There is a link between AstraZeneca’s Covid-19 vaccine and very rare blood clots in the brain but the possible causes are still unknown, a senior official for the European Medicines Agency (EMA) said in an interview published on Tuesday.

“In my opinion we can now say it, it is clear that there is an association with the vaccine. However, we still do not know what causes this reaction,” Marco Cavaleri, chair of the vaccine evaluation team at the EMA, told Italian daily Il Messaggero when asked about the possible relation between the AstraZeneca shot and cases of brain blood clots.

Cavaleri added that the EMA would say there is a link although the regulator would not likely be in a position this week to give an indication regarding the age of individuals to whom the AstraZeneca shot should be given. He did not provide evidence to support his comments.

AstraZeneca was not immediately available for comment. It has said previously its studies have found no higher risk of clots because of the vaccine.

The regulator has consistently said the benefits outweigh the risks as it investigates 44 reports of an extremely rare brain clotting ailment known as cerebral venous sinus thrombosis (CVST) out of 9.2 million people in the European Economic Area who have received the AstraZeneca vaccine.

The World Health Organisation has also backed the vaccine.

The EMA said last week that its review had at present not identified any specific risk factors, such as age, gender or a previous medical history of clotting disorders, for these very rare events. A causal link with the vaccine is not proven, but is possible and further analysis is continuing.

A high proportion among the reported cases affected young and middle-aged women but that did not lead EMA to conclude this cohort was at risk from AstraZeneca’s shot. Some countries, including France, Germany and the Netherlands, are suspending the use of the vaccine in younger people while the investigations continue.