It remains uncertain how well the vaccine works in those older than 55, a crucial group because most serious illness, hospitalizations and deaths from covid-19 occur among the oldest patients.
Researchers also are still studying which dose regimen can produce the greatest protection.
Still, the results show a safe, well-tolerated and effective vaccine, and one that is cheaper — at $2 or $3 a dose — and easier to manufacture, transport and store than its competitors, Maria Deloria Knoll of the John Hopkins Bloomberg School of Public Health wrote in a commentary accompanying the article in the Lancet.
The researchers said they are now submitting their data to regulators for approval to deploy the vaccine in mass immunization campaigns in Britain, Brazil, India and countries in Europe, among other places.
The AstraZeneca vaccine, if approved, will compete with others, including one produced by the American pharmaceutical giant Pfizer and its German partner BioNTech that is reported to be 95 percent effective.
The Pfizer vaccine was approved for emergency use in Britain, and the first injections in a massive campaign began there Tuesday. It is now being reviewed by the Food and Drug Administration, with possible approval by week’s end. Another vaccine, made by U.S.-based Moderna, is also more than 90 percent effective.
The Pfizer vaccine requires special handling and must be kept on dry ice or in high-end freezers at sub-Antarctic temperatures of minus 70 degrees Celsius. The Moderna vaccine can be stored in a freezer for six months or a refrigerator for up to one month.
The AstraZeneca-Oxford vaccine, however, can be stored long term at ordinary refrigerator temperatures, which could ease its distribution throughout the world.
In the Lancet, the AstraZeneca and Oxford scientists reported on the interim results from two clinical trials run in Britain and Brazil involving 11,636 participants. Half got the vaccine and half received a placebo.
For the volunteers who got two full doses, the vaccine was 62 percent effective in protecting them from covid-19 symptoms.
There also was an intriguing, but still mysterious, result: A subgroup that mistakenly — or serendipitously, as the researchers put it — got a half-dose and then a full dose achieved 90 percent protection from coronavirus symptoms.
But this subgroup was too small to prove such high efficacy. The scientists said their clinical trials are ongoing and more data will be forthcoming.
The Lancet article said the pooled results of the two different dose regimens produced the 70 percent effective rate, but even that number needs unpacking.
After AstraZeneca and Oxford provided news releases and media interviews last month, offering some data, the researchers were criticized for overemphasizing the 90 percent effectiveness claim and evading questions about the lower-dose regimen, which was the result of a measurement error during manufacture.
“The data released today add a lot of flesh to the bones of the earlier press release,” said Ian Jones, a professor of virology at the University of Reading.
The vaccine is “undoubtedly safe and is also capable of preventing disease, so its general rollout would impact the epidemic. Its cost and availability are also powerful positives,” he said. “But further trial data might be needed to explain why the lower-dose group was significantly better protected than the standard-dose group.”
The AstraZeneca and Oxford researchers stressed that there were no deaths, hospitalizations or severe disease observed in the vaccinated groups from three weeks after the first dose. AstraZeneca said the vaccine should exceed regulatory thresholds.
However, the vaccine’s effectiveness in older age groups could not be assessed, because only 12 percent of the participants in the trials were over age 55 and only 4 percent were over 70, too few to give a statistically robust answer.
In addition, none of the vaccine recipients over age 55 got the low dose-regular dose regimen that produced the most impressive results of 90 percent efficacy.
The vaccine’s value to older people will be determined in future analyses after more cases have accrued in this age range, the scientists said.
Mene Pangalos, a vice president of research and development at AstraZeneca, told reporters that the company has produced results “sufficient to get approval around the world.”
He said, “I do think a vaccine at 62 percent is a viable, effective vaccine.”
Pangalos said it will be up to regulators to decide to whom to give the vaccine — everyone or just those under 55, for example — and how much to give them, the two standard doses or a low dose followed by a standard dose.
Pascal Soriot, chief executive of AstraZeneca, said that the company would seek early approval from regulatory authorities around the world and that its global supply chains “are already up and running, ready to quickly begin delivering hundreds of millions of doses on a global scale at no profit.”
Soriot acknowledged that more studies were needed to understand how well the vaccine works among older people. He said ongoing clinical trials in the United States have enrolled more seniors.
“The older group is very important, but we also need to vaccinate younger groups, too,” Soriot said.
“It’s good they’ve published it. They’re being transparent,” said Peter Jay Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine.
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.
But the complexity of the AstraZeneca trial — which used different dosages, different intervals between the first and second shot, and different control groups — may mean that more data needs to be gathered to satisfy regulators.
The regulatory situation in the United States for AstraZeneca is not clear because an ongoing, 30,000-person trial is not expected to report results until late January or early February.
Moncef Slaoui, chief science adviser for the U.S. government’s vaccine development effort, Operation Warp Speed, said in an interview Monday that he expected that the vaccine might begin to be available in the United States at the end of February based on those projections.
The Oxford vaccine could be a powerful tool for the world and the United States, where there are already concerns about securing sufficient supplies. But even if doses are scarce, scientists are debating whether there might be confusion or hesitation among the public if one vaccine is less effective than others.
After Pfizer supplies the United States with the first 100 million doses, it is unclear whether the country will have access to substantially more doses until June or July.
“We’re going to need it for the U.S.,” Hotez said. “I don’t think we’re going to be able to vaccinate a significant percentage of the U.S. population with the [Pfizer and Moderna] vaccines.”
Johnson reported from Boulder, Colo.