Scientists Finally Solve the Mystery Behind Rare COVID Vaccine Blood Clots

A rare but serious clotting disorder linked to certain COVID-19 vaccines and natural adenovirus infections has puzzled scientists for years.

An international team of researchers from McMaster University (Canada), Flinders University (Australia), and Universitätsmedizin Greifswald (Germany) has identified why a very small number of people developed serious blood clots after receiving certain COVID-19 vaccines or after a natural adenovirus infection. Their findings point to an unexpected mistake by the immune system, which in rare cases targets the wrong molecule.

The study, published in the New England Journal of Medicine, explains how the body can sometimes produce harmful antibodies that attack its own blood proteins, leading to vaccine-induced immune thrombocytopenia and thrombosis (VITT). The researchers pinpointed the exact viral component capable of triggering this response under unusual conditions.

They also described a previously unknown biological pathway showing how a normal immune defense can shift into a harmful reaction. This insight may help scientists better understand other rare, antibody-driven side effects linked to infections, medications, or environmental exposures.

“This study shows, with molecular precision, how a normal immune response to an adenovirus can very rarely go off‑track. By identifying the exact viral protein involved and the specific antibody change that drives this misdirection, we now understand not only what happens in VITT but why,” says Theodore Warkentin, corresponding author of the study and professor emeritus in the Department of Pathology & Molecular Medicine at McMaster University.

“What’s exciting is that we can now point to a specific viral component that can be redesigned. It means future adenoviral vaccines can keep all their advantages while sidestepping the rare immune misfire that causes VITT,” he adds.

The discovery

The researchers determined that VITT can develop after repeated exposure to adenovirus, whether through vaccination or natural infection, but only in individuals who carry a particular inherited version of an antibody gene (IGLV3-21*02 or *03). Since this gene variant is present in up to 60 percent of the population, it cannot alone explain why the complication remains extremely rare.

The process begins with the immune system responding to an adenovirus protein known as protein VII (pVII). This viral protein closely resembles part of a human blood protein called platelet factor 4 (PF4). In exceptionally rare instances, while the immune system is producing antibodies against pVII, a single mutation can occur in one antibody-producing cell.

This mutation (called K31E) replaces one positively charged amino acid with a negatively charged one. Although the change involves just a single building block, it is enough to redirect the antibody’s focus from pVII to PF4. When the altered antibody binds to PF4, it activates platelets, leading to the abnormal clotting and reduced platelet counts characteristic of VITT.

Importantly, the scientists found the same K31E mutation in every VITT patient antibody they analyzed. When they reversed this mutation in laboratory-engineered antibodies, the harmful clotting activity disappeared. This confirmed that the mutation is essential for the condition to develop.

To uncover this mechanism, the team used advanced laboratory techniques. They sequenced antibodies from patients with VITT, analyzed their structures using mass spectrometry, and created engineered versions to observe how the antibodies changed and behaved. The findings were further validated in a humanized mouse model. In these experiments, antibodies carrying the VITT-associated mutation caused clotting, while the “back-mutated” antibodies did not.

“Many people know that mutations in DNA explain things like congenital abnormalities or cancer, but to have an immune cell that is making its expected antibodies triggered by a virus abruptly change its reactivity against a self-protein due to a specific mutation is a spectacular finding that is unprecedented in the scientific literature,” says Warkentin.

Why it matters

This discovery answers five long‑standing questions about VITT:

1. Why adenoviral‑vector vaccines – and natural adenovirus infection – can trigger it
2. Why PF4 is the target (mimicry between pVII and PF4)
3. Why VITT is extraordinarily rare (it requires a specific, chance mutation in a predisposed person);
4. Why does the incidence differs between populations (the involved antibody gene is more common in people of European ancestry) and
5. Why many cases occurred after a first vaccine dose (it stems from boosting pre‑existing anti‑pVII immunity from low baseline antibody levels).

Just as importantly, the discovery provides a practical roadmap for vaccine developers to design even safer vaccines without losing the global advantages of adenoviral vaccine technology.

Reference: “Adenoviral Inciting Antigen and Somatic Hypermutation in VITT” by Jing Jing Wang, Linda Schönborn, Theodore E. Warkentin, Luisa Müller, Thomas Thiele, Lena Ulm, Uwe Völker, Sabine Ameling, Sören Franzenburg, Lars Kaderali, Ana Tzvetkova, Alex Colella, Tim Chataway, Chee Wee Tan, Bridie Armour, Alexander Troelnikov, Lucy Rutten, James McCluskey, Roland Zahn, Tom P. Gordon and Andreas Greinacher, 11 February 2026, New England Journal of Medicine.
DOI: 10.1056/NEJMoa2514824

The study was supported by the American Society of Hematology (Global Research Award), Competence Center for Genomic Analysis (Kiel), Deutsche Forschungsgemeinschaft (DFG), DFG Research Infrastructure Next Generation Sequencing (NGS) Competence Center / NGS Competence Network, Else Kröner‑Fresenius Stiftung, European Medicines Agency, Flinders Foundation (Health Seed Grant), Gates Foundation, National Health and Medical Research Council (Ideas Grant) and Universitätsmedizin Greifswald (Gerhard Domagk Research Program).

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