The European Union's drug regulatory agency said Thursday that the AstraZeneca vaccine doesn't increase the overall incidence of blood clots and that the benefits of using it outweigh the possible risks, paving the way for European countries to resume dispensing the shots.
Several European nations, including Germany, France, Italy and Spain, had suspended their use of the vaccine over the past week following reports of clots in a few dozen of the millions of people across the continent who have gotten the shot. The question was whether the vaccine had anything to do with those clots.
Italian Premier Mario Draghi welcomed the finding by a committee of the European Medicines Agency and immediately announced Italy would resume administering AztraZeneca's vaccine as early as Friday.
Germany, France and others had said they would wait for the EMA's determination before deciding whether to start using the vaccine again.
"The committee has come to a clear scientific conclusion," said the head of the EMA, Emer Cooke. "This is a safe and effective vaccine."
She added: "If it were me, I would be vaccinated tomorrow."
However, she said the agency "still cannot rule out definitively a link" between certain 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.
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. The EMA focused on very rare types of clots associated with low levels of platelets, which can lead to severe bleeding.
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 immunize 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.
Sabine Straus, the head of EMA's safety committee, said experts found "no evidence of a quality or a batch issue" linked to any particular manufacturing site. She said the experts will continue to examine reports of rare blood clots and ensure that health care professionals and the public are kept informed.
Two rare types of blood clots -- one type appears in multiple blood vessels, the other in vessels that drain blood from the brain -- have been reported in at least 25 people, and at least nine deaths among them have been recorded, out of the 20 million who have received at least one dose of the AstraZeneca vaccine in Britain and the EU.
Most cases were in people under 55, and the majority were women, the EMA said.
But the EMA said the overall number of clotting events of various sorts reported after the rollout of the vaccination campaigns was lower than what would be expected in the general population.
The World Health Organization has said that there is no evidence to suggest the vaccine was to blame for the clots and that the benefits of immunization far outweigh the potentially small risk. The drugmaker likewise said it found no increased risk of clots.
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 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.
Like the EMA, however, British regulators followed its counterparts on the continent in urging people to seek medical attention if they suffer headaches that last for more than four days after vaccination or bruising beyond the injection site after a few days.
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.
"Rarely has an immunization program been more important to the well-being of a population," said Pauline Paterson, co-director of the Vaccine Confidence Project team at the London School of Hygiene & Tropical Medicine.
In yet another setback, however, 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.
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.
"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."