Question explored with the scientific record
Is there evidence of covid vaccination causing blood clots
Yes. The evidence is clear: COVID-19 vaccines, especially the adenovirus-vector types (AstraZeneca, Johnson & Johnson), cause a rare but serious clotting syndrome called vaccine-induced immune thrombotic thrombocytopenia (VITT).
The mechanism is now well-documented. The vaccine triggers the body to make antibodies against platelet factor 4 (PF4), a protein normally stored inside platelets. These anti-PF4 antibodies then activate platelets through their FcγRIIa receptors, causing them to clump together and form clots while also being consumed, leading to the paradoxical combination of thrombosis and low platelets (thrombocytopenia) [13][14]. This is the same mechanism as heparin-induced thrombocytopenia, but without the heparin.
The strongest signal is with the adenovirus-vector vaccines. A systematic review of 450 VITT cases found cerebral venous sinus thrombosis (CVST) in 317, pulmonary embolism in 17, and a mortality rate of 30% [11]. The estimated incidence for AstraZeneca was about 13.6 cases per million doses, and for Johnson & Johnson about 3.2 per million [12]. The mRNA vaccines (Pfizer, Moderna) have a much lower risk, but cases are still reported, including after booster doses [15][1][3][4][5][6][7][8][9].
The evidence here is dominated by case reports and case series, which can show that an event happens but cannot calculate the exact population risk. The large population studies in this retrieval do show a small excess risk. A UK study of 19.6 million AstraZeneca recipients found a 10% increase in venous thromboembolism 8-14 days after vaccination [16]. A US veterans study found about 1.4 excess VTE cases per million vaccinated [17]. A Korean study found vaccination was associated with a reduced risk of pulmonary thromboembolism overall, but that comparison is complicated because the vaccinated group was healthier to begin with [10].
The key comparison that matters for a real decision is vaccine risk versus COVID-19 infection risk. The same UK study found that COVID-19 infection increased VTE risk by nearly 14-fold, compared to the 1.1-fold increase from the AstraZeneca vaccine [16]. The risk of CVST is estimated to be up to ten times higher from COVID-19 infection than from vaccination [2]. For most people, the clotting risk from the disease dwarfs the risk from the shot.
| Clotting Event | Risk from COVID-19 Infection | Risk from Adenovirus Vaccine | Risk from mRNA Vaccine |
|---|---|---|---|
| Venous thromboembolism (VTE) | ~14-fold increase (IRR 13.86) [16] | ~1.1-fold increase (IRR 1.10) [16] | Not significantly elevated [16] |
| Cerebral venous sinus thrombosis (CVST) | Up to 10x higher than vaccine [2] | ~13.6 per million doses (AstraZeneca) [12] | Lower, but cases reported [1][3][4] |
| VITT (clots + low platelets) | Not applicable | Well-documented syndrome [11][12] | Rare cases reported [15] |
My call: the vaccines cause clotting, especially VITT with adenovirus-vector types, but the risk is far lower than the clotting risk from a COVID-19 infection for most people. The burden of proof has been met for causation, but the absolute risk is small. Confidence: high.
Sources used 17
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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 -
Extensive Cerebral Venous Thrombosis following by COVID-19 Vaccine with Concomitant COVID-19 Infection
A case of cerebral venous sinus thrombosis with concurrent COVID-19 infection is reported in the post-vaccine setting; cited evidence suggests CVST risk is higher with COVID-19 infection than with vaccination, supporting a high index of suspicion.
DOI: 10.17576/mh.2024.1901.25 -
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 After Pfizer-BioNTech COVID-19 (BNT162b2) Vaccination
This case report describes a 61-year-old male who developed cerebral venous sinus thrombosis (CVST) following the Pfizer-BioNTech COVID-19 vaccination, highlighting the need for awareness of this rare side effect amidst ongoing vaccination efforts.
DOI: 10.3988/jcn.2021.17.4.573 -
Pulmonary Embolism and Inferior Vena Cava Thrombosis in a Young Male Patient after mRNA-1273 (Moderna) Immunization: A Case Report
This case report describes a 27-year-old male who developed pulmonary embolism and inferior vena cava thrombosis following the second dose of the mRNA-1273 (Moderna) vaccine, highlighting a rare adverse event associated with COVID-19 vaccination.
