Data from the 27 E.U. member states by the European Center for Disease Prevention and Control shows that over all, 80 percent of vaccine doses distributed to the bloc have already been administered. That share drops to 65 percent for AstraZeneca, however, suggesting that many of its doses are sitting unused.

Yet it is hard to predict how serious a blow the latest twist in the AstraZeneca saga — and the new Johnson & Johnson concerns — will be to E.U. vaccination efforts, as officials in Brussels have made big if belated efforts to turbocharge the second-quarter supply of doses.

The European Union is poised to receive at least 300 million doses of various vaccines, three times what it got in the first quarter. Two hundred million are slated to come from Pfizer/BioNTech. Moderna is expected to deliver 35 million doses. Another 55 million doses are due of the Johnson & Johnson jab, and 70 million from AstraZeneca.

In the rosiest scenario, the European Union could get up to 360 million doses by June.

On Thursday, after Spain’s government changed the age threshold for the AstraZeneca shot, two-thirds of people called up for vaccination in Madrid did not show up, Antonio Zapatero, the regional health minister, told a news conference on Friday.

He attributed the no-show by 18,200 people to “confusion” generated by Spain’s central government, which said on Wednesday that the AstraZeneca vaccine should be given only to people over 60. Before this change, Mr. Zapatero said, the rate of abstention was 2 percent.

In Belgium, where the use of the AstraZeneca vaccine has also been limited, the authorities said they did not expect major delays in the overall rollout, but they are still concerned about the confusion that the rare blood clotting issue is causing.

Yves Van Laethem, a top epidemiologist who is the country’s Covid task force spokesman, said he expected a two-week delay that would mostly affect younger age groups in late summer. He said the E.U. regulator guidance had only partly helped in clarifying the situation.

The European Medicines Agency’s opinion “wasn’t very clear, and it is also part of the problem,” Dr. Van Laethem said in an interview. “When you say, ‘We don’t apply limitations, but we just say there are serious side effects,’ there is part science and part diplomacy in that.”

He said the limited effect that the new AstraZeneca issues would have on Belgian’s rollout was in large part because the country had ordered big shares of other vaccines.

Although all E.U. countries have been offered a chunk of each vaccine approved in the bloc so far — AstraZeneca, Johnson & Johnson, Moderna and Pfizer — many opted to forgo parts of their share of more expensive or cumbersome vaccines like Pfizer and Moderna early on, instead favoring the AstraZeneca jab.

“In Britain or Eastern Europe, a big part of the campaigns are based on AstraZeneca,” Dr. Van Laethem said.

Wealthier bloc members like Denmark, France, Germany and the Netherlands can better compensate for the loss of confidence in AstraZeneca, because they acquired extra doses of other vaccines — especially Pfizer — through a secondary market after poorer E.U. nations gave theirs up.

Those countries — including Bulgaria, Croatia, Latvia and Slovakia — are likely to be less able to quickly offer alternatives.

Dr. Van Laethem, the Belgian immunologist, said that the national and European authorities needed to better communicate the costs and benefits of taking the AstraZeneca dose versus and the other authorized vaccines.

Experts worry that even limited concerns over one vaccine’s unlikely side effects can affect people’s overall willingness to be immunized.

“The main thing is to make people understand that the problem is the virus,” he said. “We have to vaccinate people — the risk linked to the virus is higher than those rare side effects.”

Raphael Minder contributed reporting from Madrid and Constant Méheut from Paris.

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