Several European countries had suspended the use of the vaccine over the past week following reports of rare types of blood clots occurring in a small number of the millions of people who had received the shot across the continent.

Germany, France and others had said they wanted to wait for the European Medicines Agency's determination before deciding whether to resume using the vaccine.

"Our scientific position is that this vaccine is a safe and effective option to protect citizens against COVID-19," said the head of the EMA, Emer Cooke.

She added: "If it were me, I would be vaccinated tomorrow."

However, she said the agency "cannot rule out definitively a link" between the rare types of blood clots and the vaccine. The EMA recommended adding a description of these cases to the vaccine leaflets so health workers and patients would be aware.

Also read: EU regulator 'convinced' AstraZeneca benefit outweighs risk

Clots that form in the arms, legs or elsewhere can break free and travel to the heart, brain or lungs, causing strokes, heart attacks or other deadly blockages.

While many countries have continued to use the AstraZeneca vaccine, there are concerns the debate could seriously undermine confidence in the shot, which is key to efforts to vaccinate the world's population, especially in poorer countries.

In Europe, where there is a choice of vaccines, the safety questions have complicated an already slow rollout across the 27-nation EU at a critical moment when infections are rising at an alarming rate again in many countries, in part because of mutant versions of the coronavirus.

Blood clots have been reported in at least 37 people, and at least four deaths among them have been recorded, out of the 17 million who have received at least one dose of the AstraZeneca vaccine in Europe.

The World Health Organization has said that there is no evidence to suggest the vaccine was responsible and that the benefits of immunization far outweigh the potentially small risk. The drugmaker likewise said it found no increased risk of clots.

Also read: Major European nations suspend use of AstraZeneca vaccine

AstraZeneca is expected to apply in the coming weeks for U.S. authorization for its vaccine. The U.S. now relies on vaccines from Pfizer, Moderna and Johnson & Johnson.

On Thursday, ahead of the EMA's announcement, Britain's drug regulatory agency said there was no evidence that AstraZeneca's vaccine causes clots, and it urged people to continue getting the shot. Britain was not among the countries that suspended use of the vaccine.

The pause in vaccinations with AstraZeneca's formula comes as tens of thousands of new daily cases have prompted new lockdown measures in Italy, caused hospitalizations in France to spike and led German officials to announce a third surge of COVID-19 has begun.

In yet another setback, British officials said Thursday that the country is facing a shortfall in vaccine deliveries that may delay the start of shots for people under 50 in the coming weeks.

Figures from the European Centers for Disease Prevention and Control this week show there are about 7 million unused doses of the AstraZeneca across the EU.

Also read: Why countries are halting the AstraZeneca shot

The German government defended its decision to suspend use of the vaccine, saying it was based on expert advice. Government spokeswoman Ulrike Demmer said Wednesday that the move could "strengthen trust" in the vaccines.

"Concerns are taken seriously and examined. And as soon as these concerns are cleared up, a vaccine can be used again without hesitation," she said.

Germany will rely on the EMA decision for how to proceed, Health Ministry spokesman Hanno Kautz said. Other countries including France have also indicated they will follow the advice issued Thursday.

Clinical trials are typically done in tens of thousands of participants. As a result, extremely rare side effects often aren't detected until vaccines are used in many millions of people, long after they have reached the market.

For example, it took nearly a year after vaccination campaigns began against the 2009 swine flu pandemic for European officials to notice an increase in narcolepsy in children and teenagers who got the GlaxoSmithKline vaccine.

Also read: No reason to stop using AstraZeneca jab, says WHO

"It's right to investigate any potential signals of problems, but you can do that while you continue immunization," said Michael Head, a senior research fellow in global health at the University of Southampton. "If we pause the vaccine rollout every time there's a possible signal, it won't be much of a rollout."

Head cautioned that there are costs to going slowly: The longer the coronavirus is allowed to circulate widely, the more chance it has to mutate into a deadlier version.

"People may well be more hesitant to take an AstraZeneca vaccine when immunization resumes," he said. "And this is at a time when we need to stop the virus circulating to the reduce the chances of further variants emerging."