DOI: 10.4046/trd.2022.0050 -
Pulmonary Embolism after Vaccination with the COVID-19 Vaccine (Pfizer, BNT162b2): A Case Report
This case report describes a 58-year-old man who developed pulmonary embolism and deep vein thrombosis following vaccination with the Pfizer COVID-19 vaccine (BNT162b2), highlighting potential adverse effects associated with mRNA vaccines.
DOI: 10.3390/vaccines11061075 -
Pulmonary Embolism after Moderna Vaccination in Kidney Transplant Patients: Two Case Reports and Literature Review
This study presents two case reports of kidney transplant patients who developed pulmonary embolism after receiving the Moderna COVID-19 vaccine, highlighting the need for awareness of thromboembolic risks in immunocompromised individuals post-vaccination.
DOI: 10.3390/vaccines10060868 -
Deep venous thrombosis and pulmonary embolism after COVID-19 mRNA vaccination
This study reports a case of deep venous thrombosis and pulmonary embolism occurring in a 24-year-old male patient with thrombophilia shortly after receiving the second dose of the Pfizer BioNTech mRNA COVID-19 vaccine, highlighting potential risks associated with vaccination in…
DOI: 10.1007/s00277-021-04743-1 -
Case of Covid Mrna Vaccine Linked Antiphospholipid Syndrome
This case report presents a 22-year-old male who developed antiphospholipid syndrome and a large pulmonary embolus following COVID mRNA vaccination, suggesting a potential autoimmune reaction linked to the vaccine.
DOI: 10.33140/ijdmd.07.01.07 -
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 -
COVID-19 vaccine-induced immune thrombotic thrombocytopenia: a review
This systematic review investigates the prevalence, clinical characteristics, and management of vaccine-induced immune thrombotic thrombocytopenia (VITT) following COVID-19 vaccination, highlighting its strong association with viral vector-based vaccines and the critical need fo…
DOI: 10.7774/cevr.2023.12.4.265 -
Vaccine-induced thrombotic thrombocytopenia: the elusive link between thrombosis and adenovirus-based SARS-CoV-2 vaccines
This paper reviews the association between adenovirus-based COVID-19 vaccines and the rare syndrome of vaccine-induced thrombotic thrombocytopenia (VITT), discussing its epidemiology, pathogenesis, and management strategies.
DOI: 10.1007/s11739-021-02793-x -
Immune-Mediated Platelet Activation in COVID-19 and Vaccine-Induced Immune Thrombotic Thrombocytopenia
A review of immune-mediated platelet activation in COVID-19 and vaccine-induced immune thrombotic thrombocytopenia (VITT), summarizing evidence that anti-PF4 antibodies activate platelets via FcγRIIA and discussing therapeutic implications.
DOI: 10.3389/fimmu.2022.837629 -
FcγRIIa - dependent platelet activation identified in COVID-19 vaccine-induced immune thrombotic thrombocytopenia-, heparin-induced thrombocytopenia, streptokinase- and anisoylated plasminogen-streptokinase activator complex-induced platelet activation
A narrative review examining how FcγRIIa-dependent platelet activation underlies vaccine-induced immune thrombotic thrombocytopenia (VITT) and parallels heparin-induced thrombocytopenia, streptokinase, and APSAC-induced platelet disorders, using comparative analysis to better un…
DOI: 10.3389/fcvm.2023.1282637 -
Vaccine-Induced Immune Thrombotic Thrombocytopenia following BNT162b2 mRNA COVID-19 Booster: A Case Report
This case report describes a previously healthy 28-year-old man who developed vaccine-induced immune thrombotic thrombocytopenia (VITT) following his third dose of the BNT162b2 mRNA COVID-19 vaccine, presenting with severe thrombotic events and responding well to intravenous imm…
DOI: 10.3390/vaccines11061115 -
COVID-19 vaccination–related cardiovascular complications
This narrative review finds COVID-19 vaccination is associated with rare cardiovascular complications, especially mRNA vaccine myocarditis in young men and adenoviral vaccine VITT/VTE, but myocarditis and thrombosis are more common after SARS-CoV-2 infection and benefits outweig…
DOI: 10.36011/cpp.2023.5.e17 -
COVID-19 vaccination and venous thromboembolism risk in older veterans
In a retrospective cohort of US veterans aged 45 or older, SARS-CoV-2 vaccination was associated with only a trivial increased risk of venous thromboembolism, much lower than the VTE risk associated with COVID-19.
DOI: 10.1017/cts.2022.527