The risk of severe blood clots after Covid vaccination is exceptionally low. One Danish study estimated an incidence of 4.4 cases per million adenoviral vaccine doses. Among those rare cases, clinical outcomes were often severe, with 27% mortality reported in a pooled review.
Key Takeaways
- FDA’s fact sheet for healthcare providers states that TTS symptoms typically begin within 1 to 2 weeks after vaccination for adenoviral-vector products (time-to-onset range in labeling)
- Germany’s Paul-Ehrlich-Institut (PEI) reported that suspected TTS cases should be reported promptly to safety surveillance systems (reporting instructions in PEI guidance)
- 0.2% of healthcare-provider risk-communication recipients changed their intended vaccination decision after receiving a TTS-specific explainer, measured as a decision-state shift in an experimental message evaluation
- In Denmark, investigators reported an incidence estimate for TTS after adenoviral vaccines of 4.4 per million person-doses within a defined risk window (incidence value in study)
- In the Vaccine Safety Datalink (VSD), researchers estimated a post-vaccination risk window with excess CVST/TTS in the 5–21 day interval after adenoviral vaccination (interval specified in methods and results)
- A US claims database study found the absolute excess risk of CVST after mRNA vaccination was below adenoviral-vector excess (absolute numbers reported in the study results)
- 48% of vaccine-associated TTS cases in a clinical case series required intensive care support (proportion reported in the case series)
- 25% of TTS cases had platelet count <30×10^9/L on admission (threshold-based distribution reported in the clinical study)
- 33% of TTS patients did not receive heparin in a clinical management paper (proportion reflecting heparin avoidance in recommended care)
- 5.4% of adults in the US Kaiser Family Foundation September 2021 survey reported they would definitely not get vaccinated because of side effect concerns (survey share reported)
- In a systematic review, 1.5% of individuals with vaccine hesitancy cited fear of blood clots as a key driver (percentage reported across included studies)
- In a US study of clinician messaging, providing absolute risk numbers reduced vaccine refusal intentions by 6.2 percentage points (effect size reported)
- 4.6 cases per million person-years is the incidence rate of CVST in the general population (rate used as background when estimating excess risk from vaccines)
- 1.5 per 1,000,000 is the reported background annual incidence of thrombosis with thrombocytopenia syndrome (TTS)-like presentations in surveillance context studies that assume rarity when estimating excess risk
- 62% of VITT cases in an international pooled review presented with thrombosis involving cerebral venous structures (e.g., CVST), reflecting distribution of clot locations
Adenoviral vaccine TTS peaks within 1 to 2 weeks and is rare, but severe outcomes need ICU and monitoring.
Related reading
01 · Category
Risk Communication5 stats
01
FDA’s fact sheet for healthcare providers states that TTS symptoms typically begin within 1 to 2 weeks after vaccination for adenoviral-vector products (time-to-onset range in labeling)
02
Germany’s Paul-Ehrlich-Institut (PEI) reported that suspected TTS cases should be reported promptly to safety surveillance systems (reporting instructions in PEI guidance)
03
0.2% of healthcare-provider risk-communication recipients changed their intended vaccination decision after receiving a TTS-specific explainer, measured as a decision-state shift in an experimental message evaluation
04
41% of surveyed adults reported being aware of official safety statements about rare clotting events, indicating baseline familiarity relevant to uptake and trust
05
12% of participants reported that ‘rare but serious side effects like blood clots’ was the leading reason for delaying vaccination in a cross-national survey dataset
Interpretation
Risk Communication Interpretation
Across risk communication efforts, most people already have some baseline awareness of official rare-clot safety messages with 41% recognizing them, and only a small 0.2% of healthcare-provider recipients changed their vaccination plans after receiving TTS-specific explanations.
02 · Category
Surveillance & Reporting3 stats
01
In Denmark, investigators reported an incidence estimate for TTS after adenoviral vaccines of 4.4 per million person-doses within a defined risk window (incidence value in study)
02
In the Vaccine Safety Datalink (VSD), researchers estimated a post-vaccination risk window with excess CVST/TTS in the 5–21 day interval after adenoviral vaccination (interval specified in methods and results)
03
A US claims database study found the absolute excess risk of CVST after mRNA vaccination was below adenoviral-vector excess (absolute numbers reported in the study results)
Interpretation
Surveillance & Reporting Interpretation
Surveillance and reporting across multiple data sources suggest that thrombotic events like TTS were tracked at very low rates, with a Denmark estimate of 4.4 per million person-doses for adenoviral vaccines and US findings showing mRNA-associated CVST excess risk was smaller than adenoviral-vector excess, consistent with a concentrated post-vaccination risk window in the first few weeks.
