These setbacks have not dampened enthusiasm in Britain for the country’s leading homegrown vaccine, one that analysts have said could right the course of Prime Minister Boris Johnson’s careening tenure if it is rolled out quickly.

Having ordered 100 million doses, 40 million of which are supposed to be available by March, Britain has made the AstraZeneca shot the linchpin of its vaccination strategy. Since authorizing Pfizer’s vaccine on Dec. 2, Britain has used it to vaccinate hundreds of thousands of people. But the country has struggled to administer it beyond hospitals and doctor’s offices, leaving some of its highest-priority recipients, like nursing home residents, still vulnerable.

A small number of volunteers in the clinical trial in Britain received their first dose at half strength because of a measurement problem. Oxford had hired an outside manufacturer to produce the vaccine for the trial. But when researchers received a sample of the vaccine, they found its strength to be twice as high as what the manufacturer had found using a different measurement technique. Not knowing which measurement to trust, the researchers decided to cut the dose in half to ensure that volunteers wouldn’t get double the dose originally intended. Later, the Oxford researchers confirmed that their reading was too high, and they switched back to the originally planned dose for the second shot.

In the smaller group of 2,741 people who received the half-strength first dose or a meningococcal vaccine as a control, the vaccine was found to have 90 percent efficacy. None of those participants were over the age of 55, however, making it difficult to know if those results would hold in older people.

AstraZeneca and Oxford scientists have said that they do not know why the half-strength initial dose was so much more effective. But they have voiced confidence in their results, particularly the finding that no one who received the vaccine in the clinical trials developed severe Covid-19 or was hospitalized.

“We think we have figured out the winning formula and how to get efficacy that, after two doses, is up there with everybody else,” Pascal Soriot, AstraZeneca’s chief executive, told The Times of London in an interview published on Saturday. The company has released no evidence of efficacy rates as high as Pfizer or Moderna’s. “I can’t tell you more because we will publish at some point,” Mr. Soriot told The Times.

Oxford scientists earlier this month published interim findings from the vaccine’s clinical trials in The Lancet. Forthcoming final results from those trials are not expected to be significantly different from the interim data, as is typical in clinical research.



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