Researchers believe an initial half dose may be beneficial because it more accurately mirrors how the body would see the virus in the real world.
Professor Sarah Gilbert, the lead researcher of the vaccine development programme and professor of vaccinology at the University of Oxford, said: “We’ll be digging into the details of exactly why we get a better efficacy with a half dose, but it may be because that better mimics what happens with a real infection.
“What we’ve always tried to do with the vaccine is fool the immune system into thinking that there’s a dangerous infection so we get new memory and it’s there waiting and ready if the pathogen itself is then encountered.
“It could be that, by giving a small amount of the vaccine to start with, following up with a big amount, that’s a better way of kicking the immune system into action and giving us the strongest immune response and the most effective immune response.”
The vaccine also appears to protect against hospitalisation and severe disease in people who do get the virus, and researchers said they were seeing early signs that the jab prevented people transmitting the virus to other – which would make it the first vaccine to show it also stops the spread.
In contrast, results from the Pfizer and Moderna vaccines have only shown that they stop people from picking up the virus.
AstraZeneca said there were four million dozes already in vials ready for distribution, but regulators must give the green light before distribution can begin. That process is likely to take several weeks, but the first jabs are expected to take place in Britain this year.
Mass vaccination centres are being planned, with care home residents and staff getting the jab first, followed by healthcare workers and the over-80s.