Scientists finally solve mystery behind rare COVID vaccine blood clots

In 2021, only months after the first COVID-19 vaccines were rolled out, concerns emerged over an exceptionally rare but sometimes fatal side effect linked to two adenovirus-based shots — one developed by AstraZeneca and the other by Johnson & Johnson. Both vaccines were associated with unusual blood-clotting events, prompting temporary suspensions worldwide. Ultimately, the vaccines were withdrawn from the market, though questions about the mechanism behind the rare clotting disorder persisted.

Now, an international team of scientists from McMaster University, Flinders University and Universitätsmedizin Greifswald says it has uncovered why a small number of people developed severe blood clots after receiving certain COVID-19 vaccines or following natural adenovirus infections, as reported by Nature.

The findings by the research team from Australia, Canada and Germany, published in The New England Journal of Medicine, suggest that in rare instances, the immune system mistakenly targets one of the body’s own blood proteins. This misguided response leads to vaccine-induced immune thrombocytopenia and thrombosis (VITT), a condition marked by low platelet counts and dangerous clot formation.

The researchers identified the specific viral component capable of triggering this reaction under unusual conditions and described a previously unknown biological pathway that shows how a normal immune defence can shift into a harmful autoimmune response. The discovery may also help scientists better understand other rare, antibody-driven side effects associated with 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.

According to the team, VITT appears to develop after repeated exposure to adenovirus — whether through vaccination or natural infection — but only in individuals who carry a specific inherited variant of an antibody gene, known as IGLV3-21*02 or *03. Although this gene variant is found in up to 60 per cent of the population, researchers note that its presence alone does not explain why the complication remains exceedingly rare.

By Nazrin Sadigova