So far, most of the reported cases have occurred in women under the age of 60, the regulator said, but it said it still has not been able to train if certain groups were more vulnerable than others.
EU national authorities can make their own decisions on whether to use the data to restrict vaccine use in certain sections of their population, officials said.
The vaccine produced by AstraZeneca and Oxford University has been under intense scientific control since the beginning of last month, when a 49-year-old nurse died of complications due to blood clots following her vaccination in Austria. Other cases followed throughout Europe, leading some countries to suspend their use of the vaccine until scientific studies were conducted.
Concerns are centered on a rare condition called cerebral venous sinus thrombosis or CVST, a blood clot that prevents blood from draining from the brain. Regulators have said it occurs among those vaccinated at a rate above what they would expect to see in the normal population.
Wednesday’s announcement is the latest blow to the vaccine, which was touted as a key tool to lead the world out of the pandemic. Cheap and easier to store than others on the market. There are plans to roll it out in more than 140 countries through a program designed to ensure fair distribution.
Concerns are small. The UK has documented the severe blood clots in about 1 in 500,000 shots. However, higher rates in countries that only started using AstraZeneca’s vaccine among younger people have spurred the debate on whether the risk may be higher for certain age groups or for women. The EMA has estimated the risk for people under 60 to be approx. 1 in 100,000.
In Germany, 29 out of 31 patients with cerebral thrombosis have been women. But the European regulator has said it may be because countries like Germany started vaccinating communities as health workers that include more women.
Scientific teams in both Germany and Norway that have studied patients and blood tests have said that the unusual blood clots are caused by an overactive immune response and they can see no other likely trigger for this reaction in patients other than the vaccine.
New guidelines for diagnosis by testing for antibodies have already been circulated in countries including Germany and the United Kingdom.