Scientists solve mystery behind COVID vaccine blood clots

After years of scientific sleuthing, these researchers have uncovered the cause of a lethal blood clotting disorder linked to certain COVID vaccines.

New research has finally uncovered how a rare blood clotting condition can occur after some COVID19 vaccines, or after a natural adenovirus (common cold) infection.

At the height of the COVID-19 pandemic, some Australians, and others from across the world, developed vaccine-induced immune thrombocytopenia and thrombosis (VITT) - most notably after receiveing the Oxford-AstraZeneca vaccine, which was used in Australia.

Now an international team, led by Flinders University, have found that in a small number of people, the immune system can accidentally confuse a normal adenovirus protein with a human blood protein termed platelet factor 4 (or PF4).

When this happens, the body produces an antibody that triggers clotting. This mistake only occurs in extremely rare cases, but understanding the exact cause means vaccine developers can now adjust the adenovirus protein to prevent the problem entirely, helping to make future vaccines even safer.

Flinders University researcher, Dr Jing Jing Wang says that the discovery provides a clear blueprint for vaccine developers.

“By modifying or removing this specific adenovirus protein, future vaccines can avoid this extremely rare reaction while continuing to provide strong protection against disease,” says Dr Wang.

The new paper is part of an international collaboration, which included experts from Greifswald University, and charts a pathway from the first recognition of VITT to the identification of the exact molecular trigger.

How researchers became detectives - and cracked the case together

Dr Wang and Professor Tom Gordon, Head of Immunology at SA Pathology in South Australia, led a previous study in 2022 which cracked the molecular code of the PF4 antibody and identified a genetic risk factor related to an antibody gene termed IGLV3.21*02.  This finding linked VITT cases across countries and helped form a long‑term collaboration with Greifswald University, led by Professor Andreas Greinacher.

In separate research in 2023, Professor Ted Warkentin from McMaster University in Canada described a virtually identical disorder with the same PF4 antibody that was fatal in some cases after natural adenovirus (common cold) infection.

In 2024, another paper led by Flinders, Greifswald and McMaster Universities confirmed that the antibodies from vaccine‑related cases and infection‑related cases were indistinguishable. This established that the adenovirus itself, not a specific vaccine component, was the likely source of the trigger. But the precise molecular mechanism remained unknown.

Professor Tom Gordon says that the new research published in the New England Journal of Medicine represents the culmination of years of international 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,” says Professor Gordon.

Dr Wang said the team’s molecular analysis provided the breakthrough needed to understand the mechanism.

“This was the missing link that explains how a normal immune response can, in very rare cases, become harmful," she says.

Immunologist Professor James McCluskey from the University of Melbourne and the Peter Doherty Institute described the research as a major scientific achievement.

“It is a brilliant piece of molecular sleuthing, the culmination of a body of work that unravels the genetic and structural basis for how a normal immune response to a virus protein leads to pathogenic autoimmunity,” says Professor McCluskey.

With the precise trigger now identified, vaccine developers can modify the pVII protein in adenovirus‑based vaccines to eliminate this rare risk.

Researchers say the findings will help ensure future vaccines built on this technology remain effective, accessible and even safer, particularly in regions where adenovirus‑vector vaccines are critical tools for disease prevention.