ROME (AP) — A top official at the European Medicines Agency says there is a causal link between AstraZeneca’s coronavirus vaccine and rare blood clots, but that it’s unclear what the connection is and the benefits of taking the vaccine still outweigh the risks of getting COVID-19.

Marco Cavaleri, head of health threats and vaccine strategy at the Amsterdam-based agency, told Rome’s Il Messaggero newspaper on Tuesday that the EU medicines regulator is preparing to make a more definitive statement on the topic this week.

Based on the evidence to date, Cavaleri said there’s a clear association between the AstraZeneca vaccine and the dozens of rare blood clots that have been reported worldwide amid the tens of millions of vaccine jabs that have been given out.

“It is becoming more and more difficult to affirm that there isn’t a cause-and-effect relationship between AstraZeneca vaccines and the very rare cases of blood clots associated with a low level of platelets,” Cavaleri was quoted as saying.

Asked about Cavaleri’s comments, the EMA press office said its evaluation “has not yet reached a conclusion and the review is currently ongoing.” It said it planned a press conference as soon as the review is finalized, possibly Wednesday or Thursday.

AstraZeneca did not immediately respond to a request for comment.

Last month, more than a dozen countries, including Germany, suspended their use of AstraZeneca over the blood clot issue. Most EU nations restarted on March 19 — some with age restrictions — after the EMA said the benefits of the vaccine outweighed the risks of not inoculating people against COVID-19. At the time, the EMA recommended the vaccine’s leaflet be updated to tell people about the rare clots.

Any further doubts about the AstraZeneca vaccine would be a setback for the shot, which is critical to Europe’s immunization campaign and a linchpin in the global strategy to get vaccines to poorer countries. The AstraZeneca vaccine is cheaper and easier to use than rival vaccines from Pfizer and Moderna and has been approved for use in over 50 countries and groups, including by the 27-nation EU and the World Health Organization. U.S. authorities are still in the process of evaluating the vaccine.

Cavaleri said while EMA was prepared to declare a link, further study was still needed to understand why and how the phenomenon occurs.

“Certainly the information about the product will be updated, affirming that these adverse events are linked to the vaccine. It will be declared in a very clear way,” he was quoted as saying.

He said the rare blood clots, including some in the brain, coupled with a low level of blood platelets that may make people at risk of serious bleeding, “seem to be the key event to study further.” Cavaleri promised more details soon, adding: “In the coming hours, we will say that the link is there, how this happens we still haven’t figured out.”

Cavaleri said the biological mechanism for how the vaccine might be causing the rare clots was still unknown and if it was linked to how the shot is made, other vaccines with similar technologies might also need to be evaluated.

Cavaleri was asked how he could arrive at such a causal conclusion given the relatively few cases of adverse events.

“Among those vaccinated there is a number of cerebral thromboses with a low level of platelets among young people that is higher than what we would have expected. This we have to say,” he was quoted as saying.

But he stressed the risk-benefit analysis remained positive for the AstraZeneca jab, even for young women who appear to be more affected by the clots.

“Let’s not forget that young women also end up in intensive care with COVID. So we need to do very meticulous work to understand if the risk-benefit analysis remains for all ages,” he was quoted as saying.

He said the EMA was in a difficult situation, given the different virus outbreaks in each of the EU’s 27 nations.

“Certainly, many people would like EMA to resolve the question for everyone, but it’s not that easy,” he said. “In Italy there are still around 500 people dying a day, in Norway nearly no one. These factors justify a different approach.”

He ruled out a preventive therapy to address the rare blood clots, saying there is still too much unknown about the phenomenon.

Even after the March 19 restart, the Dutch and German governments suspended the jabs for people under 60 and some individual Europeans have been shying away from the shot.

The president of Romania’s national vaccination committee, Valeriu Gheorghita, said Tuesday that since March, 207,000 people in Romania had canceled their AstraZeneca vaccine appointments and another 92,000 simply didn’t show up.

“It is a high percentage, a third of people scheduled who did not show up,” Gheorghita told reporters.

British Prime Minister Boris Johnson declined to be drawn directly into the latest warnings about the vaccine, which was developed at Oxford University, but urged people to look at the advice from Britain’s independent Medicines and Healthcare Regulatory Agency.

“Their advice to people is to keep going out there, get your jab, get your second jab,” he said during a visit Tuesday to an AstraZeneca facility in Macclesfield, in northwest England.

Last week, the MHRA said seven people had died in the U.K. due to blood clots after getting the AstraZeneca jab. It said it wasn’t clear if the shots are causing the clot and that it was undertaking a “rigorous review” into the reports. The agency said it had identified 30 blood clot cases out of 18.1 million AstraZeneca doses given by March 24.

Johnson said the rapid rollout of vaccines in the U.K. has helped turn around the situation this year and given Britain the opportunity to roll back lockdown restrictions just as many other countries in Europe are putting them back up amid another surge of the virus.