There is only a 50% chance that the Oxford coronavirus vaccine will work because cases in the UK are decreasing so quickly, warned one of the scientists behind it.
The Jenner Institute at the University of Oxford and the Oxford Vaccine Group started developing a COVID-19 vaccine in January using a virus taken from chimpanzees.
But with the number of coronavirus cases in the UK dropping every day, there may not be enough people to test it, according to the institute's director, Professor Adrian Hill.
He told the Sunday Telegraph: "It's a race against the disappearing virus and against time. We said earlier in the year that there was an 80% chance of developing an effective vaccine by September.
"But at the moment, there is a 50% chance that we will not be successful. We are in the bizarre position of wanting COVID to stay, at least for a while."
On Saturday, 282 people died of the virus in the UK in hospitals, nursing homes and the community.
The number has dropped dramatically since the height of the outbreak in Britain, when nearly a thousand people died every day in hospitals alone.
Testing of the vaccine – officially known as ChAdOx1 nCoV-19 (pronounced Chaddox One) – began with an initial testing phase on 160 healthy volunteers between 18 and 55 years of age, to see if it could effectively fight the virus .
The study is scheduled to advance to a second and third phase, which will involve testing up to 10,260 people and expanding the age of participants to include children and the elderly.
But if insufficient people are able to catch the virus, scientists will not have enough evidence to prove it is effective and launch it for use on the NHS.
ChAdOx1 nCoV-19 is produced from ChAdOx1, a weakened version of the common cold virus (adenovirus) that causes infections in chimpanzees.
The virus was engineered not to harm humans, but it also contains part of the coronavirus to trigger the body's immune response to COVID-19's ear proteins, which it uses to enter human cells and multiply.