How effective is the AstraZeneca vaccine really?

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% 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.

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 some 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 FDA 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 U.S. trial remains underway.

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 Oxford-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.

While the overall data included more than 23,000 participants, fewer than 2,800 received the smaller first dose — a much smaller sample size than in Pfizer and Moderna’s trials. Nearly 9,000 participants got the larger first dose.

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. Fewer participants received the smaller first dose, giving their results less statistical power.

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 approval, it planned to have 4 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.

Cheng said the regimen involving a half-dose would not significantly complicate the supply chain.

The FDA 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 refrigerators or in special coolers for up to 15 days, but otherwise needs ultracold storage.

Andrew Pollard, the Oxford researcher in charge of the British trial, 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. Pollard said the Oxford scientists hoped to submit the results for publication in a peer-reviewed journal within 24 hours.

AstraZeneca’s vaccine is designed to genetically alter an adenovirus found in chimps so that it harmlessly mimics the coronavirus and provokes an immune response. A vaccine deploying that technology 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 Azar, 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% of all doses to lower- and middle-income countries, and 33% of all doses globally, according to projections from the British research firm Airfinity, which tracks deals between vaccine manufacturers and governments.