Leading UK scientists racing to develop a Covid-19 vaccine are concerned that as the virus diminishes in Britain their tests may not be effective.
Researchers at the University of Oxford's Jenner Institute and the Oxford Vaccine Group are warning that their initial hope of an 80 per cent success rate for the vaccine has now dropped to 50 per cent.
"It is a race, yes. But it's not a race against the other guys. It's a race against the virus disappearing, and against time," Professor Adrian Hill told the Telegraph newspaper.
“We said earlier this year that there was an 80 per cent chance of developing an effective vaccine by September. But at the moment, there’s a 50 per cent chance that we get no result at all.
“We’re in the bizarre position of wanting Covid to stay, at least for a little while. But cases are declining."
It comes as the number of deaths in the UK related to Covid-19 increased by 118 on Sunday, bringing the total to 36,793 since the outbreak began.
The number of people dying after testing positive for the virus has fallen steadily since the peak of the country’s outbreak last month, when almost 1,000 people were dying daily.
The researchers are now testing an experimental vaccine with a goal of immunising 10,000 people to determine if the shot works.
The team began vaccinating more than 1,000 volunteers in a preliminary study designed to test the shot’s safety.
While awaiting results, the team announced they were expanding the testing to 10,260 people across Britain, including older people and children.
The announcement comes as Chinese scientists developing a similar vaccine reported promising results from their own early testing, seeing hoped-for immune reactions without serious side effects in 108 vaccinated people.
Andrew Pollard, head of the Oxford Vaccine Group, says if it all goes smoothly it is hoped that from autumn the vaccine will be approved for use on a wider scale.
However, he said there are still many challenges ahead, including how long it will take to prove the vaccine works – particularly since transmission has dropped significantly in Britain – and any potential manufacturing complications.
The Oxford trial involves one of around a dozen experimental Covid-19 vaccines in the early stages of human testing. It remains unclear, however, that any will ultimately prove safe and effective.
Drugmaker AstraZeneca has said it has secured its first agreements to produce 400 million doses of the Oxford-developed vaccine.
The AstraZeneca investment hopefully will make the vaccine available globally, including in developing countries, said Lawrence Young of the University of Warwick.
But Mr Young repeated that the shot's effectiveness is unclear at this stage, citing research with monkeys.
“This raises serious questions about the ability of the vaccine to protect against infection in humans and to prevent virus transmission,” he said in a statement.
“We need to be urgently exploring other vaccine candidates.”