LONDON (AP) — The European Union's drug regulator will announce the conclusions of its investigation into the possible connection between AstraZeneca's coronavirus vaccine and rare blood clots later Wednesday, including recommendations that could have far-reaching effects on the use of the shot that is key to global efforts to end the pandemic.

Earlier this week, a senior official from the European Medicines Agency said there was a causal link between the AstraZeneca vaccine and the rare clots that have been seen in dozens of people worldwide, among the tens of millions who have received at least one dose of the shot.

Marco Cavaleri, head of health threats and vaccine strategy at the Amsterdam-based agency, said in comments to Rome’s Il Messaggero newspaper on Tuesday that “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.”

But Cavaleri acknowledged the agency hadn't yet figured out how exactly the vaccine might be causing these rare side effects. The agency said its evaluation “has not yet reached a conclusion and the review is currently ongoing.”

The EMA is particularly focused on two types of rare blood clots: one that appears in multiple blood vessels and another that occurs in a vein that drains blood from the brain. It is also evaluating reports of people who had low levels of blood platelets, which puts them at risk of severe bleeding.

The EMA, the World Health Organization and numerous other health authorities have said repeatedly that the AstraZeneca vaccine is safe and effective and that the protection it offers against COVID-19 outweighs the small risks of rare blood clots.

As recently as last week, the EMA said “there is no evidence that would support restricting the use of this vaccine in any population” — a response to several countries doing just that — though an expert said more brain clots were being reported than would be expected. To date, most of the cases have been reported in younger women, who are more susceptible to developing such rare clots anyway, making understanding what is causing the clots potentially more difficult.

“The problem is these clots are very unusual, and we don't really know what the background rate of them is, so it's very hard to know if the vaccine is contributing to this,” said Dr. Peter English, who formerly chaired the British Medical Association's Public Health Medicine Committee.

A full investigation would likely take months, but English said given the urgency of the continuing pandemic, regulators would likely make a quick decision.

“It's very likely we will see a suspension of the vaccine's use in certain groups while they do the further investigations to give us clearer answers," English said.

In March, more than a dozen countries, mostly in Europe, suspended their use of AstraZeneca over the blood clot issue. Most restarted — some with age restrictions — after the EMA said countries should continue using the potentially life-saving vaccine.

The suspensions were seen as particularly damaging for AstraZeneca because they came after repeated missteps in how the company reported data on the vaccine's effectiveness and concerns over how well its shot worked in older people. That has led to frequently changing advice in some countries on who can take the vaccine, raising worries that AstraZeneca’s credibility could be permanently damaged, spurring more vaccine hesitancy and prolonging the pandemic.

English said the back-and-forth over the AstraZeneca vaccine globally could have serious consequences.

“We can’t afford not to use this vaccine if we are going to end the pandemic,” he said.

That’s because the vaccine is cheaper and easier to store than many others, is critical to Europe’s immunization campaign and a pillar of the U.N.-backed program known as COVAX that aims to get vaccines to some of the world’s poorest countries. It has been endorsed for use in more than 50 countries, including by the 27-nation EU and WHO. U.S. authorities are still evaluating the vaccine.

The governor of Italy’s northern Veneto region said any decision to change the guidance on AstraZeneca would cause major disruptions to immunizations — at a time when Europe is already struggling to ramp them up — and could create more confusion about the shot.

“If they do like Germany, and allow Astra Zeneca only to people over 65, that would be absurd. Before it was only for people under 55. Put yourself in the place of citizens, it is hard to understand anything,” Luca Zaia told reporters on Wednesday.

The latest suspension of AstraZeneca came in Spain's Castilla y León region, where health chief Verónica Casado said Wednesday that “the principle of prudence” drove her to put a temporary hold on the vaccine that she still backed as being both effective and necessary.

“If there are in fact individuals of a certain age group that could have a higher risk (of clotting) then we need to adjust its use,” Casado told Spanish public radio. “We are not questioning AstraZeneca. We need all the vaccines possible to reach the goal of 70% of the adult population.”

French health authorities said they, too, were awaiting EMA’s conclusions and would follow the agency’s recommendations, especially for the 500,000 people who have received a first dose of AstraZeneca.

Even officials in Asia said they were keen to hear the EMA's decision. On Wednesday, South Korea said it would temporarily suspend the use of AstraZeneca’s vaccine in people 60 and younger. In that age group, the country is only currently vaccinating health workers and people in long-term care settings.

The Korea Disease Control and Prevention Agency said it would also pause a vaccine rollout to school nurses and teachers that was to begin on Thursday, while awaiting the outcome of the EMA’s review.

English, the former chair of the British drug regulator, said that even rare, serious side effects are seen with established vaccines and that policymakers often decide that bigger public health goals warrants their use, citing the polio vaccine as an example. For every million doses that are given of the oral polio vaccine, about one child is paralyzed from the live virus contained in the vaccine.

On Tuesday, AstraZeneca and Oxford University, which developed the vaccine, paused a study of the shot in children while the U.K. regulator evaluates the link between the shot and rare blood clots in adults.

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Associated Press writers Joseph Wilson in Barcelona, Spain, Colleen Barry in Milan, Kim Tong-hyung in Seoul and Angela Charlton in Paris contributed to this report.

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