesis of the incidence, risk factors, and management of thromboembolic events after COVID-19 vaccination, highlighting that such events are rare overall, with higher risk signals for adenoviral-vector vaccines (VITT, CVST) compared to mRNA vac…
DOI: 10.3390/vaccines13080833 -
Venous Thrombosis within 30 Days after Vaccination against SARS-CoV-2 in a Multinational Venous Thromboembolism Registry
This study investigates the incidence and clinical characteristics of venous thromboembolism (VTE) occurring within 30 days after vaccination against SARS-CoV-2, revealing significant differences in outcomes between adenovirus-based and mRNA-based vaccines.
DOI: 10.3390/v14020178 -
The early impact of COVID-19 vaccines on major events in cardiac, pulmonary, and thromboembolic disease: a population-based study
A large population-based retrospective cohort study in Seoul, Korea, comparing short-term (1 week to 3 months) incidence of myocarditis, MI, atrial fibrillation, interstitial lung disease, pulmonary thromboembolism, deep vein thrombosis, and cerebrovascular disease between adult…
DOI: 10.3904/kjim.2025.056 -
Incidence, Clinical Presentation, and Management of Myocarditis following mRNA-Based Covid-19 Vaccines: A Brief Report
Myocarditis after mRNA Covid-19 vaccines is rare, most common in younger men after the second dose, usually mild and self-limiting, with the benefit of vaccination outweighing the risk.
DOI: 10.1159/000522216 -
Post-COVID-19 Syndrome vs. Consequences of Vaccination with Special Reference to Cardiovascular Conditions
This paper argues that post-COVID-19 syndrome and COVID-19 vaccination side effects overlap in cardiovascular manifestations through shared spike protein mechanisms, and calls for healthcare vigilance and further research on long-term vaccine risks.
DOI: 10.52768/cardiology/1002 -
Cerebral venous sinus thrombosis after mRNA-based COVID-19 vaccination
This study reports a case of cerebral venous sinus thrombosis (CVST) occurring after mRNA-based COVID-19 vaccination, highlighting the need for awareness of such adverse effects even in mild cases.
DOI: 10.1007/s10072-021-05714-0 -
Cerebral venous sinus thrombosis 2 weeks after the first dose of mRNA SARS-CoV-2 vaccine
This case report describes a 49-year-old male who developed cerebral venous sinus thrombosis two weeks after receiving the first dose of the mRNA SARS-CoV-2 vaccine, highlighting potential thromboembolic complications associated with COVID-19 vaccination.
DOI: 10.1007/s00701-021-04860-w -
Cerebral Venous Sinus Thrombosis Following an mRNA COVID-19 Vaccination and Recent Oral Contraceptive Use
This case report describes a 32-year-old female who developed cerebral venous sinus thrombosis (CVST) following her second Pfizer mRNA COVID-19 vaccination, compounded by recent oral contraceptive use, highlighting the potential hypercoagulable state induced by these factors.
DOI: 10.3390/life13020464 -
Mecanismo y diseño de vacunas para el sars-cov-2, revisión narrativa
This narrative review describes SARS-CoV-2 vaccine mechanisms and designs, reporting that mRNA vaccines (BNT162b2 and mRNA-1273) demonstrate the highest efficacy (93-95%), adenovirus vector vaccines show 66.9-91.6% efficacy, and inactivated CoronaVac shows 83.5%, with rare safet…
DOI: 10.18566/medupb.v43n1.a10