LONDON — Oxford University and AstraZeneca reported Monday that their coronavirus "vaccine for the world" was safe and 79 percent effective overall, according to data from a long-awaited clinical trial in the United States, Chile and Peru. The two-shot regimen was completely effective at preventing severe cases of illness.

The results, announced in a news release, set the stage for U.S. regulators to decide whether to authorize the easily transported $4 shot after the company submits an application in coming weeks. The two-shot regimen is already being used in many countries, but authorization by the Food and Drug Administration would open up the U.S. market and send a strong signal to the globe about the quality of the inoculation.

The AstraZeneca results are sure to prompt debate within the Biden administration about how the shot fits into the U.S. vaccine strategy, if it is authorized. The U.S. government placed an order for 300 million doses at a cost of $1.2 billion last year.

Based on the timeline for other vaccines, emergency clearance appears unlikely before May, when there will be increased supply of three already-authorized vaccines from Moderna, Pfizer-BioNTech and Johnson & Johnson.

Having a fourth would provide the United States insurance against potential manufacturing or supply mishaps involving the three other vaccines, as well as additional options for deploying shots domestically and sending some to other countries that need them. Last week, the Biden administration announced that it would loan 4 million doses of the vaccine to Mexico and Canada.

“I think it’s too early for us to declare that we are in a surplus position . . . but obviously the more we get confidence in our increased supply, the more flexibility that gives us down the road,” whether that involves vaccinating adolescents or sharing the vaccine with other countries, said Andy Slavitt, senior White House adviser on the coronavirus, during a White House news briefing Monday.

It is too soon to know how the AstraZeneca shot would fit into the U.S. vaccine portfolio, said Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases.

“The one thing that one can say for sure is that this is good for the world, because it’s a cheap vaccine. It’s got good results,” Fauci said. “They will likely be able to produce enough for a lot of different countries.”

Ruud Dobber, president of AstraZeneca’s biopharmaceuticals business unit, said the day the vaccine is authorized, the company will be ready to deliver 30 million doses to the United States, with a total of 50 million ready during that first month. While the U.S. government will determine how to use the 300 million doses already ordered, Dobber said, he would be “highly surprised” if they are not used to vaccinate Americans.

The Oxford researchers said that an independent monitoring board in the United States “reported no safety concerns among the participants receiving at least one dose of the vaccine.”

The Data and Safety Monitoring Board combed through data to look for blood clotting events similar to those that caused the vaccination effort to be suspended last week in many European countries. While the inoculations were restarted after a pause, it undermined confidence in the vaccine in Europe. The independent board found no suggestion that the vaccine carried an increased risk of clotting.

The trials included 32,449 adult participants in all age groups, most of them in the United States. The volunteers received two standard doses of the Oxford-AstraZeneca vaccine, or a placebo, at a four-week interval.

The scientists said the data shows that the vaccine is 79 percent effective against symptomatic covid-19, the disease caused by the coronavirus, and 100 percent effective against severe illness. There were five cases of severe illness in the trial, all in the group that received a placebo.

The vaccine proved just as effective in people older than 65 as it was in younger recipients. The virus has been especially lethal for older adults, and earlier trials of the vaccine were criticized for not recruiting enough older people.

“The data look good. The numbers don’t lie,” said Fauci, who was briefed on the data Sunday.

The results were reported in a pair of brief news releases from the pharmaceutical company and the university scientists, and have not yet been peer-reviewed or published.

The drug company said: “Vaccine efficacy was consistent across ethnicity and age. Notably, in participants aged 65 years and over, vaccine efficacy was 80%.”

The FDA has already given emergency authorization to vaccines from U.S. firms Pfizer, which partnered with German biotech firm BioNTech; Moderna; and Johnson & Johnson.

The Pfizer-BioNTech and Moderna vaccines, which require two doses, have shown more than 90 percent efficacy against symptomatic covid-19. The Pfizer-Bio­NTech vaccine requires special handling and must be kept at especially cold temperatures in special freezers long term.

