The European Medicines Agency said it investigated 25 cases, including nine deaths, that involved particularly rare blood clots: one type of brain clot and another condition that clogs multiple veins in the body. The agency noted that was out of the 20 million people in Europe and Britain who have received the vaccine developed by AstraZeneca and the University of Oxford.
"A causal link with the vaccine is not proven, but is possible and deserves further analysis," the agency concluded.
The EMA said a warning would be added to the product to highlight the clotting risk and that it would conduct outreach about concerning symptoms to watch for after vaccinations, especially easy bruising or bleeding and severe headaches.
“If it was me, I would be vaccinated tomorrow,” said Emer Cooke, the head of the agency. “But I would want to know if anything happened to me after vaccination, what I should do about it, and that’s what we’re saying today.”
Most of the countries that had suspended AstraZeneca shots — including Germany, France, Italy and Spain — were quick to say they would resume on Friday or next week. Though some countries, including Sweden and Norway, said they would need time to make their own scientific evaluations.
A team in Norway said earlier in the day that it had concluded that the hospitalizations of three medical workers there, including one who died, were probably caused by an immune response to the vaccine.
The decisions to pause use of the AstraZeneca vaccine had divided both the scientific community and Europe. Some experts have argued that the extremely unusual and deadly nature of blood clots detected, even if in tiny numbers, warrant caution. Others said hasty decisions will have an irreparable effect on confidence in the vaccine.
How the assurance — with caveats — from the European regulator will affect uptake remains to be seen. The director of one vaccine center in Germany said the task of rebuilding trust would be "herculean."
As part of that task, leaders of France and Britain said they would be getting their AstraZeneca shots on Friday.
French Prime Minister Jean Castex said at a news conference that he wanted to "show that we can have complete confidence."
British Prime Minister Boris Johnson, who has maintained wholehearted backing of the homegrown vaccine, said at a Thursday briefing from 10 Downing Street: "So the Oxford jab is safe and the Pfizer jab is safe. The thing that isn't safe is catching covid, which is why it's so important that we all get our jabs as soon as our turn comes."
Johnson said that the British regulator has scrutinized the data from Europe and pronounced the vaccine safe.
June Raine, the head of the British Medicines and Healthcare Products Regulatory Agency, confirmed Thursday it had received a “very small number” of reports of “an extremely rare form of blood clot” in the brain, along with lower levels of platelets following vaccination, but that such events could occur among those who haven’t been vaccinated or those who have the coronavirus.
“While we continue to investigate these cases, as a precautionary measure we would advise anyone with a headache that lasts for more than four days after vaccination, or bruising beyond the site of vaccination after a few days, to seek medical attention,” she said.
The Oxford-AstraZeneca vaccine is one of four approved in Europe, and a key part of the inoculation campaign on a continent that is staring down a third wave of the virus, with new, more contagious variants taking hold and intensive care beds filling up again.
The AstraZeneca offering is also a major tool in the fight against the coronavirus in developing countries, because it’s easier to store and handle than some other products on the market, and developers have said it will be distributed on a not-for-profit basis.
Andrew Pollard, chief investigator on the Oxford vaccine team, said he welcomed the endorsement by regulators. “Real-world data shows the clear impact of the vaccine on public health,” he said. He added it was important that public health bodies continue to monitor the impact of the vaccine, as “safety is paramount.”
Experts from the EMA said the vaccine “may be associated with” highly rare blood clots, which include 18 incidents of a rare brain clot known as cerebral venous sinus thrombosis.
There have also been seven cases of an unusual condition in which blood clots form in multiple vessels throughout the body. While such symptoms may be linked to the coronavirus itself, “we still feel that we see sufficient information to include a warning,” said Sabine Straus, head of the EMA’s safety committee.
The EMA panel said it took into account information available on Wednesday, as some countries continued to report additional cases on Thursday.
Straus said “younger women” seemed to be notably affected, but it was too early to issue specific guidance, and the agency is looking into whether there could be increased risk for those taking a contraceptive pill. Women on the pill are already at a higher risk of the rare clots in the brain.
AstraZeneca is yet to be approved in the United States but is expected to submit an application to the Food and Drug Administration when it concludes its U.S. trials, which involve more than 30,000 people.
Eric Topol, founder and director of the Scripps Research Translational Institute, predicted the problems reported in Europe would not have any impact on U.S. regulators, unless they show up in the U.S.-based trial.
Concerns in Europe were first triggered in Austria in early March, when a 46-year-old nurse died of blood clots. Another medical worker at the same hospital suffered a pulmonary embolism.
Germany, which initially said it would go ahead with its AstraZeneca vaccinations, reversed its position this week after recording eight cases of a rare type of brain clot among 1.6 million doses administered. Health authorities said they would normally expect one case in a group that size.
Health Minister Jens Spahn announced Thursday that Germany would resume AstraZeneca vaccinations, but he also stood by the earlier decision to pause, saying that the EMA was not out of step with German regulators regarding concerns about “rare but serious cases.” He said doctors should be informed of the risk of cerebral vein thrombosis for women under 55, so they can inform those people of the risks before vaccinations.
Experts at Oslo University Hospital said Thursday that they think there is a link to the vaccine and three hospitalizations at their facility. Regulators reported six cases, including two deaths, among 130,000 vaccine recipients.
“Our findings support the early theory that the patients had a strong immune response, which led to antibody creation which can ignite blood platelets and cause a thrombosis,” said Pal Andre Holme, the lead physician looking into the cases. He said he could not be certain that the vaccine was the cause, but “I see no other possibility as of today.”
Other cases of the unusual clots have been reported in nations including Italy, Denmark and Spain.
Amid the pauses, there has been fierce pushback from parts of the scientific community, particularly in Britain.
Gino Martini, chief scientist at the Royal Pharmaceutical Society, said he hoped any concerns about potential links between the AstraZeneca vaccine and blood clots “can now be discarded, given the positive verdicts.”
Ian Douglas, a professor of pharmacoepidemiology at the London School of Hygiene and Tropical Medicine, said that even if the vaccine was causing some concerning side effects, how serious and numerous those side effects are must be balanced against the vaccine’s benefits.
“We do this all the time with all medicines,” he said. “You have to weigh the risks and benefits.”
Experts have speculated that Britain may not be picking up as many similar cases because it began with vaccinations of older people, generally at lower risk of such complications and a population in which deaths might be less notable.
“I think the most likely explanation is different populations being vaccinated,” Peter Arlett, head of data at the EMA, said on Thursday.
Hagen Feucht, the vaccine center director in the German city of Ulm, said he was relieved that AstraZeneca shots would continue, otherwise there would be “real problems” with scarcity. He said he missed 550 vaccination appointments because of the pause, and expects more hesitancy.
“I believe that there are a lot of people out there who are still relatively resolute and will continue to ask for AstraZeneca,” he said. “But there are certainly more skeptics now than before.”
Booth reported from London. Quentin Ariès in Brussels and Martin Selsoe Sorensen in Copenhagen contributed to this report.
One version of this story misstated the number of inoculations in Germany. The article has been corrected.