It remains unproven how well the vaccine works in those over 55 years of age, a crucial group because most serious illness, hospitizations and deaths from covid-19 occur among the oldest patients.
Researchers also are still studying which dose regime can produce the greatest protection.
Still, the results show a “highly effective” vaccine, and one that is cheaper — at $2 or $3 a dose — and easier to manufacture, transport and store than its competitors, wrote Maria Deloria Knoll of John Hopkins Bloomberg School of Public Health in a commentary accompanying the article in 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, which 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 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.
In the Lancet, the AstraZeneca and Oxford scientists report on the interim results from two clinical trials run Britain and Brazil, involving 11,636 participants — half to got the vaccine and half received a placebo.
They found that 62 percent of volunteers who got two full doses of the vaccine were protected from covid-19 symptoms.
There were also an intriguing, but still mysterious, result: a subgroup who mistakenly — or serendipitously, as the researchers put — got a half-dose and then a full dose achieved 90 percent protection from covid-19.
But this subgroup was too small to prove such high efficacy. 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 regimes produced the 70 percent effective rate.
The researchers stressed that there were no deaths, hospitalizations or severe disease observed in the vaccinated groups from three weeks after first dose. AstraZeneca said the vaccine should exceed regulatory thresholds.
However, the vaccine effectiveness in older age groups could not be assessed — because only 12 percent of participants in the trials were over 55, too few to give a statistically robust answer.
The vaccine’s value to the older people will be determined in future analyses after more cases have accrued in this age range, the scientists said.
“It’s good they’ve published it. They’re being transparent,” said Peter Jay Hotez, dean for the National School of Tropical Medicine at Baylor College of Medicine.
The vaccine appears to be effective and has advantages, including the ability to be stored long-term at ordinary refrigerator temperatures, which could ease its distribution throughout the world.
But the complexity of the 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 understand 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.
It could be a powerful tool for the world and for the United States, where there are already concerns about securing sufficient supply. After Pfizer supplies the U.S. with the first 100 million doses it has ordered, it is unclear whether the country will have access to substantial more doses 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.