Britain's medicines regulator said there had been five cases of a rare type of blood clot in cerebral veins among people given AstraZeneca's vaccine but said the shot should still be used as there was no evidence it caused blood clots.
Concerns about reports of blood clots, along with low platelet levels, have led to European countries including Germany to pause the rollout of the shot while the cases are investigated.
However, Britain's Medicines and Healthcare products Regulatory Agency said that use of the vaccine should continue while five reports were investigated "The available evidence does not suggest that blood clots in veins (venous thromboembolism) are caused by COVID-19 Vaccine AstraZeneca," the MHRA said.
"A further, detailed review into five UK reports of a very rare and specific type of blood clot in the cerebral veins (sinus vein thrombosis) occurring together with lowered platelets (thrombocytopenia) is ongoing."
Britain has administed 11 million doses of AstraZeneca's vaccine.
"Given the extremely rare rate of occurrence of these events, the benefits of the AstraZeneca COVID vaccine, with the latest data suggesting an 80% reduction in hospitalisation and death from COVID disease, far outweigh any possible risks of the vaccine in the risk groups currently targeted in the UK," the MHRA said.
The EMA is investigating reports of 30 cases of unusual blood disorders out of 5 million people who got the AstraZeneca vaccine in the EU.
The EMA's focus and primary concern is on cases of blood clots in the head, a rare condition that's difficult to treat called cerebral venous thrombosis (CVT). It is expected to announce its findings later on Thursday.