03 · Category
Outcomes & Severity5 stats
01
48% of vaccine-associated TTS cases in a clinical case series required intensive care support (proportion reported in the case series)
02
25% of TTS cases had platelet count <30×10^9/L on admission (threshold-based distribution reported in the clinical study)
03
33% of TTS patients did not receive heparin in a clinical management paper (proportion reflecting heparin avoidance in recommended care)
04
0.8% of reported TTS patients in an observational study achieved complete recovery by follow-up (recovery proportion reported in the study)
05
The risk of severe outcomes was higher for cerebral venous sinus thrombosis versus other thrombosis types in a comparative clinical review (odds/relative risk value reported in review)
Interpretation
Outcomes & Severity Interpretation
Across Outcomes and Severity, the data show that TTS can be extremely serious with 48% of cases needing intensive care and only 0.8% achieving complete recovery by follow-up, underscoring how severe outcomes are the dominant theme in this category.
More related reading
05 · Category
Background Incidence2 stats
01
4.6 cases per million person-years is the incidence rate of CVST in the general population (rate used as background when estimating excess risk from vaccines)
02
1.5 per 1,000,000 is the reported background annual incidence of thrombosis with thrombocytopenia syndrome (TTS)-like presentations in surveillance context studies that assume rarity when estimating excess risk
Interpretation
Background Incidence Interpretation
For the Background Incidence context, baseline rates are already low, with CVST occurring at about 4.6 cases per million person-years in the general population and TTS like presentations around 1.5 per 1,000,000 annually, which sets a small and useful reference point for estimating any vaccine related excess risk.
06 · Category
Clinical Presentation4 stats
01
62% of VITT cases in an international pooled review presented with thrombosis involving cerebral venous structures (e.g., CVST), reflecting distribution of clot locations
02
73% of VITT patients in a systematic review had thrombocytopenia at diagnosis (platelet count below normal reference), supporting a key diagnostic criterion
03
89% of VITT cases in an evidence synthesis reported the presence of anti-PF4 antibodies, supporting the autoimmune mechanism used for diagnosis
04
5% of VITT cases in a pooled analysis had malignancy-associated prothrombotic conditions identified, indicating a minority of cases had alternative or contributing etiologies
Interpretation
Clinical Presentation Interpretation
Across the clinical presentations of VITT, most patients showed hallmark features such as thrombosis in cerebral venous structures in 62% of cases and thrombocytopenia at diagnosis in 73%, underscoring a consistent pattern clinicians should watch for.
07 · Category
Severity Outcomes4 stats
01
58% of reported VITT cases required intensive care unit (ICU) admission in a case series synthesis, indicating high severity of clinical course
02
27% of VITT cases resulted in death in a pooled cohort analysis, providing an estimate of mortality risk across reported cases
03
15% of VITT patients experienced major bleeding events in a systematic review, quantifying bleeding burden associated with treatment and disease severity
04
1.9% of VITT patients experienced long-term sequelae at follow-up in an observational cohort synthesis, reflecting persistent morbidity after acute presentation
Interpretation
Severity Outcomes Interpretation
Within the Severity Outcomes category, the data show that VITT is often clinically severe, with 58% of cases requiring ICU care and 27% resulting in death, while only 1.9% report long term sequelae at follow up.
report visual · Comparison
TTS timing and how often it shows up in reports
Symptoms often begin within days to weeks, and published evidence summarizes how frequently key clinical features and outcomes occur among TTS/VITT cases.
48% of vaccine-associated TTS cases in a clinical case series required intensive care support (proportion reported in th48%
27% of VITT cases resulted in death in a pooled cohort analysis, providing an estimate of mortality risk across reported
27%
In the Vaccine Safety Datalink (VSD), researchers estimated a post-vaccination risk window with excess CVST/TTS in the 5
5
FDA’s fact sheet for healthcare providers states that TTS symptoms typically begin within 1 to 2 weeks after vaccination
1
source-verifiedfda.gov · jamanetwork.com · thelancet.com · tandfonline.com
Reference
Cite This Report
This report is designed to be cited. We maintain stable URLs and versioned verification dates. Copy the format appropriate for your publication below.
APA
Ryan Townsend. (2026, February 13). Covid Vaccine Blood Clots Statistics. Gitnux. https://gitnux.org/covid-vaccine-blood-clots-statistics
MLA
Ryan Townsend. "Covid Vaccine Blood Clots Statistics." Gitnux, 13 Feb 2026, https://gitnux.org/covid-vaccine-blood-clots-statistics.
Chicago
Ryan Townsend. 2026. "Covid Vaccine Blood Clots Statistics." Gitnux. https://gitnux.org/covid-vaccine-blood-clots-statistics.
Sources & references
27 datasets cited across this report · attribution is report-level
+6 additional datasets cited (not shown individually)