Researchers Uncover Cause of Rare Blood Clots Linked to COVID Vaccines

Adenovirus-based vaccines remain an important tool, so understanding this rare immune reaction is crucial.

Published on Mar. 10, 2026

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Researchers have uncovered the cause of the rare but dangerous blood clots that occurred in a small number of people after receiving COVID-19 vaccines that used a modified adenovirus to deliver the vaccine payload, such as the AstraZeneca vaccine. The condition, known as vaccine-induced immune thrombocytopenia and thrombosis (VITT), happens when the immune system mistakenly attacks a protein called platelet factor 4, leading to the formation of harmful blood clots. The researchers found that VITT is triggered by a rare mutation in antibodies that normally recognize an adenoviral protein, which then allows those antibodies to bind extremely tightly to platelet factor 4.

Why it matters

Understanding the cause of VITT is crucial because adenovirus-based vaccines remain an important tool, especially for future pandemics. VITT is very rare, but the researchers note that similar immune reactions can also be triggered by viral infections like adenovirus and cytomegalovirus, not just vaccines. Knowing the underlying mechanism could help modify future vaccines to avoid this rare side effect.

The details

The researchers studied 100 VITT patients from around the world and found that virtually all of them shared a distinctive pattern in their antibodies. They discovered that the antibodies involved in VITT begin as antibodies that recognize an adenoviral protein called protein VII, likely from previous adenovirus infections. A rare mutation then allows these antibodies to bind extremely tightly to platelet factor 4, triggering the dangerous blood clots. The researchers also found that most VITT patients carry a particular immune gene variant that shapes the structure of their antibodies in a way that enables this mutation.

  • The COVID-19 vaccine rollout began in late 2020, and a small number of people began developing VITT in the months that followed.

The players

Richard Buka

Haematology Registrar and Clinical Research Fellow at the University of Birmingham.

Samantha Montague

Postdoctoral Research Fellow at the Institute of Cardiovascular Sciences, University of Birmingham.

AstraZeneca

A pharmaceutical company that developed one of the COVID-19 vaccines that used a modified adenovirus to deliver the vaccine payload.

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What they’re saying

“Understanding exactly how VITT happens means scientists may now be able to modify future vaccines to avoid triggering this rare immune reaction.”

— Richard Buka, Haematology Registrar and Clinical Research Fellow, University of Birmingham

“The views expressed are those of the author(s) and not necessarily those of the NIHR or the BHF.”

— Samantha Montague, Postdoctoral Research Fellow, Institute of Cardiovascular Sciences, University of Birmingham

What’s next

The researchers hope that their findings will help inform the development of future adenovirus-based vaccines to minimize the risk of triggering VITT.

The takeaway

This research provides crucial insights into the rare but serious side effect of VITT associated with some COVID-19 vaccines, which could help improve the safety of adenovirus-based vaccines going forward and ensure they remain an important tool for combating future pandemics.