sitting on some 30 million doses of the AstraZeneca vaccine, which has yet to be approved by the Food and Drug Administration.

Even Britain, which moved quickly to procure vaccines and has been rapidly administering doses to most people over the age of 50, has been forced to shift its strategy in part to deal with dips in supply.

Starting late last year, British regulators chose to allow an increased gap between the two doses of vaccine required for maximum protection: up to 12 weeks, rather than the three weeks used in clinical trials.

That has allowed Britain to give initial protection to about 25 million people. But many of those will soon need a second dose, putting pressure on the system and leading officials to warn that distribution in April will move slower.

restore faith in the AstraZeneca vaccine.

Concerns about it arose from reports of a small number of people who developed dangerous blood clots or abnormal bleeding after receiving the shot.

Different reports about potential side effects were conflated and a growing number of countries halted use of the shot.

The most concerning came from Norway and Germany, involving cases where blockage develops in veins that drain blood from the brain in patients who also had lowered platelets.

After several cases came to the attention of Paul Ehrlich Institute, the agency in charge of vaccine safety in Germany, the use of the vaccine was halted. A cascade of other European countries quickly followed suit. .

Klaus Cichutek, the head of the institute, said that it had acted after seven cases of cerebral venous thrombosis had occurred four to 16 days after vaccination.

An analysis by the institute suggested that only a single case would normally be expected among the 1.6 million people who received the vaccine in that time window.

The institute said that it had convened a group of experts on Monday and that they had agreed unanimously that a link to the vaccine was not implausible and should be investigated.

Ms. Cooke said that it was important to closely monitor all side effects.

“We are also launching additional investigations to understand more about these rare cases,” she said. But the regulatory agency found that it the benefits of the vaccine — at a moment when thousands are still dying around the world every day — outweighed the risk.

“If it was me,” Ms. Cooke said. “I would be vaccinated tomorrow.”

Reporting was contributed byNick Cumming-Bruce from Geneva and Gaia Pianigiani from Rome.

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