The EU drug regulator advised on 11 November listing blood clots in the brain as a rare side effect of AstraZeneca's COVID-19 after observing cases that were not linked to a low platelet count.

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"Events of cerebrovascular venous and sinus thrombosis (CVST) without thrombocytopenia have been observed very rarely following vaccination with Vaxzevria," an update read. The European Medicines Agency said 458 cases of vaccinated people developing a stroke followed by a brain bleed were observed up to September 30. The majority of cases occurred within the first four weeks of vaccination. Some cases had a fatal outcome. The agency said that CVST should be added to the product information as a side effect of "unknown frequency." Blood clots forming in the brain while the blood platelet count is low was listed as a "very rare" side effect of the vaccine in April.