The Oxford University vaccine trial is heading into hospitals amid fears that Covid-19 is not prevalent enough in wider society, a leading scientist has revealed.
John Bell, regius professor of medicine at Oxford University, said more than 1,000 people had been vaccinated in the first phase of the project and that, so far, things were going well and the drug looked safe.
However, as researchers wait for an "efficacy signal" that will establish whether those who have been given the vaccine can ward off the virus, Prof Bell admitted there was a risk that there may not be enough "active disease" to infect people, prompting the team to employ different tactics.
"The disease is on the wane and there is a risk that we won't be enough active disease to catch people," he told BBC Radio 4's Today programme.
"We're doing some clever things. We have good data now on how much disease is around. But the population that is still at pretty high risk are healthcare workers.
"So they will be moving, or will have already moved into the healthcare worker population, because there the disease prevalence is about four per cent. So they should be able to get a signal from those individuals, we hope."
The university has struck a deal with AstraZeneca, the UK's largest drugmaker, to ensure its vaccine can be manufactured at scale.
However, they have admitted that if the peak of disease transmission arrived before the vaccine was ready for trials, the studies would be challenging.
Conducting trials when the peak has subsided means that so many people will have developed a natural immunity that the amount of transmission will have dropped and those still not immune will take longer to be exposed to the virus.
Asked about the ethics of infecting volunteers with Covid-19, Prof Bell admitted that, while such an approach had been discussed, it was not ideal.
He said that "in reality" researchers did not want to know if the vaccine worked in healthy 21-year-old males as they were very unlikely to die from the virus.
"You want to know if it works in a 78-year-old, unhealthy, frail, vulnerable person," he added. "So it's fraught with all kinds of problems.
"Look, we're not that far away from an answer if we just keep our heads down. I think that’s the safest bet."