When reviewing the number of thromboembolic events in over 11 million people who have been given the vaccine, the MHRA concluded there is no evidence that blood clots in veins are occurring more than would be expected in the absence of vaccination. The agency is still reviewing five U.K. reports of sinus vein thrombosis occurring together with thrombocytopenia, which has been reported in fewer than one in a million people vaccinated and can also occur naturally.

The EMA's Pharmacovigilance Risk Assessment Committee (PRAC) concluded there was no increase in the overall risk of thromboembolic events with AstraZeneca's vaccine. However, the PRAC added that, for very rare cases of serious thromboembolic events with thrombocytopenia, a causal link with the vaccine is not proven, but it is possible, and deserves more analysis. Additionally, the PRAC said there's no evidence of a problem related to specific batches of the vaccine or to particular manufacturing sites.

AstraZeneca said it will work closely with health authorities to ensure appropriate use of its vaccine and will implement the PRAC's recommendations, including updating product information. The firm also said it will work to understand the nature and relevance of these events to ensure safe delivery of its vaccine.

When analyzing its safety database of tens of millions of records, the vaccine did not show these events occurred any more commonly than would be expected in millions of people, AstraZeneca said.