Still, experts say the vaccine seems likely to be approved, despite some confusion in the results and lower levels of protection than what other vaccine candidates have shown.
“What we can see looks reasonable, but it’s a bit more complicated than what we’ve seen so far,” said Dr. Buddy Creech, a Vanderbilt University researcher helping to test two other vaccines. “If this had been the first report out, the field would have still been excited to have a vaccine.”
The medical journal Lancet on Tuesday published partial results from tests of the vaccine in the United Kingdom, Brazil and South Africa – safety results on 23,745 participants and protection levels on 11,636.
They’re hard to interpret because a mistake led some participants to get a half dose followed by a full one rather than two full doses as intended.
Researchers claim the vaccine protected against disease in 62% of those given two full doses and in 90% of those initially given the half dose. However, independent experts have said the second group was too small – 2,741 people – to judge the possible value of that approach and that more testing is needed.
The half-dose group also didn’t include anyone over 55, and among others in the study, only about 20% were in that age group.
It’s unclear if the results will be enough to lead regulators in the United Kingdom and elsewhere to approve its use right away.
“We have no safety concerns about the vaccine,” with no hospitalizations or severe disease among those who received it, and results from all study locations consistently showing benefit, said one study leader, Oxford’s Andrew Pollard.
“The only way we get the pandemic behind us is if we get doses of vaccine out there,” he said.
AstraZeneca’s Mene Pangalos called the results “very compelling” and said they “clearly show we have an effective vaccine” that meets criteria for approval around the world. “I really believe this vaccine will have a big impact on the pandemic,” he said.
Associated Press writers Maria Cheng and Danica Kirka in London contributed to this report.