public confusion about a vaccine that appears to be highly effective.

In early September, the company quietly halted its global trials after a participant in Britain fell ill. But American regulators did not find out until the story broke publicly. Subsequently, the company’s slowness to provide the F.D.A. with evidence that its vaccine was not linked to any illnesses kept it grounded for nearly seven weeks. AstraZeneca has said it shared data in a timely manner.

By late November, the company was again riding high: It released results from early clinical trials, including in Britain, showing that the vaccine was up to 90 percent effective.

But those results, too, were quickly clouded by uncertainty. AstraZeneca later acknowledged that there had initially been confusion over the vaccine dosage received by some study participants, making it more difficult to interpret the findings.

Britain, which has long championed the homegrown vaccine, authorized the shot in late December, relying on the earlier clinical trial results. The European Union’s medicine regulator did the same, but a month later.

E.U. officials said that the delay had partly resulted from a back-and-forth between the regulators and AstraZeneca over the quality of the data.

And even after the vaccine was authorized, a number of European countries initially restricted it to younger people, citing a lack of sufficient data about its efficacy in older people. That problem was supposed to be resolved by the American trial, in which older people were better represented.

Neither European nor British regulators gave any indication on Tuesday that the problems with AstraZeneca’s American data would have any impact on rollouts there. Those agencies relied on a separate set of data from non-American trials to authorize the vaccine.

“We are in contact with the company regarding this further information,” the European Medicines Agency said in a statement on Tuesday, “and E.M.A. will assess the data concerned as soon as the company submits it to us.”

Matina Stevis-Gridneff contributed reporting from Brussels.

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