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Since rare but severe clotting was seen in some people following vaccination with AstraZeneca's Covid-19 vaccine, researchers worldwide have been grappling to understand why the clotting syndrome, known as thrombosis with thrombocytopenia" (clotting with a low platelet count), occurs.

Melissa Davey brings you up to speed here:

Related: AstraZeneca blood clotting: what is this rare syndrome and how is it caused?