A rare but serious complication following COVID-19 vaccination – and, it turns out, sometimes even natural adenovirus infection – has been explained at a molecular level by researchers. The condition, known as vaccine-induced immune thrombocytopenia and thrombosis (VITT), involves dangerous blood clots and low platelet counts. A new study published in the New England Journal of Medicine identifies a specific antibody mutation and a key viral protein that, in rare instances, trigger this immune misdirection, offering a path toward safer vaccine design.
For years, scientists have been working to understand why a small number of individuals developed VITT after receiving adenoviral vector-based COVID-19 vaccines, such as those produced by Johnson & Johnson and AstraZeneca. The new research, led by McMaster University, pinpoints the adenovirus protein VII (pVII) as a critical starting point. This protein closely resembles a human protein, platelet factor 4 (PF4). In susceptible individuals, the immune system can mistakenly target PF4 instead of the virus, leading to the formation of dangerous clots.
The breakthrough lies in identifying a specific mutation – K31E – that occurs in antibody-producing cells. This tiny change, altering just one amino acid, is enough to redirect the antibody’s focus from pVII to PF4. “This study shows, with molecular precision, how a normal immune response to an adenovirus can very rarely go off-track,” explains Theodore Warkentin, corresponding author of the study and professor emeritus in the Department of Pathology & Molecular Medicine at McMaster University. “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.”
The researchers discovered that VITT isn’t simply linked to the vaccine itself, but can also occur after a natural adenovirus infection. However, the condition only develops in people with a specific inherited version of an antibody gene (IGLV3‑21*02 or *03). While up to 60 percent of the population carries this gene variant, it’s not enough on its own to cause VITT; the K31E mutation is the crucial second step. The study detected this K31E mutation in all VITT patient antibodies examined and when researchers reversed the mutation in the lab, the dangerous activity disappeared, confirming its central role.
This discovery builds on years of research into VITT. In 2021, Warkentin co-authored the first paper identifying the syndrome. That initial research laid the groundwork for understanding the clinical presentation of VITT. In 2023, his team showed that natural adenovirus infection could also trigger the same PF4-reactive antibodies, a finding published in the New England Journal of Medicine that pointed toward adenovirus as the underlying cause. Further research in 2024 revealed that vaccine- and virus-induced cases share an identical antibody “fingerprint,” as detailed in another NEJM publication. This latest study, published in 2025, finally pinpoints pVII and the K31E mutation as the mechanism.
The implications of this research extend beyond simply understanding VITT. Warkentin believes this discovery provides a “roadmap for vaccine developers to design even safer vaccines without losing the global advantages of adenoviral vaccine technology.” By redesigning the adenoviral vector to minimize the similarity between pVII and PF4, future vaccines could significantly reduce the risk of this rare but serious complication.
The research also answers several key questions about VITT: why adenoviral-vector vaccines and natural adenovirus infection can trigger it, why PF4 is the target, why the condition is so rare, why its incidence differs between populations (the involved antibody gene is more common in people of European ancestry), and why many cases occurred after a first vaccine dose (due to boosting pre-existing immunity).
The team utilized cutting-edge tools to unravel the mechanism, including antibody sequencing, mass spectrometry to map antibody structures, and the creation of lab-engineered antibodies to observe their behavior and mutations. They also confirmed their findings in a humanized mouse model, demonstrating that the “back-mutated” antibody did not cause clotting.
“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,” Warkentin said.
While VITT remains exceptionally rare, this research offers reassurance and a clear path forward for improving vaccine safety. Researchers will continue to monitor the long-term effects of adenoviral vaccines and explore strategies to further minimize the risk of this adverse event. The next step involves translating these findings into concrete vaccine modifications and evaluating their effectiveness in clinical trials.
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