Question explored with the scientific record
Conversation so far
- 1 Oxford–AstraZeneca COVID-19 vaccine side effacts?
- 2 How likely is VITT after the first vs second AstraZeneca dose?
Answered on this page ↓
How likely is VITT after the first vs second AstraZeneca dose?
The short version: VITT is overwhelmingly a first-dose problem with AstraZeneca, but the retrieved evidence does not give a clean first-versus-second dose incidence ratio.
The clearest signal comes from the case series themselves. A 2025 comparison of 220 ChAdOx1 VITT cases found they all occurred after the first dose [1]. A 2021 BMJ case report describes a fatal VITT case after dose one [5]. A 2021 Haematologica analysis of 49 cases found 79% were women and only 4% were over 60, with a 40% mortality rate [4]. A 2025 review states VITT risk signals are higher in females under 60 after the first dose of adenoviral-vector vaccines [2].
The incidence numbers in this retrieval are per million doses, not per dose number. Scotland reported 16.34 CVST cases per million ChAdOx1 doses [2]. The UK pooled rate ratio for CVST within 28 days post-vaccination was 1.93 [2]. But none of these records break down the rate by dose number. That is a real gap.
The mechanism supports the first-dose pattern. VITT is an antibody response to platelet factor 4 [6]. The first exposure primes the immune system; the retrieved records simply do not quantify how much lower the second-dose risk is. One 2022 case report documents a 29-year-old male with fulminant portomesenteric thrombosis after an adenoviral vector vaccine, but does not specify dose number [3].
| Group | CVST rate per million doses | Notes |
|---|---|---|
| ChAdOx1 (Scotland) | 16.34 | [2] |
| BNT162b2 (Scotland) | 12.60 | [2] |
| Ad26.COV2.S (US VAERS) | 3.83 TTS per million | [2] |
My call: VITT is a first-dose phenomenon in the retrieved case data, but the exact first-versus-second dose incidence ratio was never quantified in these records. Confidence: moderate.
Sources used 6
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Comparison of vaccine-induced immune thrombocytopenia and thrombosis cases following two adenovirus-vectored COVID-19 vaccines
A descriptive comparison of 86 Ad26.COV2.S and 220 ChAdOx1 nCoV-19 VITT cases found no strong evidence of a clinically relevant difference in patient characteristics or outcomes, with observed differences attributed to vaccination demographics and reporting or detection differen…
DOI: 10.1038/s43856-025-00891-x -
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 -
Transjugular intrahepatic portosystemic shunt, local thrombaspiration, and lysis for management of fulminant portomesenteric thrombosis and atraumatic splenic rupture due to vector-vaccine-induced thrombotic thrombocytopenia: a case report
This case report details the diagnosis and management of a 29-year-old male who developed fulminant portomesenteric thrombosis and atraumatic splenic rupture due to vaccine-induced immune thrombotic thrombocytopenia following vaccination with an adenoviral vector vaccine against…
DOI: 10.1186/s13256-022-03464-x -
A new enemy is emerging in the fight against the SARS-CoV-2 pandemic
This study investigates vaccine-induced immune thrombotic thrombocytopenia (VITT) following the ChAdOx1 nCoV-19 vaccination, highlighting its clinical features, pathogenesis, and implications for vaccine administration in different age groups.
DOI: 10.3324/haematol.2021.279186 -
ChAdOx1 nCOV-19 vaccine-induced immune thrombotic thrombocytopenia and cerebral venous sinus thrombosis (CVST)
A case report of vaccine-induced immune thrombotic thrombocytopenia with cerebral venous sinus thrombosis after ChAdOx1 nCOV-19 vaccination, illustrating PF4‑IgG positivity and fatal outcome despite IVIg and non‑heparin anticoagulation.
DOI: 10.1136/bcr-2021-243931 -
Unique features of vaccine-induced immune thrombotic thrombocytopenia; a new anti-platelet factor 4 antibody-mediated disorder
This study reviews the unique features, pathophysiology, clinical presentation, and diagnostic challenges of vaccine-induced immune thrombotic thrombocytopenia (VITT), a disorder associated with adenoviral vector vaccines, highlighting its similarities and differences with hepar…
DOI: 10.1007/s12185-022-03516-4