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.
However, the EMA later said in a statement that its review of the vaccine was ongoing and it expected to announce its findings on Wednesday or Thursday. An AstraZeneca spokesman declined to comment on the matter.
"In my opinion, we can now say it, it is clear that there is an association (of the brain blood clots) with the vaccine. However, we still do not know what causes this reaction," Marco Cavaleri, chair of the vaccine evaulation team at the EMA, told Italian daily Il Messagero.
Cavaleri provided no evidence to support his comment.
The EMA has repeatedly said the benefits of the AstraZeneca shot 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 vaccine.
The World Health Organization has also backed the vaccine.
AstraZeneca has said previously its studies have found no higher risk of clots because of its vaccine.
Cavaleri said the EMA would say in its review that there is a link but was not likely to give an indication this week regarding the age of individuals to whom the AstraZeneca shot should be given.
Some countries, including France, Germany and the Netherlands, have suspending the use of the vaccine in younger people while the investigations continue.
In response to Cavaleri's comments, the Amsterdam-based EMA said in a statement on Tuesday: "EMA's Pharmacovigilance Risk Assessment Committee (PRAC) has not yet reached a conclusion and the review (of any possible link) is currently ongoing."
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, the agency said.
A high proportion among the reported cases affected young and middle-aged women but that did not lead EMA to conclude this cohort was particularly at risk from AstraZeneca's shot.
Scientists are exploring several possibilities that might explain the extremely rare brain blood clots that occurred in individuals in the days and weeks after receiving the AstraZeneca vaccine.
European investigators have put forward one theory that the vaccine triggers an unusual antibody in some rare cases; others are trying to understand whether the cases are linked with birth control pills.
But many scientists say there is no definitive evidence and it is not clear whether or why AstraZeneca's vaccine would cause an issue not shared by other vaccines that target a similar part of the coronavirus.
In a separate interview, Armando Genazzani, a member of the EMA's Committee for Medicinal Products for Human Use (CHMP), told La Stampa daily that it was "plausible" that the blood clots were correlated with the AstraZeneca vaccine.
The AstraZeneca vaccine is based on a modified chimpanzee adenovirus vector, ChAdOx1, developed at Oxford University, and is one of several adenovirus-vector COVID-19 vaccines. The current vaccine rollout represents the first use of viral vector vaccines on such a global scale.