LONDON — AstraZeneca on Monday became the third pharmaceutical company to announce positive results from late-stage trials of a coronavirus vaccine, saying that its candidate, developed by Oxford University, was "highly effective."

Scientists and politicians alike hailed the third straight week of buoyant scientific news as a sign that, even as coronavirus cases surge to devastating levels in many countries, an end to the pandemic is in sight.

The AstraZeneca candidate was 70 percent effective on average, with up to 90 percent efficacy in a smaller group that got a lower dosage. Pfizer and its German partner BioNTech and Moderna have each reported vaccines that were 95 percent effective in clinical trials. Different trial designs make direct comparison complicated, but even with somewhat lower protection, the AstraZeneca vaccine may be a more realistic option for much of the world, as it is likely to be less expensive and does not need to be stored at subzero temperatures.

Peter Piot, director of the London School of Hygiene & Tropical Medicine, who was instrumental in the battle against AIDS, said the positive results from three vaccine candidates cannot be overestimated.

He said that “2020 will be remembered for the many lives lost from covid-19, lockdowns and the U.S. election. Science should now be added to this list.” He added that “the only way to stop covid-19 in its tracks is having multiple effective and safe vaccines that can be deployed all around the world and in vast quantities.”

“I’m totally delighted,” said Hildegund C.J. Ertl, a vaccine expert at the Wistar Institute in Philadelphia. “What it tells me is this virus can be beaten quite easily: 90 to 95 percent efficacy is something we’d dream about for influenza virus, and we’d never get it.”

In years when the flu vaccine is a good match for the strain in circulation, it ranges between 40 and 60 percent effective.

The Oxford-AstraZeneca team said in a video conference with journalists that its candidate offered 90 percent protection against the coronavirus when a subject received half a dose, followed with a full dose one month later. Efficacy was lower — 62 percent — when subjects received two full doses a month apart.

Andrew Pollard, chief investigator of the Oxford trial, said the findings showed that the vaccine would save many lives.

“Excitingly, we’ve found that one of our dosing regimens may be around 90 percent effective, and if this dosing regimen is used, more people could be vaccinated with planned vaccine supply,” he said.

Britain has preordered 100 million doses — which at a dose and a half per person would cover most of its population. The United States has ordered 300 million, though both countries have hedged their bets and signed multiple vaccine deals.

The AstraZeneca results have yet to be peer-reviewed or published, and outside scientists said that much depended on seeing those in detail. Many questions remain, including why the lower-dose regimen was more effective. It is also unclear whether the vaccine can reduce transmission of the coronavirus by people who have no symptoms, which would have repercussions for how soon people could stop wearing masks. No one yet knows how long immunity from the vaccine will last.

Sarah Gilbert, a lead Oxford researcher, cautioned that the dose-and-a-half regimen would have to be studied more closely to be fully understood. But she said that the first half-dose might be priming a person’s immune system just enough and that the booster would then encourage the body to produce a robust defense against infection.

AstraZeneca and Oxford have been conducting Phase 3 clinical trials worldwide, with the most recent data coming from an interim analysis based on 131 coronavirus infections in Britain and Brazil among nearly 23,000 volunteers, with half receiving the vaccine and half getting a placebo.

The company said it will present the results to Britain’s health-care products regulators immediately and will discuss its latest data with U.S. regulators as soon as possible. It is still only midway through its U.S. clinical trial, though, and now wants to adjust the format to further assess the half-dose shot plus booster.

But Michael Kinch, director of the Center for Research Innovation in Business at Washington University in St. Louis, said there needs to be more transparency about the findings. He said the reliance on foreign data, the lack of clarity about which dose should be considered by regulators, and the fact the trial found the stronger 90 percent efficacy in a small subset of people — fewer than 3,000 received the dose-and-a-half regimen — raised concern.

“Under normal circumstances, I would say this would not be looked at favorably,” Kinch said.

World markets have rallied on optimistic vaccine news, although AstraZeneca shares were down Monday on the London Stock Exchange.

Because the vaccine is in production, if approved, the first 4 million doses could be ready in December, and 40 million could be delivered in the first quarter of 2021, company executives said. By the spring, the company and its global partners in Brazil, India, Russia and the United States could be producing 100 million to 200 million doses a month.

At a news conference Monday evening, British Prime Minister Boris Johnson said, “With a favorable wind we should be able to inoculate the vast majority of the people who need the most protection by Easter.”

Johnson — who had the virus in the spring and spent several days in an intensive care unit — told Parliament on Monday that vaccines are “edging ever closer to liberating us from the virus, demonstrating emphatically that this is not a pandemic without end.”

No participants who received the vaccine developed severe cases of the coronavirus or required hospitalization, AstraZeneca said Monday. The drugmaker also said that no “serious safety events” were reported in connection with the vaccine, which was typically “well tolerated” by participants regardless of their dosing levels or ages.

The vaccine uses a harmless cold virus that typically infects chimpanzees to deliver to the body’s cells the genetic code of the spike protein that dots the outside of the coronavirus. That teaches the body’s immune system to block the real virus.

Although the reason the regimen with an initial half-dose worked better remains unclear, Ertl said that it could be related to the fact that the body’s immune system can develop a defense mechanism to block the harmless virus that’s used to deliver the spike protein’s code. Giving a smaller initial dose may lessen those defenses, and make the vaccine more effective.

Several other vaccines in late-stage development use a similar technology, harnessing a harmless virus to deliver a payload that will teach the immune system how to fight off the real thing — including a Johnson & Johnson vaccine, a Russian vaccine being developed by the Gamaleya Research Institute and a vaccine made by CanSino Biologics in China.

The Pfizer and Moderna vaccines use messenger RNA, a new vaccine platform never approved in a medical product for people.

Although the AstraZeneca results indicate somewhat lower efficacy, the vaccine can be stored and transported at normal refrigerated temperatures for up to six months. That could make it significantly easier to distribute around the world than Pfizer’s vaccine, which must be stored at minus-70 degrees Celsius, or Moderna’s, which is stable in refrigerated conditions for only 30 days and must be frozen at minus-20 degrees Celsius after that.

Gilbert and her team began developing the AstraZeneca vaccine in a small laboratory at Oxford on a shoestring budget. The university kicked in $1.3 million and then chose AstraZeneca as a manufacturing partner.

“We wanted to ensure there wouldn’t be any profiteering off the pandemic,” said Louise Richardson, the university’s vice chancellor, so that their vaccine would be widely distributed “and wouldn’t just be for the wealthy and the first world.”

The scientists said that although it appeared to be a race, or a competition, among the front-running vaccine developers, no one company could produce the millions of doses needed to end the pandemic.

“We don’t have enough supply for the whole planet,” Pollard said, adding that the important message is that there are at least three highly effective, safe vaccines that also appear to work well among the elderly, and that they are produced using different technologies, ensuring the quickest route to manufacture the billions of doses that will be needed.

Pollard said that it is “unclear why” the different vaccines were producing different results, and that he and the scientific community are awaiting full data from all the clinical trials. He said different studies were also using different end points to describe efficacy.

“At this moment, we can’t fully explain the differences,” Pollard said. “It’s critical to understand what everyone is measuring.”

Johnson reported from Boulder, Colo. Antonia Noori Farzan in Washington contributed to this report.