Even without delays, however, the vaccine is still a long way from being widely available. Regulators must assess the study data and decide whether to authorize the vaccine. AstraZeneca must ramp up production and work with government officials to roll out doses. And in the first weeks and months after the vaccine is authorized, it is expected to be available only to the highest-priority groups, likely health workers first, followed by other vulnerable groups.
Ms. Cheng said the regimen involving a half-dose would not significantly complicate the supply chain.
The F.D.A. has said that it expects a Covid-19 vaccine to prevent disease or decrease its severity in at least half of people who are vaccinated. That is similar to the effectiveness of seasonal flu vaccines most years. More interim findings from other leading vaccine makers, including Johnson & Johnson, are expected soon.
AstraZeneca’s vaccine is expected to come with relatively simple storage requirements, which would be an asset once it gets rolled out. Moderna’s vaccine can be kept for up to a month at the temperature of an ordinary refrigerator. Pfizer’s can be kept for up to five days in conventional refrigerators, or in special coolers for up to 15 days, but otherwise needs ultracold storage.
“Our goal is to make sure we can have a vaccine that was accessible everywhere,” Professor Pollard said. “I think we’ve managed to do that.”
The data released on Monday came from AstraZeneca’s Phase 2/3 clinical trial in Britain and its Phase 3 clinical trial in Brazil. The participants were randomly assigned to receive either the coronavirus vaccine or a meningitis vaccine as a control, followed by a booster shot about a month later.
As of early November, AstraZeneca had a total of 23,000 participants in mid- to late-stage trials in Britain, Brazil, South Africa and the United States. Results from its study in the United States are not expected to be available until next year. That trial, which aims to enroll 30,000 participants, was slowed by the safety pause that lasted over a month.