New research has clues about why Covid vaccine led to blood clots in some people

AstraZeneca in partnership with the University of Oxford developed one of the first vaccines against Covid-19. The vaccine, which used an adenovirus to smuggle instructions into human cells to make antibodies against the novel coronavirus, saved countless lives. But a problem soon emerged. A tiny proportion – about one in 50,000 – of those vaccinated developed blood clots.

This blood clot syndrome is known as vaccine-induced immune thrombotic thrombocytopenia, or VITT. In people with this condition, something goes wrong with the immune response and people make antibodies that can stick to one of the body’s own proteins.

The protein is called platelet factor 4 (PF4). We know that in the course of infection, many people make antibodies that stick to PF4 as part of the immune response, but these antibodies usually stick weakly. In VITT, antibodies form that can stick to PF4 like superglue.

The antibodies in VITT glue PF4 molecules together, forming large structures known as “immune complexes”. These complexes bind to and activate small cells called platelets that are vital for blood clotting. Normally, platelets float around in the blood in an inactive state, but once activated they spread out, get very sticky, and spew out hundreds of different chemicals.

In VITT, platelets are strongly activated and this causes...

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