But while the impact for Europe may be cushioned, it could be a different story elsewhere. The Johnson & Johnson vaccine has been an important component of vaccination plans for countries around the world.

While it has not yet been rolled out at anything near the scale of AstraZeneca’s, some regions have pivoted to the shot amid AstraZeneca shortages. The African Union recently acquired 400 million doses.

The pause on Johnson & Johnson vaccinations in the United States, along with new restrictions on the use of AstraZeneca’s shot in Europe, rattled vaccination campaigns around the world relying on those vaccines. South Africa followed the United States in pausing Johnson & Johnson shots, though its health regulator in recent days recommended resuming its use.

U.S. health officials called for a pause in the vaccine’s use on April 13. Johnson & Johnson suspended its E.U. rollout immediately afterward, just as the first shipments of the shot were arriving in the region.

U.S. regulators and scientists are still studying the original reports of the clotting disorder and sifting through any new safety reports of possible cases of the clotting disorder. That effort has so far turned up little.

Dr. Rochelle P. Walensky, the C.D.C. director, said on Monday that health officials were investigating “a handful” of new, unconfirmed reports that emerged after the pause was recommended, to determine whether they might be cases of the rare blood clotting disorder.

“Right now, we are encouraged that it hasn’t been an overwhelming number of cases, but we are looking and seeing what has come in,” she said at a White House news conference.

Carl Zimmer contributed reporting from New Haven; Noah Weiland and Sharon LaFraniere from Washington; and Benjamin Mueller from London.

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E.U. Regulator Says J.&J. Shot Should Carry Rare Clot Risk Label

BRUSSELS—The European Union’s drug regulator on Tuesday said a warning should be added to the Johnson & Johnson Covid-19 vaccine indicating a possible link to rare and unusual blood clots, but stopped short of recommending it be pulled from use, saying its benefits outweigh its risks.

“The reported combination of blood clots and low blood platelets is very rare, and the overall benefits of Covid-19 Vaccine Janssen in preventing Covid-19 outweigh the risks of side effects,” the European Medicines Agency said in a statement, referring to the division of Johnson & Johnson that develops vaccines, Janssen. The rare clots were “very similar,” the agency added, to those associated with the AstraZeneca vaccine, for which the agency made a similar recommendation.

Johnson & Johnson decided to delay its rollout in the bloc’s 27 member states last week, after regulators in the United States called for a pause on the vaccine following concerns about the rare but serious side effect.

The EMA’s recommendation is not binding, but it is the first indication of what might happen next with the European rollout of the much-anticipated, single-shot vaccine that’s already been given to nearly eight million people in the United States. The agency said that regulators in individual E.U. member states should decide how to proceed taking into account their particular case load and vaccine availability.

damage had been done. Many Europeans have been refusing to take the vaccine, and several E.U. countries have limited its use to older people.

Pfizer-BioNTech vaccine, and was negotiating a new deal for future booster shots with the company for 2022 and 2023, signaling it was going to prioritize vaccines, like Pfizer’s and Moderna’s, that use the mRNA technology.

But the Johnson & Johnson vaccine has been an important component of vaccination plans from the United States to South Africa.

U.S. health officials called for a pause in the vaccine’s use on April 13 to examine a rare blood-clotting disorder that emerged in a small number of recipients. Johnson & Johnson suspended its E.U. rollout immediately afterward. E.U. countries had just began receiving their first shipments of the vaccine, and all but Poland followed company guidance and have not begun administering it.

On Monday, federal health officials said they were investigating “a handful” of new, unconfirmed reports that have emerged since the nationwide pause of the Johnson & Johnson injections. Dr. Anthony S. Fauci, the United States’ leading infectious disease expert, said previously that he anticipated a decision about whether to resume administering the Johnson & Johnson vaccine this Friday, when an expert panel that is advising the Centers for Disease Control and Prevention is scheduled to meet.

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