BRUSSELS — Johnson & Johnson said Tuesday that it would resume the rollout of its coronavirus vaccine in Europe after the European Union’s drug regulator said that a warning should be added to the product indicating a possible link to rare blood clots, but that the shot’s benefits outweigh the risks.

The company decided to delay distribution in the bloc’s 27 member states last week, after regulators in the United States suspended use of the vaccine there amid concerns about the potential side effect.

The E.U. drug regulator’s endorsement — even with the caveat — not only clears a path for Johnson & Johnson in Europe, but could presage how the United States will handle the vaccine in the days to come.

On Friday, an advisory panel to the Centers for Disease Control and Prevention is to meet for a second to time to decide whether to recommend lifting a “pause” put on the vaccine’s use in the United States, perhaps with a similar warning.

That would free up millions of doses for a country still waging a fierce battle against the pandemic, and where almost eight million Americans have already had the Johnson & Johnson vaccine.

But some health experts worry that the headline-grabbing pause, which began over a week ago, might discourage some people from getting vaccinated, even though the risks from Covid-19 are far greater than the risk from a clot.

“You’ve put a scarlet letter on the Johnson & Johnson vaccine,” said Dr. Paul A. Offit, a vaccine expert at Children’s Hospital of Philadelphia.

After clotting concerns associated with another vaccine, produced by AstraZeneca, were reported in Europe, Dr. Offit noted, some grew leery of it, overestimating the threat. For the Johnson & Johnson vaccine, the clot risk has been put at an estimated one in a million.

“If you take a theoretical million people who are infected with Covid, five thousand will die,” Dr. Offit said. “Therefore, the benefits of this vaccine clearly outweigh its risks.”

A loss of confidence in the Johnson & Johnson vaccine may carry a particular cost.

The vaccine has been greeted warmly by many health workers because it requires just a single shot, unlike the ones from Moderna or Pfizer-BioNTech, and is easier to store than some other vaccines. It is especially useful for hard-to-reach or transient populations.

“There is no doubt in my mind that there are groups for whom this vaccine is of benefit,” Dr. Offit said, “meaning that they’re more likely to get this vaccine than the other vaccines, whether it’s because of where they live, or because they’re homebound, or it’s hard to get a second dose.”

If that changes, he said, “then you have probably elevated a rare risk above a much more common risk, and you’ve done harm.”

In clearing the way for the vaccine’s use, the E.U.’s drug regulator, the European Medicines Agency, said the blood clots, which have been reported in a very small group of people, are “very similar” to those associated with the AstraZeneca vaccine.

The E.M.A.’s recommendation is not binding, but it is the first indication of what might happen next with the European rollout of the Johnson & Johnson vaccine.

The agency said that authorities in individual E.U. member states should decide how to proceed, taking into account their particular case load and vaccine availability. Poland is the only E.U. country that defied the company’s guidance and went ahead and administered some Johnson & Johnson doses over the past week.

All the rare cases of blood clots reviewed by the European regulator were reported in the United States, but the agency proceeded with its recommendations Tuesday before U.S. regulators acted.

Europe has been here before.

In early March, regulators began receiving reports of rare blood clots in Europeans who had been vaccinated with the AstraZeneca shot, which has been widely used on the continent. E.U. and national regulators then scrambled to interpret and respond to the findings.

The E.U. regulator said it had moved faster in greenlighting the Johnson & Johnson vaccine than the U.S. authorities have because, having spent the past two months reviewing the AstraZeneca issues, it had built some confidence in how to respond.

In a statement, the agency stressed the importance of treating the potential side effect and issued guidelines to health care professionals on the lookout for the rare clotting disorder. It listed symptoms to be vigilant for, including shortness of breath, chest pain, leg swelling, persistent abdominal pain, severe and persistent headaches or blurred vision, and tiny blood spots under the skin.

The temporary suspension of the Johnson & Johnson rollout in the European Union had added to the bloc’s vaccine rollout woes, but it was not as big a blow as the AstraZeneca issues have been.

Vaccination efforts have fallen behind in Europe partly because AstraZeneca, a British-Swedish pharmaceutical company that is a major component of the region’s inoculation efforts, was unable to deliver the number of doses expected in the first quarter of the year. Then its vaccine was suspended over the blood-clotting concerns.

Even though the authorities eventually declared that the benefits of the AstraZeneca vaccine outweighed risks, and advised E.U. members to use it, the damage had been done.

Many Europeans have been refusing to take it, and several E.U. countries have limited its use to older people, as most of those affected by the rare clots were younger than 60.

Confronted with supply shortages after the AstraZeneca and the Johnson & Johnson disruptions, the European Union last week announced it was increasing its supply of the Pfizer-BioNTech vaccine, and was negotiating a new deal for future booster shots with the company for 2022 and 2023.

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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