It was a lucky mistake. Researchers in Britain had been meaning to give volunteers the initial dose at full strength, but they made a miscalculation and accidentally gave it at half strength, Reuters reported. After discovering the error, the researchers gave each affected participant the full strength booster shot as planned about a month later.

Fewer than 2,800 volunteers got the half-strength initial dose, out of the more than 23,000 participants whose results were reported on Monday. That’s a pretty small number of participants on which to base the spectacular efficacy results — far fewer than in Pfizer’s and Moderna’s trials.

For years, Oxford researchers have been testing their chimpanzee adenovirus vaccine, ChAdOx1, on a number of other diseases including Ebola and Zika. Although none of those studies have reached the final, so-called Phase 3 trials, they have allowed researchers to examine the safety of the vaccine platform. The researchers have not found any serious side effects.

When the researchers adapted ChAdOx1 for Covid-19, their early clinical trials also did not turn up any adverse reactions. In Phase 3 trials, however, the testing had to be paused twice when volunteers experienced neurological problems. The Food and Drug Administration did not directly tie the vaccine to the problems, but when the agency allowed the trial to resume in the United States, it advised the company to be vigilant for any signs of similar problems.

In their announcement on Monday, AstraZeneca and Oxford said that no serious safety issues were confirmed related to the vaccine.

AstraZeneca’s vaccine has a number of advantages over other leading vaccine candidates: It’s easier to mass produce and store, and it’s also cheaper, at $3 to $4 per dose. That reflects the prices paid by governments like the United States that have placed orders for tens or even hundreds of millions of doses of the vaccine. U.S. health officials have promised that Covid-19 vaccines will be available free of charge to any American who wants one.

There’s still a long way to go.

It is not yet clear whether the results announced on Monday are enough for AstraZeneca to take the first formal step of the regulatory process: submitting an application to the F.D.A. to get emergency authorization to distribute its vaccine. AstraZeneca plans to start testing the half-strength initial dose in its continuing United States trial and to ask the agency for guidance on how to proceed. The agency is likely to advise the company to collect more data on its promising dosing plan before submitting a formal application to authorization, several vaccine experts said.

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