As a third wave of the pandemic crashes over Europe, questions about the safety of one of the continent’s most commonly available vaccines led Germany, France, Italy and Spain to temporarily halt its use on Monday.

The suspensions created further chaos in inoculation rollouts even as new coronavirus variants continue to spread.

The decisions followed reports that a handful of people who had received the vaccine, made by AstraZeneca, had developed fatal brain hemorrhages and blood clots.

The company has strongly defended its vaccine, saying that there is “no evidence” of increased risk of blood clots or hemorrhages among the more than 17 million people who have received the shot in the European Union and the United Kingdom.

“The safety of all is our first priority,” AstraZeneca said in a statement Monday. “We are working with national health authorities and European officials and look forward to their assessment later this week.”

The timing of the pause in inoculations by some of Europe’s largest countries — which followed a flurry of similar actions by Denmark, Norway and several others — could not have been worse.

Europe’s vaccine rollouts already lag far behind those in Britain and the United States, and there is dawning realization that much of the continent is suffering a third wave of infections. Leading immunologists fretted on Monday that the decision by several of Europe’s leading nations to suspend the use of AstraZeneca would make vaccination efforts even harder by emboldening vaccine skeptics in countries where they are particularly entrenched.

The European Medicines Agency and the World Health Organization warned against an exodus from vaccines that would undermine rollout efforts at a pivotal moment.

“We do not want people to panic,” the W.H.O.’s chief scientist, Soumya Swaminathan, said at a news conference, adding that no link had been found between the clotting disorders reported in some countries and Covid-19 shots. A W.H.O. advisory committee plans to meet on Tuesday to discuss the vaccine.

The European Medicines Agency, or E.M.A., said Monday that it would continue to investigate a possible connection between the AstraZeneca shots and blood clots or bleeding in the brain. But the agency said the numbers of such problems reported in vaccinated people did not seem higher than those usually seen in the general population. Germany, for instance, reported seven cases of a “rare cerebral vein thrombosis” out of 1.6 million people who received the vaccine there.

“While its investigation is ongoing, E.M.A. currently remains of the view that the benefits of the AstraZeneca vaccine in preventing Covid-19, with its associated risk of hospitalization and death, outweigh the risks of side effects,” the agency said.

The European Union bet heavily on AstraZeneca, a British-Swedish company, last year.

In France, where AstraZeneca is being relied on to accelerate the country’s vaccination campaign, and where top officials had urged people to trust the vaccine only days ago, President Emmanuel Macron called the suspension a “precaution” and expressed “hope of quickly picking them up again.”

In Italy, the police on Monday began seizing nearly 400,000 doses of AstraZeneca vaccine on the orders of local prosecutors investigating the death of a teacher who had received the vaccine.

The Italian Medicines Agency said in a statement that the suspension of the vaccine, among the most commonly distributed in the country, was “precautionary and temporary.” Its director, Nicola Magrini, said on television Monday night that “there is no reason to instill doubts at this moment and to lead people to prefer one vaccine over another.”

“We are confident that after the investigation by the E.M.A. we can pick it up,” said Cesare Buquicchio, a spokesman for Italy’s health minister.

In Germany, which had previously supported the vaccine despite other countries’ concerns, the health minister, Jens Spahn, called the decision to pause shots “a purely precautionary measure.” More than 1.6 million doses of AstraZeneca have been administered in Germany, which has relied heavily on the BioNTech-Pfizer vaccine.

But the country’s Paul Ehrlich Institute said the country decided to suspend AstraZeneca shots because cases of a “rare cerebral vein thrombosis” had been reported in the country following vaccinations.

Mr. Spahn acknowledged the seven cases of thrombosis made it very rare, but he defended the decision to pause shots as necessary to ensure trust in the vaccine moving ahead.

“For nearly everybody there is no risk, but a connection cannot be fully ruled out,” Mr. Spahn said. “That is why we decided to make this decision.”

Spain followed suit Monday night. At news conference, Carolina Darias, Spain’s health minister, said she had been in touch with European counterparts before ordering a two-week suspension of the vaccine. That should give time for the relevant medical agencies “to offer responses” about the cases of thrombosis recently detected, she said.

Throughout Europe, officials and immunologists worried that the actions would cost vital time in the race against fast-spreading variants.

“This is a catastrophe,” said Heike Werner, the minister for health in the eastern German state of Thuringia, who was already grappling with learning that her region would receive just 9,600 of 31,200 doses of AstraZeneca because of a reported shortage of supplies. “Many people are desperately waiting for this vaccine.”