The one-shot Johnson & Johnson efficacy rate is 66 percent overall and 72 percent in the United States in preventing moderate to severe cases of covid-19, according to data presented to the FDA.

Experts have warned against comparing trials conducted under different circumstances. Naor Bar-Zeev, a vaccine expert at the Johns Hopkins Bloomberg School of Public Health, said when considering protection against severe disease and death, all the vaccines look very similar. Variation in their protection against milder cases is hard to interpret, because the most efficacious vaccines were tested at a time when variants had not yet emerged. It remains an open question exactly how well any of the vaccines stop asymptomatic infections and transmission.

The Oxford-AstraZeneca shot has advantages: It can be stored in an ordinary refrigerator and is relatively cheap. The company has vowed to sell the vaccine at cost during the pandemic.

If the Oxford-AstraZeneca shot is authorized by the FDA for emergency use, it would offer the United States a bounty of vaccines — either to administer to people here or provide to countries in need. Most nations have not administered a single dose of vaccine.

Use of the Oxford-AstraZeneca vaccine was paused across Europe last week after reports of a handful of worrying blood clots. The European Medicines Agency, which regulates drugs in the European Union, declared the vaccine safe and effective and said it was not linked to a rise in the overall risk of blood clots. But the EMA did not rule out a possible link to rare cases of clotting in the brain, known as cerebral venous sinus thrombosis (CVST).

There were no cases of CVST identified in the trial after an investigation by an external safety board that consulted with an independent neurologist.

The European Medicines Agency on March 18 said the benefits of the vaccine far outweighed the risks, while adding it can't rule out a link to rare blood clots. (Joshua Carroll/The Washington Post)

The vaccine looked more effective in the U.S. trial than in Oxford-AstraZeneca trials in Britain last year, which showed 62 percent efficacy.

Andrew Pollard, a leader of the Oxford team, told The Washington Post that higher efficacy for the vaccine in the U.S. trials probably reflects a difference in testing protocols and identifying volunteers with mild or more moderate covid, meaning that the British clinical trials picked up more mild cases, thereby lowering the efficacy readout. The U.S. trials also gave the AstraZeneca vaccine at a four-week interval, but Pollard said there’s enough data now to support spacing the doses a month to three months apart.

Bar-Zeev said the differing results could stem from differences in the populations tested or when and where the trials were conducted — for example, the U.S. trials included far more older adults.

Ann Falsey, a professor of medicine at the University of Rochester School of Medicine, who was a leader of a clinical trial in the United States, said in the company’s news release: “It’s exciting to see similar efficacy results in people over 65 for the first time. This analysis validates the AstraZeneca COVID-19 vaccine as a much-needed additional vaccination option, offering confidence that adults of all ages can benefit from protection against the virus.”

Pollard declined to criticize the Europeans for pausing the rollout of his vaccine when they noted clotting cases. But he stressed that regulators concluded that the benefits of the shot outweighed any risk. Some 3,000 Europeans are dying of covid every day, Pollard said, “and the virus is still out there.”

In its first set of clinical trials in Brazil, Britain and South Africa, AstraZeneca was criticized for failing to include enough elderly volunteers to show that the vaccine works well in those at greatest risk of serious illness. In the U.S. trials, about 20 percent of participants were 65 years and older, and about 60 percent had health conditions that can make covid more life-threatening, such as diabetes, severe obesity or cardiac disease.

AstraZeneca said 79 percent of volunteers were White, 8 percent Black, 4 percent Native American and 4 percent Asian; in addition, 22 percent of the participants were Hispanic.

The U.S. trial of the Oxford-AstraZeneca vaccine took longer than many forecast. Rare neurological symptoms occurred in three people, two of whom received doses of the study vaccine, causing trials to be suspended worldwide to explore whether those might have been caused by the vaccine. There was no evidence of a connection, but the pause went on for weeks in the United States, setting the trial back.

Johnson reported from Boulder, Colo. Laurie McGinley in Washington contributed to this report.