A rare but sometimes fatal blood clotting disorder that emerged during mass COVID-19 vaccination campaigns has finally been explained at the molecular level — a finding that researchers say could make future adenoviral vector vaccines significantly safer. After years of international scientific collaboration spanning three continents, a team led by Flinders University in South Australia has identified the precise biological trigger behind vaccine-induced immune thrombocytopenia and thrombosis, or VITT.
The findings were published on February 11, 2026, in the New England Journal of Medicine, representing the third in a trilogy of landmark papers on the condition from the same international consortium, which includes researchers from Greifswald University in Germany and McMaster University in Canada.
What is VITT?
At the height of the COVID-19 pandemic, a small number of people who received adenovirus vector-based vaccines developed VITT (most notably with the Oxford-AstraZeneca vaccine), a dangerous immune response characterized by blood clots combined with low platelet counts. The condition proved fatal in some cases. In Australia, where the AstraZeneca vaccine was widely administered, VITT prompted significant public concern and contributed to shifts in national vaccination guidance.
The condition is mediated by antibodies that mistakenly attack platelet factor 4 (PF4), a human blood protein, triggering runaway clot formation. While clinicians developed protocols to recognize and treat VITT, the underlying cause — why the immune system produced these rogue antibodies in the first place — had remained unresolved.
The molecular breakthrough
The new study reveals that the confusion originates in a specific protein found in adenoviruses: the core protein known as pVII. In a small subset of people carrying a particular immunoglobulin light-chain gene variant (IGLV3-2102 or 03), the immune system generates antibodies against pVII. Through a process called somatic hypermutation — a normal part of how the immune system refines antibodies — a single critical mutation designated K31E causes those antibodies to cross-react with PF4. The result is an inadvertent autoimmune attack that drives dangerous clotting.
Critically, the researchers demonstrated that when this mutation was reversed in the laboratory, the antibody lost its harmful activity and reverted to binding pVII rather than PF4. This direct experimental confirmation effectively closes the causal loop, providing unambiguous evidence of the mechanism.
“This was the missing link that explains how a normal immune response can, in very rare cases, become harmful.”
– Dr. Jing Jing Wang, lead researcher at Flinders University
Implications for vaccine development
Adenoviral vector technology underpins a range of vaccines in development or deployment for diseases including Ebola, HIV, and other pathogens of pandemic potential. The ability to identify and eliminate a specific molecular risk factor by modifying or removing the pVII protein from future vaccine constructs gives developers a concrete engineering solution rather than an unresolved safety signal.
Professor Tom Gordon, Head of Immunology at SA Pathology and a senior collaborator on the research, described the publication as the culmination of years of careful scientific detective work. “It has been a fascinating journey with an outstanding international team of collaborators to complete a trilogy of publications in the New England Journal of Medicine to solve the mystery of this new group of blood clotting disorders, and potentially translate our discoveries into safer vaccines,” he said.
The research also clarified a puzzling parallel observation: an essentially identical disorder was documented in patients who had never been vaccinated but had contracted a natural adenovirus infection — a common cold virus. Earlier work confirmed that the antibodies in both vaccine-related and infection-related cases were molecularly indistinguishable, pointing to the adenovirus itself as the trigger, independent of any vaccine-specific component. The new study now explains precisely how that trigger operates.
Professor James McCluskey of the University of Melbourne and the Peter Doherty Institute, an independent immunologist, called the research “a brilliant piece of molecular sleuthing” that “unravels the genetic and structural basis for how a normal immune response to a virus protein leads to pathogenic autoimmunity.”
Sources and further reading:
Adenoviral Inciting Antigen and Somatic Hypermutation in VITT. The New England Journal of Medicine, February 11, 2026.
Scientists solve mystery behind COVID vaccine blood clots – Flinders University