A UK coronavirus vaccine trial now only has a 50% chance of success, a leading scientist has warned.
Professor Adrian Hill, whose team at Oxford University are racing to develop a jab, had originally hoped their efforts would be proved successful by September.
The Oxford vaccine is one of eight around the world and just one in the UK which is currently being tested in humans.
Earlier in the year, scientists said they were “80% confident” it would be proved effective by the autumn, making it one of the most promising candidates in the world.
It is widely believed that a vaccine is the only long-term way to stamp out Covid-19 and allow lockdown measures to be fully lifted.
However, if the virus is not spreading in the community, volunteers who have had the jab cannot catch it and scientists will not know if they have immunity.
More than 10,000 people – including over 70s and 5-12-year-olds – have been enrolled in the second phase of the trial.
However, as the UK emerges from the peak of its coronavirus epidemic, Prof Hill says he now expects just 50 of these volunteers to catch it.
If fewer than 20 test positive, the results will be of limited use.
“We’re in the bizarre position of wanting Covid to stay, at least for a little while. But cases are declining,” Prof Hill told the Telegraph.
Tipped to be a ‘front runner’ in the fight against Covid-19, the UK government has pinned their hopes on the Oxford project, and has already agreed to pay for up to 100 million doses.
Business Secretary Alok Sharma has also suggested that if proved to work, half of the UK population could be given the vaccine by autumn.
Earlier this week, pharmaceutical giant AstraZeneca announced a $1.2bn deal with the US government to produce an additional 400 million doses.
The team is now looking to move its trials to coronavirus hotspots overseas, where the deadly bug is still spreading.
“The US government, the UK government, loads of charities and philanthropists are all saying we’ll pay to have it manufactured, before you finish the trial. I mean, it’s a huge kind of vote of confidence in what we’re doing. It’s really flattering,” Prof Hill continued.
“But that doesn’t guarantee the result. It could be nothing or could be great or somewhere in between.
The University is expected to share results from the first stage of its trials next month, which was conducted with 1,000 volunteers in April.
Globally, there are eight potential coronavirus vaccines which have reached the stage of human trial.
Four of these are in China, while two are in the US and one in Germany.