The coronavirus vaccine developed by the Oxford University and United Kingdom-based drugmaker AstraZeneca has 90% efficacy in a small group who got a half-dose first, but only 62% among them were given two full doses, the trial data published in The Lancet showed on Tuesday. The overall vaccine efficacy across both groups was 70.4%.
Oxford University and AstraZeneca are the first Covid-19 vaccine makers to publish final-stage clinical trial results in the scientific journal. Their vaccine is seen as important to combat Covid-19 in developing countries, as it would be cheaper and easier to distribute than the two RNA-based vaccines from drugmakers Pfizer and Moderna, which need to be stored at ultra low temperatures.
AstraZeneca official 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, AP reported. “I really believe this vaccine will have a big impact on the pandemic,” he said.
Experts, however, expressed caution, saying that while the vaccine was safe and effective, AstraZeneca and Oxford University would have to do more work to achieve 90% efficacy as some 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,” Buddy Creech, a Vanderbilt University researcher helping to test two other vaccines told AP. “If this had been the first report out, the field would have still been excited to have a vaccine.”
The data also highlighted a number of unknowns, including questions about the most effective dosing regimen and how well it works in adults above the age of 55.
Dosage and age group
In the study, researchers said the vaccine protected against Covid-19 in 62% of those given two full doses and in 90% of those initially given the half dose. The question is, how could a higher efficacy be achieved from a lower initial dose?
Experts said the second group was too small – 2,741 people – to judge the possible value of that approach, and that more testing was needed.
“The basic message: that the overall efficacy across the trials that are reported here is about 70% but with a clear description of its uncertainty,” Stephen Evans, a professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine, told Reuters. “The statistical uncertainty is that the efficacy could be as low as 55% or as high as 80%.”
In contrast, the Pfizer/BioNTech and Moderna vaccines both have efficacies above 90% and are clearly more efficacious under trial conditions, Evans said.
Asked whether the half, then full dose regimen had been a mistake, Andrew Pollard, director of the Oxford Vaccine Group and chief investigator into the trials, told a news conference it had been “unplanned”.
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Pollard also conceded that the different results from the two dosage regimens had complicated matters.
“The regulators will decide exactly what their label should say,” he said, when asked whether regulators might approve the two full shots regimen first, and then potentially the half, then full shot regimen when more data is in, according to Reuters.
Moreover, the half-dose group of the Oxford vaccine did not include anyone over 55, and among others in the study, only about 12% were in that age group. Older people also enrolled later, so there has not been enough time to see whether they develop infections at a lower rate than those not given the vaccine.
This means that while the trial results found that the vaccine was safe and effective, more analysis will be needed to see how well it works in people over 55, who are particularly susceptible to Covid-19.
Vaccine better at stopping Covid-19 than slowing spread
The trial data showed that the vaccine can stop people from getting sick, but showed a limited ability to slow the spread of the virus.
Pollard, the Oxford professor who led the study, called the data “tantalising” but said it was too early to draw firm conclusions, AFP reported. He said researchers were trying to analyse the amount of virus in the swabs to determine whether vaccination reduced viral loads, which would help limit contagion.
“What would be fantastic would be vaccines that prevent transmission so you reach the point where you not only protect the people who are vaccinated but also those who can’t be vaccinated or haven’t been vaccinated yet,” he said.
Another question was whether the vaccine was capable of fighting asymptomatic infections. The data showed that the low-dose vaccine regimen was about 60% effective in reducing asymptomatic infections, but it was unclear whether the standard dose significantly reduced such infections at all.
Overall, authors analysed data from phase three clinical trials in Britain and Brazil, involving 11,636 people, alongside safety data from a total of 23,745 participants in trials in Britain, Brazil and South Africa.