AstraZeneca and Oxford Say Vaccine Is Up to 90 Percent Effective

AstraZeneca and the University of Oxford announced on Monday that their inexpensive, easy-to-produce coronavirus vaccine appears effective, the latest in a string of encouraging results this month from leading vaccine developers.

An early analysis of data from late-stage clinical trials found that AstraZeneca’s vaccine was either 62 percent or 90 percent effective, depending on the manner in which the doses were given.

While the overall efficacy of the vaccine remains unclear, the encouraging preliminary results indicate that it has the potential to become a powerful new weapon in the war on the pandemic, which has killed more than 1.3 million people worldwide since January and is still spreading rapidly.

AstraZeneca said it expected to begin distributing the vaccine this year and that it would be able to make up to three billion doses next year. At two doses per person, that would be enough to inoculate nearly one in five people worldwide.

Unlike some other leading vaccine candidates, the AstraZeneca offering can be quickly produced in vast quantities, will cost only a few dollars per dose and is easy to store for long periods. That could substantially expand the number of countries and people that have access to the vaccine.

AstraZeneca’s announcement came on the heels of Pfizer and Moderna’s releasing trial results that showed their vaccines to both be about 95 percent effective. Those vaccines, however, can only be kept outside of ultracold freezers for a few weeks, making their distribution and storage more challenging.

The price of AstraZeneca’s shot, at $3 to $4, is a fraction of the cost of some other vaccines. AstraZeneca has pledged to make it available at cost around the world until at least July 2021 and in poorer countries in perpetuity.

The varying effectiveness of AstraZeneca’s vaccine reflected differences in how doses were administered in the late-stage trials. In the dosing plan that was 90 percent effective, study participants were given a half-dose of the vaccine and then, a month later, a full dose. The vaccine was less effective when people were given a standard full dose upfront, followed a month later by another full dose. On average, the vaccine was 70 percent effective.

The analysis, which looked at data from participants in Britain and Brazil, did not turn up any serious safety issues that were confirmed to be related to the vaccine. It had come under global scrutiny after AstraZeneca temporarily paused its trials in September after a participant in Britain developed a neurological illness.

Oxford and AstraZeneca said they would submit the trial data to regulators in Britain, the European Union and Brazil and would seek emergency authorization to start distributing the vaccine in those places. British and European Union regulators have been conducting so-called rolling reviews of the vaccine, which could hasten the authorization process.

The path toward the vaccine being available in the United States is less clear. Because clinical trials in the United States were paused more than a month, longer than in Britain and several other countries, the U.S. results are not expected until next year. And AstraZeneca has not been testing the more-effective half-dose regimen in its U.S. trials. The company said it would work with the Food and Drug Administration to add it as quickly as possible to its ongoing trial.

In the meantime, AstraZeneca said, it will share its latest British and Brazilian trial data with the F.D.A. this week. The company is seeking guidance on whether it should formally submit the findings for review and authorization for emergency use even though the American trial remains underway.

Some experts said the F.D.A. was unlikely to authorize the vaccine without more data, especially because fewer than 2,800 participants in the clinical trials received the smaller first dose that yielded such strong results. That is a much smaller sample size than in Pfizer and Moderna’s trials.

“I don’t think the F.D.A. had envisioned 3,000 participants to base a decision,” said Dr. Eric Topol, a clinical trial expert at Scripps Research in San Diego.

Pfizer and Moderna’s vaccines are based on similar messenger RNA technology. It uses synthetic genetic material to stimulate cells to produce a harmless viral protein that the immune system can learn to attack.

The AstraZeneca vaccine is different. It uses a weakened version of a chimpanzee adenovirus as a delivery vehicle to ferry coronavirus genes into human cells. That trains the immune system to fight future attacks from the actual coronavirus.

The company said its early analysis was based on 131 symptomatic coronavirus cases that turned up in participants at least two weeks after they had received their second shot.

None of the vaccinated people who developed the disease required hospitalization, AstraZeneca and Oxford said.

“Today marks an important milestone in our fight against the pandemic,” AstraZeneca’s chief executive, Pascal Soriot, said in a statement. “This vaccine’s efficacy and safety confirm that it will be highly effective against Covid-19 and will have an immediate impact on this public health emergency.”

In a call with reporters on Monday morning, the Oxford scientists said they were still trying to understand why the vaccine was more effective with a smaller first dose. The first dose is supposed to prime the immune system, while the second is supposed to boost its response.

Dr. Saad B. Omer, the director of the Yale Institute for Global Health, pointed to several potential explanations for why the dosing regimens yielded different results. There could be key differences between the two groups, such as participants’ ages or their past exposure to similar viruses, that influenced how they responded.

The design of the vaccine could also play a role: The full dose could be giving participants immunity to the delivery vehicle and dampening their subsequent immune response. Sample size may also be at play. Because fewer participants received the smaller first dose, their results have less statistical power.

Dr. Omer cautioned against drawing definitive conclusions until more details were available. “I would withhold my judgment until we specifically look at more data,” he said.

Pam Cheng, an executive vice president at AstraZeneca, told reporters on Monday that if the company received regulatory authorization, it planned to have four million doses available in Britain by the end of the year. By the end of March, the company said, it would have at least 300 million doses of finished vaccine ready to distribute globally.

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 expected 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.

The relatively simple storage requirements of AstraZeneca’s vaccine are expected to make it easier to roll out than several of the other leading vaccines. 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 freezers or in special coolers for up to 15 days, but otherwise needs ultracold storage.

Andrew Pollard, the Oxford researcher in charge of the trials, told reporters on Monday that the scientists’ goal was “to make sure we can have a vaccine that was accessible everywhere.” He added: “I think we’ve actually 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 of the coronavirus vaccine, the meningitis vaccine or a placebo about four weeks later. Professor Pollard said the Oxford scientists hoped to submit the results for publication in a peer-reviewed journal within 24 hours.

A vaccine deploying the technology behind AstraZeneca’s candidate has never won approval. But the approach has been studied before, notably in a small 2018 study of an experimental vaccine against the virus that causes Middle East respiratory syndrome, or MERS. That virus is related to SARS-CoV-2, the novel coronavirus that causes Covid-19.

When Covid-19 emerged, the team of scientists at Oxford’s Jenner Institute that had been leading the work on similar coronaviruses had a head start. Once the genetic code of SARS-CoV-2 was published in early January, the Oxford team sped to adapt their platform to the new coronavirus and begin animal testing.

They also needed a development and manufacturing partner and found one in AstraZeneca in April. Safety testing in Britain began that month.

In May, the U.S. Department of Health and Human Services pledged up to $1.2 billion to help fund AstraZeneca’s development and manufacturing of the vaccine and secure at least 300 million doses if it proved to work. Alex M. Azar II, secretary of the Department of Health and Human Services, called the deal “a major milestone” in the work of Operation Warp Speed, the U.S. government’s program to fast-track Covid-19 vaccines.

AstraZeneca has struck deals to supply doses of its vaccine, once authorized, to other wealthy nations, including Britain and other nations in Europe, as well as to lower- and middle- income countries. AstraZeneca’s vaccine is on track to supply 44 percent of all doses to lower- and middle-income countries, and 33 percent of all doses globally, according to projections from the British research firm Airfinity, which tracks deals between vaccine manufacturers and governments.

Jenny Gross contributed reporting.