Out of 34 million people who received the vaccine in Britain, the European Union and three other countries, 222 experienced blood clots that were linked with a low level of platelets. The majority of these cases occurred within the first 14 days following vaccination, mostly in women under 60 years of age.

On April 7, the European Medicines Agency, the main regulatory agency, concluded that the disorder was a very rare side effect of the vaccine. Researchers in Germany and Norway published studies on April 9 suggesting that in very rare cases, the AstraZeneca vaccine caused people to make antibodies that activated their own platelets.

Nevertheless, the regulators argued, the benefit of the vaccine — keeping people from being infected with Covid-19 or those few who get it out of the hospital — vastly outweighed that small risk. Countries in Europe and elsewhere continued to give the vaccine to older people, who face a high risk of severe disease and death from Covid-19, while restricting it in younger people.

Both AstraZeneca and Johnson & Johnson use the same platform for their vaccine, a virus known as an adenovirus. On Tuesday, the Australian government announced it would not purchase Johnson & Johnson vaccines. They cited the fact that Johnson & Johnson, like AstraZeneca, used an adenovirus as the basis of its vaccine. But there is no obvious reason adenovirus-based vaccines in particular would cause rare blood clots associated with low platelet levels.

AstraZeneca has not yet applied for an emergency use authorization in the United States.

In addition to Johnson & Johnson’s vaccine, the F.D.A. has authorized the vaccines developed by Moderna and Pfizer-BioNTech. Those two vaccines use a different technology to produce immunity.

The first sign of concern about Johnson & Johnson’s vaccine came on April 9, when the European Medicines Agency announced that they were investigating reports of four cases of blood clots in people who received the Johnson & Johnson vaccine in the United States. One case occurred in the clinical trial that took place before the vaccine was authorized. Three occurred in the vaccine rollout. One of them was fatal, the agency said.

The regulators described these reports as a “safety signal” — a cluster of cases requiring further investigation. But they said it wasn’t clear if the vaccines caused the clots.

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