The announcement this week that a cheap, easy-to-make coronavirus vaccine appeared to be up to 90 percent effective was greeted with jubilation. But since unveiling the preliminary results, AstraZeneca has acknowledged a key mistake in the vaccine dosage received by some study participants, adding to questions about whether the vaccine's apparently spectacular efficacy will hold up under additional testing. Scientists and industry experts said the error and a series of other irregularities and omissions in the way AstraZeneca initially disclosed the data have eroded their confidence in the reliability of the results. The regimen that appeared to be 90 percent effective was based on participants receiving a half dose of the vaccine followed a month later by a full dose; the less effective version involved a pair of full doses. AstraZeneca disclosed in its initial announcement that fewer than 2,800 participants received the smaller dosing regimen, compared with nearly 8,900 participants who received two full doses. Moncef Slaoui, the head of Operation Warp Speed, the U.S. initiative to fast-track coronavirus vaccines, noted another limitation in AstraZeneca's data. On a call with reporters, he suggested that the participants who received the half-strength initial dose had been 55 years old or younger. The fact that the initial half-strength dose wasn't tested in older participants, who are especially vulnerable to Covid-19, could undermine AstraZeneca's case to regulators that the vaccine should be authorized for emergency use.