Roberto Burioni, a leading Italian virologist, voiced his worries on Twitter that people would now avoid the vaccine.

“I understand if you will decide not to get vaccinated, scared by inexplicable decisions,” he said. “I understand and I am sorry because you will expose yourself to a serious risk to avoid a negligible one.”

Dr. Michael Head, senior research fellow in global health, University of Southampton said “the decisions by France, Germany, and other countries look baffling.” He said that the delay in inoculations, and “the potential for increased vaccine hesitancy,” belied any new or conclusive data.

Britain authorized the AstraZeneca vaccine in late December, and it had administered 9.7 million doses as of late February.

Its medicines regulator has not reported any concerns about blood clotting for that vaccine or the Pfizer shot, saying in its latest safety report that “the number and nature of suspected adverse reactions reported so far are not unusual in comparison to other types of routinely used vaccines.”

Among the millions of people who have received the AstraZeneca shot in Britain, 14 reported cases of deep vein thrombosis and 13 reported cases of a pulmonary embolism, conditions that can both be caused by blood clots. Only one of those people died. There were 35 reported cases of thrombocytopenias, a condition involving a low blood platelet count. That also led to one death.

“We are closely reviewing reports but the evidence available does not suggest the vaccine is the cause,” Dr. Phil Bryan with a British regulatory agency said in a statement.

The World Health Organization signed off on the safety of the vaccine developed by researchers at the University of Oxford in partnership with the pharmaceutical company AstraZeneca.

The European Medicines Agency, the European Union’s regulatory authority, also approved its use, after monitoring some five million vaccinations already administered across the continent. Its guidance as of Monday remained the same.

Norwegian authorities held a news conference Monday to explain their earlier decision to suspend using the vaccine.

They said a 50-year-old patient who had died was in good health before she received the vaccine, but then suffered a fatal “intracerebral hemorrhage”

Another health care worker who died on Friday was described as being in her 30s, and dying of the same cause 10 days after receiving a shot.

The doubt, merited or not, around the AstraZeneca vaccines comes as more countries embrace or contemplate broad new restrictions — in some places for a third or fourth time in a year.

Short of widespread inoculations, and with the more easily transmissible and potentially more lethal British variant dominating infections, Italy extended tough new restrictions on movement nationwide on Monday, deepening a year’s worth of economic and psychic damage.

The quiet on the streets of Rome and elsewhere was eerily reminiscent of a year ago, when Italy became the first European country to shut down, underscoring to some how frustratingly little progress had been made in combating the pandemic.

“The second, the third wave, I have lost count,” said Barbara Lasco, 43, as she sat in a park in Milan, near the epicenter of the original European outbreak in northern Italy. “I am puzzled and disappointed; one year was enough time to keep this from happening again.”

Advances against the virus, nearly everywhere, have been maddeningly halting. In Germany, even as many nonessential stores opened last week for the first time in months, health officials called for caution.

“We are seeing clear signs: In Germany the third wave has already begun,” Lothar H. Wieler, the president of the Robert Koch Institute, Germany’s equivalent of the C.D.C., said on Friday. Since then, the daily numbers of infections have increased.

The virus is also spreading, and hospitals are again buckling, across Central Europe.

Prime Minister Viktor Orban of Hungary predicted that this week would be the most difficult since the start of the pandemic in terms of allocating hospital beds and ventilators, as well as mobilizing nurses and doctors.

In Germany, where infections are being driven by the British variant, a more lasting suspension of AstraZeneca could delay vaccination of the population by a month, according to the Central Institute for Registered Doctors.

France is hoping to stave off a new surge of infections with local restrictions, but some health officials think the time for a third national lockdown has come because intensive care units are swamped. “New decisions” will be taken in the coming days to tackle the rise in infections in France, Mr. Macron said on Monday.

Greek authorities last week reported the country’s highest daily rate of infections since mid-November, driven by the British variant; the rise prompted a reversal of plans to reopen schools and shops later this month.

Italy’s prime minister, Mario Draghi, warned on Friday that the country was facing a “new wave of contagion,” driven by more infectious variants of the coronavirus.

He has put an army general in charge of the vaccine rollout and hoped to increase inoculations from 100,000 a day to 500,000.

But that was before the AstraZeneca fears spread more widely.

On Monday, Iacopo Benini, a 32-year-old professor, had his AstraZeneca vaccination appointment canceled 20 minutes before he arrived for his shot in Milan. “Who is going to accept getting AstraZeneca now?” he said.